or shell. _Ringmer._
1_a_.--The dorsal surface of the same.
2. --Corystes[A] Stokesii (G. A. M.). _Ringmer._
3. --Corystes[440] Broderipii (G. A. M.). _Ringmer._
3_a_.--The under surface of the same species, displaying the
sternal plates and the bases of the claws, three of which
are marked _a, a, a_.
]
[440] These two crustaceans are included in the fossil genus
Notopocorystes, established by Professor M’Coy, 1849.
There are two species that appear to have some relations with
_Corystes_, a genus which includes several recent crustaceans that
inhabit our shores, and are characterised by their elongated oval
shell and four antennæ, the external pair being long, setaceous,
and furnished with two rows of cilia. The tail is folded under the
body when the animal is in repose. They have ten legs, the anterior
pair _chelate_ (_with pincers_), the others terminating in an acute
elongated nail or claw. The fossils consist of the carapace, and one
example possesses the inferior or thoracic plates and the remains of
the bases of some of the legs (see _Lign. 168, fig. 3a_).
Notopocorystes Stokesii. _Lign. 168, fig. 2._--The carapace is relatively
wider than in most species of this genus; is has a strong dorsal ridge
of irregular oblong tubercles; the union of the cephalic and thoracic
segments is marked by a transverse undulated groove; there are three or
four tubercles on the surface of each lateral portion of the former,
and one on each of the latter. The whole surface is finely granulated.
The openings left by the attachment of the peduncles of the eye remain.
Notopocorystes Broderipii. _Lign. 168, fig. 3._--This species, like
the former, has a transverse undulated furrow, indicating the union of
the cephalic with the thoracic segments; the dorsal ridge is smooth,
and there are two tubercles on each lateral cephalic portion of the
shield. The carapace is longitudinally ovate, much depressed, with
three sharp points directed forwards on each margin of the anterior
part: the whole surface is finely granulated. In the specimen _fig._
3_a_ the sternal plates, with portions of the first joints of the
claws, remain; one example (figured _Geol. S. E._ p. 169) possessed
six or seven arcuate abdominal segments, which were turned under the
body.[441]
[441] I have described these small crustaceans somewhat minutely, and
have given them specific names, in the hope of directing the attention
of collectors to these highly interesting relics, and leading to the
discovery of more illustrative examples. See Foss. South D. pp. 96, 97.
The carapace or shell of the other crustacean observed in the Sussex
Galt (_Notopocorystes Bechei_) is of an orbicular inflated form (see
_Geol. S. E._ p. 169, _fig._ 3), and ornamented with twelve or thirteen
aculeated tubercles; its margin is dentated.
In the friable arenaceous limestone of the Cretaceous formation at
St. Peter’s Mountain, near Maestricht, the cheliferous claws of a
small kind of crustacean (_Mesostylus Faujasii_, _Wond._ p. 338),
are frequently discovered (and occasionally in the Chalk of Kent and
Sussex), but with no vestige of the carapace or shell. This curious
fact is explained by the analogy existing between the fossil claws and
those of the _Pagurus_, or Hermit-crab, whose body is only covered by a
delicate membrane, the claws alone having a calcareous covering; hence
the latter might be preserved in a fossil state, while no traces of the
soft parts remained. In the fossil, as in the recent claws, the right
arm is the strongest. There is no doubt that the crustaceans to which
the fossil claws belonged possessed the same modification of structure
as the recent (anomurous) Hermit-crab, and must have sought shelter
in the shells of the mollusks with which their durable remains are
associated.
[Sidenote: FOSSIL LOBSTERS.]
Fossil Lobsters.--The _macrurous_, or long-tailed, crustaceans, as the
_Lobster_, are distinguished from those of the former divisions by the
prolonged abdomen (or tail, as it is commonly termed), which forms
a powerful instrument of locomotion, and enables the animal to dart
backwards through the water with great rapidity; and this is furnished
with an appendage or tail, which none of the ambulatory crustaceans
possess.
Of the fresh-water species, the Cray-fish (_Astacus fluviatilis_),
and of the marine, the Lobster (_Astacus marinus_), are illustrative
examples. The remains of three macrurous species occur in the London
Clay of the Isle of Sheppey, associated with congenerous crustaceans;
and the segments of the tails (_post-abdomen_) are often well
preserved.
The Chalk contains a few species of the _Macrura_, which were first
discovered in the quarries near Lewes, and are figured in _Foss. South
D._ tab. xxx. xxxi.; they are among the most rare and delicate of the
fossils of the Cretaceous strata. These remains consist of the carapace
and claws, and rarely of the tail and antennæ, and are composed of
a friable earthy crust, which, when first discovered, is of a dark
chocolate colour, but quickly changes to a pale fawn, or reddish brown,
by exposure to the air. In the specimens obtained by breaking the
stone, the crustaceous covering remains attached by its rough external
surface to one portion of chalk, and on the corresponding piece are
sharp casts of the carapace and claws, having a glossy surface covered
with minute papillæ formed by the bases of tubercles or spines. Four
species have been observed.
Enoploclytia Leachii. _Lign. 169, figs. 1, 2, 3._--This is a long
delicate crustacean, having a pair of equal, slender, anterior chelate
claws, the fingers of which are long, attenuated, and armed with a
row of obtuse cylindrical spines. The surface of the hand-claws is
muricated, or covered with short erect aculeated tubercles. The pincers
in the specimen figured _Lign. 169, fig. 3_, are shorter than in
most examples. The carapace is elongated and sub-cylindrical, with a
dorsal ridge and two lateral furrows, indicating the normal division
of the cephalic and thoracic segments of the shield; the antennæ are
long, filiform, and setaceous (_bristly_), and are placed on squamous
peduncles (see _Lign. 169, fig. 2_).
There appear to have been five legs on each side; the anterior or
chelate pair are the most usual relics of this animal; of the other
claws and the branchiæ, but obscure indications have been obtained.
The abdominal segments are arcuate, and six or seven in number; their
surface is granulated; the appendage, or tail, is foliaceous and
marginate, with a few longitudinal ridges (see _Lign. 169, fig.
1_).[442] The claws of these crustaceans may be easily recognised by
their general aspect, and the length and straightness of the fingers or
pincers. In most examples traces remain of the tendinous expansion of
the muscles of the moveable claw (see _Lign. fig. 3_).
[442] See Foss. South D. p. 221, _et seq._ for further details.
[Illustration: Lign. 169. Fossil Crustaceans: _nat._ _Chalk._ _Lewes._]
Fig. 1.--Abdominal appendage, or tail, of Astacus[443] Leachii (G. A. M.),
with three abdominal segments.
2.--Part of the head of _Astacus Leachii_, with remains of the
squamous peduncles of the antennæ, and of the long setaceous
antennæ.
3.--Chelate claw, with part of the carpus, or wrist, of _Astacus
Leachii_.
4.--Chelate claw of Astacus[443] Sussexiensis (G. A. M.).
[443] These fossil Lobsters are now to be referred to Prof. M’Coy’s
genus Enoploclytia.
Enoploclytia Sussexiensis. _Lign. 169, fig. 4._--The claws of this
species are readily distinguished from the former by their broader and
stronger proportions, and spinous character; the pincers are strong and
tuberculated, and the moveable finger is more curved and shorter than
its opponent. The entire crust of this lobster is muricated, or beset
with spines and sharp tubercles.[444]
[444] Foss. South D. tab. xxx. _fig._ 3. In the beautiful work of
the late Mr. Dixon, on the Fossils of Sussex, there are figures of
very fine specimens of these two species of Enoploclytia (plate
xxxviii*.) with detailed descriptions by Prof. Bell (p. 344); but
unfortunately, from inadvertence, no reference is made to my former
drawings and descriptions of these fossil Lobsters of the Chalk,
and new names are given by the Professor; _E. Leachii_ appearing as
"Palæastacus macrodactylus," and _E. Sussexiensis_ as "P. Dixoni." I
need scarcely remind my readers that according to the established rules
of Nomenclature the _earlier specific names_ must be preserved; and
that, Prof. M‘Coy having already indicated the distinction of these
fossils from the recent _Astacus_ by providing an appropriate generic
appellation, these new names are altogether inadmissible.
Claws of other _Astacidæ_ have been found in the Chalk of Sussex and
Kent; one species in particular is distinguished from those previously
described by its short curved pincers, and granulated surface;[445] and
remains of other species of _Enoploclytia_, from near Cambridge and
Maidstone, are described by Prof. M’Coy.
[445] This species may be named _Astacus cretosus_, to indicate its
geological habitat.
The "Lobster-clays" in the Lower Greensand, or Neocomian, beds at
Atherfield afford numerous fine specimens of a small long-clawed
crustacean, allied to _Astacus_; a fine specimen is figured in the
_Geology of the Isle of Wight_, title-page vignette, and see page 232.
One or more species, apparently distinct from those of Atherfield, have
been discovered by Mr. Beckles in beds of clay that appear to occupy
the line of junction between the Wealden and Greensand, on the Sussex
coast, between Pevensey and Bexhill.
Imperfect claws of _Astacidæ_ have also been found at other places in
the Shanklin Sand; and I have collected from the Galt at Ringmer, the
abdominal segments of a small species, resembling _Meyeria ornata_,
_Lign. 170_.
[Illustration: Lign. 170. Fossil Crustacean. _Speeton Clay._
_Yorkshire._
Meyeria ornata: _nat._ (_Phillips and M’Coy._)]
The carapaces of two or three small _Astacidæ_, sometimes with the
abdominal segments attached, as in the beautiful fossil figured in
_Lign. 170_, are found in the Speeton Clay, near Scarborough. In
this example the post-abdomen and its appendages are entire, and
traces remain of the antennæ and some of the feet. These specimens
are commonly imbedded in masses of indurated clay, as the fern-leaves
in the Carboniferous ironstone, and are discovered by splitting the
nodules through their longest diameter. The carapace of another small
species (_Glyphea rostrata_)[446] occurs in the Oolite of Scarborough.
My friend, the Rev. J. B. Reade, has also discovered an example of this
species in that rich repository of organic remains, the Kimmeridge
Clay, at Hartwell, Bucks.
[446] This is the _Astacus rostratus_ of Prof. Phillips’s Geol. York.
vol. i. tab. 4, _fig._ 20.
A remarkable macrurous crustacean (_Eryon Cuvieri_) is found in the
Jurassic limestone of Solenhofen. Perfect specimens of this species
are occasionally seen in collections; it is distinguished by its very
large, flat, oval shell, with the front lateral margins strongly
dentated, and by its short setaceous antennæ; the front claws are as
long as the body, and armed with pincers; the post-abdomen consists of
six segments, terminating in a caudal appendage or tail.
In the United States several fossil crustaceans have been noticed in
the Cretaceous strata of New Jersey; some of which are said to be
related to _Pagurus_, and others to _Astacus_.
Fossil Prawns and Shrimps, of exquisite beauty, are found in the
lithographic limestone of Pappenheim: a specimen from that locality
(_Palæmon spinipes_) is figured _Wond._ p. 513: see also Frontispiece
of this work.
A large crustacean of the Shrimp family has been discovered by the Earl
of Enniskillen in the Lias of Lyme Regis.[447] Other specimens also of
Macrura, more or less perfect, have been obtained from the same rich
mine of organic remains: especially some in which the _branchiæ_, or
respiratory organs, remain; and a portion of the post-abdomen, or tail,
of a Cray-fish, as large as the common species.
[447] This beautiful fossil is figured and described by Mr. Broderip,
Geol. Trans, second series, vol. v. pl. xii. under the name of _Coleia
antiqua_.
[Sidenote: FOSSIL ISOPODOUS CRUSTACEANS.]
Isopodous Crustaceans. _Lign. 171._--Isopoda (_equal-feet_) is the term
applied to an order of crustaceans in which the body is composed of a
distinct head, and seven rings, each having a pair of equal feet; the
common _Oniscus_, or wood-louse, is a familiar example of a terrestrial
_Isopod_. This order includes many genera and species, some of which
nearly approach the extinct family of crustaceans (Trilobites) whose
remains abound in the palæozoic strata; and the parasitical Isopod,
_Bopyrus_, that infests the common Prawn, is closely related to certain
genera of Trilobites, hereafter described.
[Illustration: Lign. 171. Archæoniscus Brodiei. _Purbeck limestone._
_Vale of Wardour._ (_Drawn by S. P. Woodward, Esq._)
Impressions of the upper and under surfaces.]
One species of fossil _Isopod_ has been found in green fissile marl,
at Montmartre; and another in fine-grained limestone, probably, from
Pappenheim.[448] No fossil remains of this order had been noticed
in the British strata, until the discovery of the _Archæoniscus_ in
the Purbeck strata by the Rev. P. B. Brodie. The quarry in which
these relics were found is situated at Dallards, near the village of
Dinton, about twelve miles west of Salisbury. They are principally
distributed in a bed of light-brown and grey limestone, in the lower
part of which are numerous fresh-water bivalves (_cyclades_), and a
few small oysters. These Isopods often occur in clusters (see _Lign.
171_); the lenses of the compound eyes are sometimes detectable in the
limestone, and, rarely, attached to the head; traces of legs have been
observed, but no antennæ. In the same stratum the _elytron_ (sheath,
or wing-case) of a coleopterous insect was discovered. Mr. Brodie has
obtained specimens an inch and a half in length, and an inch broad.
These fossils appear to have been deposited tranquilly at the bottom of
the water which they inhabited, since they are usually found imbedded
with their legs downwards, and generally well preserved.[449]
[448] Nat. Hist. Crust. Foss. p. 138.
[449] See Brodie’s Fossil Insects, pp. 3 and 10.
The _Archæonisci_ also occur in the Purbeck insectiferous limestone of
Durlstone Bay, near Swanage, and have been discovered in strata of a
similar character at the Ridgway railway-cutting between Dorchester and
Weymouth, by the Rev. Osmond Fisher, of Dorchester.
Entomostraca.--The Crustaceans that we have above noticed belong to the
Sub-class Malacostraca; and we have now to describe some fossil genera
belonging to various divisions of the Entomostracous Crustaceans.
[Illustration: Lign. 172. Fossil. Limulus, in a Nodule of Ironstone:
1/2 _nat._
_Carb._ _Coalbrook Dale._]
Fig. 1. Limulus Rotundatus, the imprint of _fig._ 3.
2.--The nodule before it was broken.
3.--Limulus Rotundatus, (the corresponding portion of the nodule,
_fig._ 1,) showing the entire form of the carapace.
[Sidenote: FOSSIL LIMULUS.]
The Limulus (Mollucca or King Crab) is a genus belonging to that
Division of the Entomostraca termed Pœcilopoda and is abundant in
the seas of India and America. The carapace is crustaceous and of
a semilunar form, the head and thorax are blended together, and the
superior abdominal shield, which is composed of confluent segments,
appears like one piece, and has an indistinct trilobed character; the
last segment is prolonged into a three-edged, sharp, styliform weapon.
The Limulus has two reniform, compound eyes, composed of facets of a
peculiar form. The gills are disposed on lamelliform processes. It is
distinguished from all other crustaceans by the mastication of its food
being performed by the first joint of the thoracic legs which surround
the mouth, instead of by jaws.
[Illustration: Lign. 173. Limulus trilobitoides.
_Coal Measures._ _Coalbrook Dale._]
Very fine examples of a fossil species of this remarkable genus are
occasionally obtained from the lithographic stone of Solenhofen.[450]
In England three small species have been discovered in nodules of
ironstone and indurated clay, in the Carboniferous strata of Coalbrook
Dale, by Mr. Prestwich.[451] In one example (_L. Anthrax_[452]) two of
the legs are seen extending from under the body; in another species the
sharp, pointed process of the tail is well developed (_Buckl._ p. 396,
and tab. xlvi″.; see also _Lign. 173_). The specimen figured _Lign.
172_ is one of several examples which I obtained by breaking up nodules
from that celebrated locality. _Fig. 2_ represents the nodule unbroken,
and without any indication of its contents; by a well-directed blow
it was separated into two equal portions, _figs._ 1 and 3, in which
the carapace of the crustacean, and its marginal appendages, are well
displayed. The rounded form of the carapace, and the membrane which
appears to connect the spines, separate this species from all others.
[450] Hist. Crust. Foss. pl. xi. _fig._ 6.
[451] See Mr. Prestwich’s Memoir on the Geology of Coalbrook Dale,
Trans. Geol. Soc. second series, vol. v. part 3.
[452] Trans. Geol. Soc. second series, vol. v. pl. xli. figs. 1-4.
The Eurypterus and Pterygotus are palæozoic crustaceans, of large size.
They are regarded by Prof. M‘Coy and Mr. Salter as belonging to the
Pœcilopoda, and as differing from the Limulus chiefly in having the
segments of the abdomen freely articulating with each other.[453] The
_Eurypterus_ was first described by Mr. Dekay, in the United States,
probably from the Carboniferous system. The head is round, the thoracic
and cephalic portions of the carapace being blended together, and the
abdominal region is formed of eleven segments, with a caudal appendage.
It has two depressed, lunated eyes, remote from each other, and eight
feet, the anterior pairs furnished with _branchiæ_, and the hindmost
pair relatively larger than in any other crustacean. Two American
species are described; the one is five, and the other about four inches
long.[454] In the Carboniferous strata at Burdie-house, near Edinburgh,
and of Kirton, near Glasgow, a large species of this curious genius has
been found by Dr. Hibbert and Dr. Scouler; the length of some specimens
being estimated at from twelve to eighteen inches.[455]
[453] See Salter on Pterygotus, Quart. Journ. Geol. Soc. vol viii p.
387.
[454] Dr. Harlan, in Trans. Geol. Soc. Pennsylvania, vol. i. p. 96.
[455] See the elegant Memoir on the Fresh-water Limestone of
Burdie-house, near Edinburgh, by Samuel Hibbert, M.D. F.E.S.E.
Other species of _Eurypterus_ have been noticed in the Upper Silurian
rocks of Kendal, Westmoreland,[456] and of Kington, Radnorshire; and in
the Devonian of Russia.
[456] Pal. Foss. Cambridge, Appendix.
Pterygotus.--In the Old Red sandstone of Forfarshire, and other parts
of Scotland, the remains of this remarkable crustacean have been long
known to the quarry-men by the name of "_petrified Seraphims_;" from
an imaginary resemblance of the expanded post-abdomen to the usual
representations of those ideal beings! This genus is characterised by
the angular carapace, which forms a lozenge-shaped shield; and the
appendage of the post-abdomen, which, instead of being divided into
segments, as in most animals of this class, is a continuous flap. The
eye-pits on the carapace are like those of Eurypterus, but are very
large. The claws resemble those of the common lobster. The external
crustaceous covering is ornamented with circular and elliptical
markings, producing an imbricated or scaly appearance, the imprints
of which gave rise to the enigmatical "Seraphims" of the Forfarshire
sandstone. Some specimens indicate a total length of four feet![457]
[457] The Old Red Sandstone, or New Walks in an Old Field, by Hugh
Miller, p. 147. There are specimens in the British Museum from the
quarries of Carmylie.
Besides this Devonian species (_P. anglicus_), there is also a Silurian
species (_P. problematicus_), from Herefordshire.[458]
[458] See Quart. Journ. Geol. Soc. vol. viii. p. 386.
* * * * *
Several fossil genera of the Entomostracous Crustaceans belong to the
_Phyllopoda_ (_leaf-feet_), which constitute a subdivision of the
Branchiopoda (_gill-feet_). Of these perhaps the most remarkable is
the Dithyrocaris, first discovered by Dr. Scouler in the Carboniferous
shale near Paisley. This genus is allied to the recent _Apus_, and,
like it, has a broad, flat, thin carapace, easily divisible down the
middle of the back, and a lengthened tail or post-abdomen, with a
trifid termination. Six species have been found in the Carboniferous
deposits of the British Isles.
Ceratiocaris and Hymenocaris, which are related to the recent
_Nebalia_, are also of the Phyllopod group, and maybe said to resemble
a shrimp-like animal partly enclosed in a bivalve carapace, while its
tail is exposed, and either protruded, or turned beneath the body.
Of the first-mentioned genus there are three Upper Silurian species;
and of _Hymenocaris_ Mr. Salter has described a single species (_H.
vermicanda_) from the Lower Silurian (Cambrian) of North Wales.
The _Limnadiadæ_,--another Phyllopodous family, of which the recent
_Limnadia_ is the type,--are well represented in the older rocks.
These Crustaceans are also bivalved, but the body is wholly enclosed.
They are of small size. Estheria, a genus closely related to Limnadia,
occurs plentifully in the Wealden of Sussex and Germany, and in
the Lias of Westbury. Mr. Bean also has noticed a species (_E.
concentrica_) in the Oolite shell of Gristhorpe Bay; and the _E.
minuta_[459] is characteristic of the Keuper division of the Trias in
England and Europe.
[459] This was formerly described as a _Posidonomya_.
Leperditia and Beyrichia are other fossil genera belonging to the
_Limnadiadæ_. These little crustaceans were gregarious in their
habit, and, like Estheria, occur locally in great profusion. They are
characteristic of the Silurian deposits; the former being an abundant
Upper Silurian fossil in Europe and North America, and the latter,--a
very minute form,--both in the Upper and Lower Silurian.
The next group of Entomostraca that we have to notice belongs
to the Lophyropoda (_feet crested with bristles_), namely, the
Ostracoda (_shelled_). The recent genera, _Cypris_, _Cythere_, and
_Cypridina_,[460] are types of the three families of this group, and
are the existing representatives of numerous closely related forms that
occur in a fossil state.
[460] The student should consult Dr. Baird’s elegant and elaborate work
on the Natural History of the British Entomostraca (published by the
Ray Society), for information on the characters and habits of these
interesting little crustaceans and their numerous allies
[Illustration: Lign. 174. Fossil Cyprides. _Wealden._ _Sussex and Isle
of Wight._]
Fig. 1.--Cypris Valdensis: _magn._; showing the ventral aspect of
the united valves. _Cooksbridge._
1_a_.--Side view of the same, _magn._ The three small figures are
of the _nat. size_.
2.--Cypris Fittoni, _magn._ _Langton Green._
3.--Cypris tuberculata, _magn._ _Langton Green._
3_a_.--View of the same, showing the lower edges of the valves
united: the three small figures, _nat._
4.--Cypris granulosa, _magn._; view showing the hinge-line.
5.--A piece of Weald Clay, with Cyprides and Shells; _nat._
Cypris. _Lign. 174._--The animals of this genus belong to those
Crustaceans in which the covering of the body is not divided into
transverse segments, but consists of a large dorsal shield, having
the form of a bivalve shell. They are often very minute, and numerous
kinds swarm in our lakes and pools. The species of an allied genus,
named _Cythere_, the shells of which cannot always with certainty be
distinguished in a fossil state from those of _Cypris_, inhabit salt
or brackish waters. As the living _Cyprides_ are interesting objects
for microscopic examination, they are commonly shown in the exhibitions
of the hydro-oxygen microscope, and their appearance is therefore well
known. Two recent species are figured in _Ly._ p. 183. These animals
have the body enclosed in a bivalve, horny case, the two pieces being
united by a hinge-line. They have four feet, and two pairs of antennæ,
with a pencil of cilia at the extremities; and one compound eye. They
swim with rapidity, by means of their ciliated antennæ, and crawl about
actively on the water-weeds and other subaqueous surfaces. Like the
other crustaceans, they frequently moult or cast their cases, and the
surface of the mud spread over the bottoms of still lakes is often
covered with their exuviæ. The largest living Cypris (_C. clavata_)
does not exceed one-eighth of an inch in length. The fossil cases or
shells of Cyprides are found in considerable variety and in prodigious
numbers in certain Tertiary and Secondary strata, which appear to have
been deposited by fresh-water; as, for example, in the lacustrine marls
of Auvergne (_Ly._ p. 183), and the fluviatile clays and limestones of
the south-east of England. They have not been observed in any decidedly
marine beds; but Mr. Lonsdale discovered in the Chalk, by means of the
microscope, cases of crustaceans, that belong to the genus _Cythere_
(_Ly._ p. 26, figs. 21-24), the recent species of which inhabit the
sea. One or more species of Cypris have been observed in the older
British Tertiaries,--by Sir C. Lyell, in Hordwell Cliff (_Geol. Trans._
2d ser. vol. iii. p. 288), and by Mr. Prestwich at Hempstead Cliff
(_Brit. Assoc._ 1846); but in the Pleistocene deposits the Cypris and
its congeners abound, and of these Mr. R. Jones has enumerated nine
species (_Annals Nat. Hist._ 2d ser. vol. iii.). In many districts on
the Continent, the Eocene marls and clays abound in these remains.
Some of the fresh-water Tertiary strata of France contain myriads of a
Cypris (named _C. faba_, from its bean-like form) which was formerly
supposed, but erroneously, to be identical with a species found in
the Wealden. The laminated marls of Auvergne contain, between every
layer, countless myriads of the shells of Cyprides, through a depth of
several hundred feet; although each lamina of marl scarcely exceeds
the thickness of paper. Near Clermont, the green _cypridiferous_ marls
approach to within a few yards of the granite which forms the borders
of that ancient basin (_Ly._ p. 184). In the eastern portion of the
basaltic districts of India, Mr. Malcolmson has discovered two species
of Cypris, associated with fresh-water univalves and bivalves.[461]
In England the principal deposits of the Cyprides, are the clays
and limestones of the Wealden and the Isle of Purbeck,[462] to the
composition of which the relics of these minute beings have largely
contributed. Entire layers of their cases are found in the laminated
clays and marls on the southern shores of the Isle of Wight, at
Atherfield and Sandown Bay, where some of the Wealden beds emerge from
beneath the lower division of the Shanklin sand. Upon splitting the
clay in the direction of the laminæ, the exposed surfaces are found to
be covered with these minute bodies; as in the specimen, _Lign. 174,
fig. 5_.
[461] Geol. Trans. 2d ser. vol. v. pl. xlvii.
[462] On account of the distinctive characters of their
carapace-valves, M. Bosquet has suggested the appellation _Cypridea_ as
a generic name for the Cyprididæ of the Wealden and Purbeck beds.
The appearance of four characteristic Wealden Cyprides, of their
natural size and magnified, is shown in _Lign. 174_. The one named _C.
Valdensis_, or Wealden Cypris, by Dr. Fitton and Mr. Sowerby (fig.
1), is the most frequent, and occurs in numerous localities in Kent
and Sussex. A Cypris having the case studded with relatively large
tubercles (fig. 3), is found in many of the finer sandstones of
Tilgate forest; another, with the shell tuberculated, but divided by
a transverse ridge (fig. 2), indicating a rudimentary condition
of the segments which characterise the class, is certainly distinct
from the former, and can have no name more appropriate than that of
_C. Fittoni_. The other beautiful species (_C. granulosa_, fig. 4)
has the surface of the case covered with granules. One more species
has been observed in the Weald clay, at Sandown Bay and Atherfield
(by Mr. Lonsdale); it is distinguished by a short conical spine on
each valve, and is therefore named _C. spinigera_. In the Weald clay
at Resting-oak-hill, near Cooksbridge (_Geol. S. E._ p. 187), _C.
Valdensis_ is so abundant, that every thin flake is covered with its
white calcareous shells; and upon breaking the nodules and septaria
of reddish-brown ironstone which occur in that locality, myriads of
beautiful sharp casts of the cases are observable in almost every
fragment. They are associated with fresh-water bivalves and univalves
(_Cyclas_ and _Paludina_), and minute scales of fishes. The sandstone
at Langton Green, near Tunbridge Wells, which contains casts and
impressions of several species of fresh-water shells, abounds in
_Cyprides_; and the layers of argillaceous ironstone, interstratified
with the sandstone in one of the quarries, are particularly rich in
these remains. The surface of a recently broken slab is often covered
by minute, polished, oblong, convex bodies, which are the casts of
cypridean carapaces.
The Sussex marble is largely composed of the remains of these minute
crustaceans. Upon examining thin polished slices of this limestone
under the microscope, the cavities and interstices of the shells
are found to be filled with the shields of Cyprides, entire or in
fragments; and some specimens of the Purbeck marble equally abound in
these remains. The Purbeck marls, as well as the limestones, often
abound with Cyprides. According to Prof. E. Forbes, these belong to
several species, all more or less distinct from those of the Wealden
(_Brit. Association_, 1850). The Cyprides of the Wealden of Germany
have been figured and described by Dr. Dunker; and several of the
British forms are found there.
As the recent species inhabit still lakes, or gently running streams,
and not the turbulent waters of estuaries, we cannot doubt that the
strata in which these animals so largely predominate were deposited in
lakes or bays, communicating with the river which transported to their
present situation the bones and other remains of the colossal reptiles
of the Wealden. And the beds of fresh-water snails, with scarcely any
intermixture of other organic remains but the Cyprides, which are
spread over extensive areas in the Wealden and Purbeck districts,
appear to afford corroborative proof of this inference.
Four species of minute bivalved Entomostraca from the Carboniferous
deposits have been referred to _Cypris_: viz. _C. arcuata_, Bean, from
the Coal-shale at Newcastle; _C. inflata_, Murchison, Coal-measures,
near Shrewsbury; _C. Scoto-Burdigalensis_, Hibbert, Coal-measures at
Burdie-house, and Coal-shale at Derry; and _C. subrecta_, Portlock,
also from Derry, Tyrone.
Cythere.[463]--This animal differs but little from Cypris except in
having an additional pair of feet. In the Subgenus Cythereis the
valves are thick, oblong, and strongly hinged; thus differing from the
thin and more or less oval valves of the true Cythere and of Cypris.
Several species of _Cythere_ and its sub-genera occur in the Tertiary,
Cretaceous, Oolitic, Liassic, Permian, and Carboniferous deposits.[464]
[463] For description and illustration of this genus, see Baird’s
British Entomostraca, p. 163, &c.
[464] Consult M. Bosquet’s Memoir on the Tertiary Entomostraca of
Belgium and France; Mr. T. R. Jones’s Monograph of the Cretaceous
Entomostraca (Palæontographical Society), and of the Permian
Entomostraca, in Prof. King’s Monograph of the Permian Fossils
(Palæont. Soc.); and Prof. M’Coy’s Synopsis of the Characters of the
Mountain Limestone Fossils of Ireland.
Three species of Entomostraca, very closely related to Cypridina,[465]
from the Carboniferous rocks of Belgium, hare been figured and
described by M. De Koninck; one species from the Carboniferous rocks
of Ireland, by Prof. M’Coy;[466] and two species from the Cretaceous
limestone of Maestricht,[467] by M. Bosquet. The genera _Cyprella_ and
_Cypridella_ have been established by M. De Koninck for the reception
of some allied forms found in the Carboniferous strata of Belgium; and
Entomoconchus (M‘Coy) and _Daphnoidia_ (Hibbert) are allied British
Carboniferous forms.
[465] See Baird’s British Entomostraca, p. 176, &c.
[466] Under the name of _Daphnia primæva_.
[467] Under the generic appellation of _Cyprella_.
Trilobites.--Among the numerous petrifactions which are found in the
limestones in the neighbourhood of Dudley, in Staffordshire, there
are certain fossil bodies which, from their extraordinary form and
appearance, have for more than a hundred and fifty years been objects
of great interest to the naturalist, and of wonder to the general
observer, and have long been provincially termed _Dudley insects_, or
_locusts_.[468] By the earlier naturalists these fossils were referred
to fishes, to molluscs, and to insects, before their real character was
discovered. The most common type consists of a convex, oblong body,
divided transversely into three principal parts, and longitudinally
into three lobes, by two deep, parallel furrows; this last character
suggested the name _Trilobita_, or _Trilobites_, by which the family is
now distinguished by naturalists.
[468] Lhywd. Philos. Trans. for the year 1698.
[Illustration: Lign. 175. Trilobites. _Silurian Limestones._
Fig. 1.--Illænus perovalis. (_Murch. Sil. Syst._)
2.--Trinucleus Lloydii. (_Murch. Sil. Syst._)
3.--Calymene Blumenbachii: the upper surface of the
carapace, viewed from above. Dudley.
4.--The same, coiled up, and seen in profile.
4_a_.--The same, seen in front.
]
These fossils are the carapaces, or shells, of crustaceans, belonging
to an extinct family,[469] which comprises many genera, and numerous
species. Mortimer, Da Costa, Guettard, and Linnæus recognised the
crustacean character of this interesting, but obscure class of organic
remains: their true affinities, however, were first scientifically
determined by Alex. Brongniart.[470] Many memoirs on the _Trilobites_
have since been published by eminent Continental and American
writers;[471] and much light has been thrown on the subject by the
labours of Martin,[472] Parkinson,[473] Stokes, Phillips,[474]
König,[475] Dr. Buckland,[476] Sir E. I. Murchison,[477] and others.
The beautiful illustrations and interesting description of the
Trilobites in Dr. Buckland’s Bridgewater Treatise (_Bd._ p. 389, and
pl. xlv. xlvi.) must have rendered the reader familiar with the most
important facts relating to these extinct beings; but subsequent
discoveries have thrown additional light on their structure and natural
affinities. The works of M. Burmeister,[478] Mr. Salter,[479] Prof
M’Coy[480] and especially of M. Barrande,[481] should be carefully
referred to by the student in this branch of Palæontology.
[469] The Trilobites appear to have been related more nearly to the
Phyllopoda than to any other division of the Crustacea.
[470] Hist. Nat. Crust. Foss. Burmeister on Trilobites; Introduction.
[471] A Monograph on the Trilobites of North America, by Jacob Green,
M.D. Philadelphia, 1832.
[472] Petrificata Derbiensia.
[473] Organic Remains of a Former World, vol. iii. See Pictorial Atlas.
[474] Geology of Yorkshire; and Palæozoic Fossils of Devon.
[475] Icones Fossilium Sectiles.
[476] Bridgewater Treatise.
[477] Silurian System, chap, xlvii.
[478] Die Organisation der Trilobiten. 1843. Translation: Ray Society.
[479] In the Quarterly Journal of the Geological Society, and in the
Memoirs and Decades of the Geological Survey of Great Britain.
[480] Palæozoic Fossils in the Cambridge Museum.
[481] The Silurian System of Bohemia: the Trilobites.
In the Trilobites the head is distinct, and without antennæ, and the
feet are supposed to have been rudimentary, soft, and membranaceous:
the essential characters which separate them from all other
crustaceans, except _Bopyrus_ (a parasite on the branchiæ of the common
prawn), are, according to Mr. Macleay, the deficiency of antennæ,
and of lateral posterior abdominal appendages, and the presence of
evanescent feet. Like other crustaceans, the Trilobites were subject
to the process of metamorphosis during their early stages of life;
and M. Barrande has ascertained that one species, the _Saö hirsuta_,
appeal’s in no less than twenty different stages of development. In
its earliest, embryonic condition, it is a simple disk, and it passes
through various stages until it becomes a perfect adult trilobite,
having seventeen free thoracic segments and two caudal joints.
No less than ten genera and eighteen species were instituted by
palæontologists on _some_ of the forms only which this one species
presents in its different stages of metamorphosis, before M. Barrande’s
laborious and long-continued investigations gave him an insight into
the true relations of these various conditions of the same animal
to one another. This talented and indefatigable palæontologist has
arrived at like results with other Trilobites, and has been enabled
to add greatly to our knowledge of the natural history and geological
distribution of this interesting group of crustaceans. See Transact, of
the Sections, Brit. Assoc. 1849 and 1850; and _Trilob. Bohême_, pl. vii.
The _Trilobites_ have been arranged in numerous genera, the names of
which in a few cases are expressive of natural characters, but in
others have reference to the obscurity that still invests some parts of
the organization of these animals.[482]
[482] As, for example, Asaphus, _obscure_; Calymene, _concealed_;
Agnostus, _unknown_.
Calymene Blumenbachii. _Lign. 175, figs. 3, 4._--This is the
Trilobite so well known as the Dudley locust, or insect. It consists
of an ovate, convex, trilobed crustaceous shell, or case, and is found
either expanded, as in _Lign. 175, fig. 3_, with its under surface
attached to, and blended with, the limestone (_Wond._ p. 789); or
coiled up like an Oniscus, or wood-louse, as in _figs._ 4 and 4_a_.
The head is large, convex, rounded in front, with a broad border, and
divided into three lobes by two longitudinal depressions. The eyes are
two in number, compound, and have numerous facets; they are situated
on the sides of the head, remote from each other. The carapace is
deeply trilobed by two longitudinal furrows; the thoracic portion is
composed of thirteen segments; the caudal shield is small and nearly
semicircular. This species is from one to four inches in length. It
occurs from the Lower Llandeilo rocks up to the Upper Ludlow inclusive.
The structure here described may be regarded as the normal type, but
numerous and important modifications prevail in the different genera.
[Illustration: Lign. 176 Homalonotus dephinocephalus.
(_Reduced from_ pl. vii. _Sil. Syst._) _Upper Silurian._ _Dudley._]
In the genus Homalonotus, _Lign. 176_, the thoracic portion of the
carapace is but obscurely lobed, and consists of thirteen segments;
the abdominal is distinct from the thoracic, and formed of nine rings;
it terminates in a prolonged point. The _H. Herschelii_ is a large
Trilobite, very plentiful in the Upper Silurian schists of the Cape of
Good Hope.
In another genus, Asaphus (_Geol. Surv. Decade 2_), the carapace is
wide and much depressed; the middle lobe distinct, the cephalic portion
rounded in front, and terminating posteriorly in a sharp process on
each side. The eyes are compound, and each contains upward of six
thousand lenses, many of which remain in some examples.[483] Some
American species belonging to this group are of a gigantic size, as,
for example, the _Isotelus gigas_ (of Mr. Dekay), which is eighteen
inches long. In the Isotelus[484] the body is of an oval shape, and the
posterior angles of the head are rounded; the thorax is composed of
eight segments.
[483] Mem. Geol. Surv. Decade 2, pl. v. p. 2, note.
[484] _Isotelus_, i. e. equal extremities.
Another division of the Trilobites has the body contracted, and very
thick, and the abdomen large and scutiform, without any segmentary
divisions; the small crustacean (_Illænus perovalis_, _Murch._) _Lign.
175, fig. 1_, will serve to illustrate these characters.
The Trilobite called Bumastus by Sir R. Murchison (from its grape-like
form) presents a very curious modification of the normal type. Both the
head and caudal extremity are rounded, with no distinct longitudinal
furrows; and the whole surface of the carapace is covered by extremely
thin, apparently imbricated, lamellæ, the edges of which are undulated,
and the intermediate spaces studded with minute dots. The eyes are
smooth, and not granulose, as in Calymene. This genus is known in
England by the name of the Barr Trilobite, from its occurrence in the
limestone near Barr, in Staffordshire; it is sometimes five inches
long, and three and a half wide (_Geol. Surv. Decade 2_, pl. iii. and
iv.).
The genus Ogygia (_Bd._ pl. xlvi. fig. 9) is characterised by the
elliptical and depressed form of the carapace, its nearly balanced
extremities, and the prolongation of the buckler, or cephalic portion,
on each side, into slender spikes, distinct from the body; the thoracic
and abdominal regions are divided by two deep, longitudinal furrows,
into three lobes; there is also a straight, longitudinal groove, in
the front of the buckler (see figures and descriptions of _O. Buchii_,
_Geol. Surv. Decade 2_). The Trilobites of this genus are found in the
Lower Silurian rocks of North Wales and Ireland; they occur also in
great abundance in the slate rocks of Angers, and some species are more
than a foot in length.
Some species of the genus Phacops have long, pointed caudal appendages,
as the _P._ (formerly _Asaphus_) _caudatus_ (_Lign. 177_; _Bd._ pl.
xlv. figs. 10, 10′; and _Geol. Surv. Decade 2_, pl. i.). The eyes are
often well preserved, and each contains about 240 spherical lenses.
[Illustration: Lign. 177. Phacops caudatus. _Upper Silurian._ _Dudley._
The caudal extremity.]
Trinucleus. _Lign. 175, fig. 2_; _Ly._ fig. 432.--This genus
comprises several small forms which are found in the Lower Silurian
rocks of England, and occur in the equivalent deposits of Sweden,
Norway, and Russia.[485] In the _Trinucleus_, the cephalic shield is
obtuse, trilobed, rounded, and terminating in lateral spikes; and
its margin is marked by numerous pit-like depressions. There are
six body-rings or thoracic segments. The caudal shield is large and
somewhat triangular. There are no distinct eyes.
[485] Murch. Sil. Syst. p. 217.
Paradoxides. _Lign. 178._--The Trilobites of this genus are easily
recognised by the ends of the lateral segments of the thorax and
abdomen terminating in deflected points, which extend in spikes beyond
the membrane they supported, and particularly those near the tail,
which are much elongated; whereas in the other genera the lateral
points of the segments are united by a membrane, which often forms
a border beyond them. The cephalic buckler is semicircular, and its
lateral angles are lengthened out behind into two strong spines; it
is divided on the median line into four protuberances, by transverse
grooves. The thorax consists of from sixteen to twenty segments; the
abdominal buckler is generally very small and rounded. The animals of
this genus have the body much depressed, and the lateral lobes wider
than the middle lobe: some species are of considerable size, attaining
several inches in length.
[Illustration: Lign. 178. Paradoxides Bohemicus: _nat._
_Silurian._ _Bohemia._]
A very peculiar form of Trilobite (_Brontes flabellifer_, _Ly._ p. 348)
is found in the Devonian strata of the Eifel and South Devonshire; the
head, or cephalic region, is narrow, and has two lunated eyes; the
thoracic region is trilobed and short, and composed of about ten small
articulations; the abdominal very small, and bordered by segments,
which radiate and form a wide, fan-shaped expansion. Other species of
this genus occur in the Silurian rocks.
With regard to the under surface of the Trilobites much remains to
be known. No decided indications either of antennæ or extremities
have been discovered. In an American specimen, Mr. Stokes detected
a plate,[486] which appears to be a _labrum_, or upper mandible or
lip, resembling that of _Apus cancriformis_. This animal has a similar
labrum, "and lateral influted terminations of the shelly segments
of the body, with a distinctly trilobed _pygidium_ (tail or caudal
portion), and a prolonged tail: the feet being foliaceous, and the
abdomen merely covered by a membrane."[487] In the upper or dorsal
surface of the carapace the Trilobites approach certain _Isopoda_,
particularly in the characters of the buckler and eyes. Mr. Macleay
states that among the existing crustaceans there are certain genera
which individually possess some one or more of the characters, which
have been thought peculiar to the extinct Trilobites. Thus the
_Serolis_ (_Bd._ pl. xlv. fig. 6), and the _Bopyrus_, have a trilobed
form; the female _Cymothoæ_ have the coriaceous margin of the body,
and in some species are without eyes as are many of the Trilobites;
while the eyes of the males of some _Cymothoæ_ are composed of large
facets, and are situated on the back of the head, wide apart, as in
the _Calymene_; rudimentary feet, and the absence of antennæ occur in
_Bopyrus_; and lastly, the _Sphæroma_ has an onisciform body, and the
power of rolling itself up into a ball, like the Calymene (_Lign. 175,
fig. 4_). The analogy between the Bopyrus and the Barr Trilobite is
so close, that if the latter had a body with thirteen equal segments,
and short crustaceous feet, it would be in every essential particular
a male _Bopyrus_.[488] Burmeister regards the Trilobites as being
related to the _Branchipus_. From the absence of eyes in the female,
and their presence in the male of certain recent genera of crustaceans,
it is not improbable that a similar character may have prevailed in the
Trilobites, and that certain fossils referred to different genera, from
the presence or absence of eyes, may have been the males and females of
the same species.
[486] Geol. Trans, vol. ii. p. 208. See also _Bd._ pl. xlv, _fig._ 12;
and Burmeister, pl. vi.
[487] Murch. Sil. Syst. p. 665.
[488] Ibid. p. 667.
The habits of the Trilobites, as deducible from Mr. Macleay’s
exposition of their structure and affinities, must have resembled those
of the _Cymothoadæ_, some of which, like the Calymenes, coil themselves
up, and are not parasitical; while their close affinity to Bopyrus, and
the apparent absence of distinct crustaceous feet, imply that they were
to a certain degree sedentary. The flat under surface of their bodies,
and the lateral coriaceous margin of several species, which is so
analogous to that of the multivalve shell _Chiton_, render it probable
that they adhered by a soft, articulated, under surface, to the rocks
or sea-weeds. Their instruments of progression are unknown; whether
they moved by means of membranaceous feet, or by the undulations of
setigerous segments, like the earth-worm, or by wrinkling the under
surface of the abdomen, like the Chiton, are questions yet to be
determined. It is evident, from their longitudinally trilobed form, and
lateral coriaceous margin, that they had the power of firmly adhering
to flat surfaces; and while thus sedentary the thin but hard dorsal
crustaceous shell would protect them from the attacks of their enemies.
"The Trilobites, probably, like the Chitones, adhered in masses one
upon another, and thus formed those conglomerations of individuals
which are so remarkable in certain rocks; but it is not likely that
they were parasitical, since almost all the existing parasites that
adhere to other animals, have strong feet, armed at their extremities
with hooks for that purpose."[489] From the form of the _labrum_ of
the mouth (_Barrande_, pl. i. and ii A) it is inferred that they were
carnivorous, preying on naked mollusks, or on the annelides, with which
their remains are associated.
[489] Murch. Sil. Syst. p. 669.
As the compound eyes of the _Trilobites_[490] are similar to those
of existing crustaceans and insects (see _Wond._ p. 792), the highly
interesting and important fact is established, that the mutual
relations of light to the eye, and of the eye to light, were the
same in the remote epoch when the Trilobites flourished, as at the
present time; and that the condition of the waters of the sea, and
the atmosphere, and the relation of both these media to light, have
undergone no change through the countless ages that have elapsed since
the deposition of the Silurian strata.[491]
[490] The compound eyes in many specimens remain in a high state of
preservation. M. Barrande in the eye of a _Brontes palifer_ counted
30,000 lenses. See also Barrande, pl. iii.
[491] See Dr. Buckland’s eloquent and instructive commentary on this
subject, _Bd._ pp. 401-404.
* * * * *
Geological Distribution of Fossil Crustaceans. We have seen that the
Tertiary strata contain the remains of many of the highest organized
crustaceans; a few brachyurous, macrurous, and entomostracous genera
appear in the Cretaceous, Oolitic, and Liassic formations; whilst
the Isopodous _Archæoniscus_ and several species of Cypris occur in
the Wealden and Purbeck deposits. Some few Entomostraca have been
enumerated from the Trias and Permian.
One species of macrurous decapod has been found in the Muschelkalk of
Germany; and Mr. Prestwich’s "Apus dubius" (_Geol. Trans._ 2d ser. vol.
v. pl. xli. _fig._ 9), and Dr. Ick’s crustacean, noticed in _Journ.
Geol. Soc._ vol. i. p. 199, both from the Coal Measures of England,
are probably true Decapodous Crustaceans. With these exceptions not
a species of the numerous tribes of Crabs, Lobsters, &c. has been
observed in the older formations, though composed of such enormous
thicknesses of marine detritus, and containing countless myriads of the
relics of the inhabitants of the ocean.
A few species of _Limulus_, several _Cytheres_ and allied genera, and a
few Trilobites (_Phillipsia_ and _Griffithides_) belong to the strata
of the Carboniferous System. In the Devonian System we find some minute
entomostracans, the gigantic _Pterygotus_, and various Trilobites
(_Brontes_, _Cheirurus_, _Homalonotus_, _Phacops_, &c.); but it is the
Silurian rocks that constitute the grand mausoleum of those ancient
beings, the Trilobites.
There are about thirty genera of Trilobites found in the Silurian rocks
of Great Britain and Ireland. Many of these are common to the Upper
and Lower Divisions of that system; and some of them are met with both
in the Silurian and in the Devonian rocks, as _Phacops_, _Brontes_,
_Cheirurus_, _Harpes_, and _Homalonotus_. The _Calymene Blumenbachii_
(_Lign. 175_, _figs._ 3 and 4) ranges through the Ludlow and Wenlock,
to the Bala and Llandeilo formations. The _Phacops caudatus_ also
(_Lign. 177_), the _Cheirurus bimucronatus_,[492] and the _Encrinurus
punctatus_[493] extend from the Ludlow, to the Llandeilo formation.
The Upper Silurian rocks exclusively contain some peculiar forms, as
_Encrinurus variolaris_, _Bumastus Barriensis_, and several species
of _Acidaspis_. And the Lower Silurian has several distinct genera,
namely, the _Trinucleus_ (_Lign. 175, fig. 2_), _Ogygia_, _Agnostus_,
_Asaphus_, _Olenus_, _Remopleurides_, &c. One species of _Pterygotus_,
and one of _Eurypterus_, the _Ceratiocaris_ and _Hymenocaris_, and
several species of the minute bivalved Entomostraca (_Leperditia_ and
_Beyrichia_) are all that remain to be enumerated as constituting, in
company with the _Trilobites_, the Crustacean fauna of the ancient
Cambrian and Silurian seas.[494]
[492] In Murchison’s Silurian System this form is figured (pl. xiv.
figs. 8 and 9) and described as Paradoxides bimucronatus.
[493] This is described and figured as Asaphus tuberculatus in
Buckland’s Bridgewater Treatise, pl. xlvi. _fig._ 6.
[494] We have also to refer to the indications of the existence of
other large Silurian Entomostraca afforded by the magnificent series of
fossil foot-tracks lately brought to England by W. E. Logan, Esq., and
obtained by that gentleman from the Potsdam Sandstone (Lower Silurian)
of Eastern Canada. These foot-marks and trails have been determined by
Prof. Owen as being most probably referable to some large Crustaceans
of the _Limulus_ Group, and are named by him _Protichnites_. (See
drawings and descriptions in the Quarterly Journal of the Geological
Society, vol. viii.)
On Collecting Fossil Crustaceans.--The Crabs and Lobsters of the
argillaceous tertiary strata are generally imbedded in nodules of
indurated clay and septaria. On the shore beneath the cliffs on the
north of the Isle of Sheppey, and near Southend, specimens may be
observed in the nodules that have been exposed to the action of the
waves, the attrition to which they have been subjected having partially
worn away the surrounding stone, and displayed the enclosed fossils.
In these examples the carapace is occasionally seen on one side, and
the pair of pincer-claws on the other face of the boulder; the other
feet and the plates of the thorax may sometimes be developed in such
examples by chiselling away the enveloping mass. In the laminated marls
of the tertiary and other deposits, in which the minute crustaceans,
as the _Cyprides_ abound, thin slabs covered with these relics may
be easily extracted; and many of the tertiary clays and sands yield
_Cytheres_, together with _Foraminifera_ and other minute fossils, on
careful washing and examination with a lens.
The Chalk crustaceans, particularly those which are muricated, or beset
with spines and tubercles, as the _Enoploclytia Sussexiensis_ and _E.
Leachii_ (_Lign. 169_), require considerable patience and dexterity to
develope successfully. The crustaceous covering of the carapace and
claws adheres firmly to the chalk by the rough external coat, while
the inner, smooth, glossy surface as readily separates. Hence, upon
breaking a block of chalk containing portions of these crustaceans,
we find one piece exhibiting a chalk cast of the claw or carapace,
covered with tubercles or papillæ, that have been moulded in the bases
of the spines of the crust; and on the other portion the crustaceous
shell imbedded by its outer surface, and presenting the internal glossy
lining, beset with circular depressions, which are the bases of the
spines. This crust is exceedingly friable, and will flake off by a
very slight touch. To obtain specimens with the external characters,
it is necessary to proceed with great caution; and when indications of
a crustacean are observed in a block, the chalk should be chiselled
or sawn off to within half an inch of the surface of the fossil, and
the remainder of the stone be cleared away, piece by piece, by means
of a penknife or graver. By this process the fossils figured _Foss.
South D._ pl. xxix. xxx. xxxi. were developed. When a fine specimen
has been broken, and the shell is attached to one piece of the stone
and the cast to the other, it is possible to obtain an illustrative
example of the external surface, by cementing the pieces accurately
together with very thin hot glue; and, when firmly consolidated, the
chalk may be removed, and the spines, tubercles, and papillæ of the
crustaceous covering be developed by the method previously described. A
thin coating of mastic varnish will give durability to the crust, and
improve its appearance; but the rich brown colour it possesses when
first exposed soon disappears. The Crustaceans of the Galt are often
found amongst the argillaceous and pyritous nodules flung aside in
heaps where the Galt is used for brick-making.
The _Cytheridæ_ of the Chalk, Galt, Oolite, &c. are to be obtained by
disintegrating the matrix in water, and examining the debris, after
sifting, under a lens.
The Limuli of the Coal-measures often form the nuclei of clay nodules,
as in the example figured _Lign. 172_, in which _fig. 2_ represents the
nodule without any external indication of its contents, and _figs._ 1,
and 3, the same broken, and displaying the crustacean. Traces of the
legs, branchiæ, and other appendages, should be diligently sought for
in fossils of this kind, for they are more likely to be detected in
such specimens than in those found in limestone. It is possible that
polished sections of the coiled up examples of Trilobites (_Lign.
175, fig. 4_) would throw some light upon the nature of the hitherto
undiscovered organs of locomotion and respiration of this extinct order
of Crustaceans.
A FEW BRITISH LOCALITIES OF FOSSIL CRUSTACEANS.
Abberley. Silurian: _Trilobites_ and _Beyrichia_.
Aberystwith, neighbourhood of. Silurian: _Trilobites_.
Arundel, Sussex. Chalk-pits in the vicinity. Astacidæ and Cytheridæ.
Atherfield, Isle of Wight. Wealden: _Cyprides_ in clay (_Lign. 174_).
Barr, Staffordshire; limeworks at Hay Head. Silurian: _Trilobites_,
particularly of the genus _Bumastus_.
Bewdley, Shropshire. Silurian: _Trilobites_.
Bolland, Yorkshire. Carboniferous Limestone: _Trilobites_ (_Cyclus_,
_Phillipsia_).
Burdie-house, near Edinburgh. Fresh-water coal-measures. _Cyprides_
and _Eurypteri_.
Burham, near the banks of the Medway, Kent. Quarry of Mr. W. Lee, a
good section of the lower Chalk: fine _Crustaceans_.
Coalbrook Dale. Coal-measures and Silurian. _Limuli_ and _Trilobites_.
Coniston, Lancashire. Silurian: _Trilobites_.
Dinley, Wilts. Purbeck. _Isopoda_ (_Lign. 171_) and _Cyprides_.
Dover. In the lower Chalk, _Astacus_ (_Enoploclytia_) _Sussexiensis_,
&c.
Dudley. Upper Silurian: _Trilobites_ in abundance.
Durlstone Bay, near Swanage. Purbeck: _Cyprides_ and _Isopoda_.
Folkstone, Kent. In Galt: small Crabs (_Lign. 168_), and numerous
_Cytheres_.
Grays, Essex. Pleistocene: _Cyprides_.
Gristhorpe Bay, Yorkshire Oolite: _Astacidæ_.
Hastings, Sussex, neighbourhood of. Wealden: _Cyprides_.
Hollington, near Hastings. Wealden: _Cyprides_.
Hordwell Cliff, Hampshire. Upper Eocene: _Cyprides_.
Kildare, Ireland. Carboniferous and Silurian: _Trilobites_.
Langton Green, near Tunbridge Wells. Wealden: _Cyprides_.
Lewes, Sussex. In the Chalk-pits of the vicinity: Astacidæ (_Lign._
169), and other Crustaceans.
Llandeilo, Caermarthenshire. Lower Silurian: _Trilobites_, _Trinuclei_.
Lyme Regis, Dorset. Green Sand: Hoploparia. Lias: _Coleia_.
Malvern Hills. Lower Silurian. _Trilobites_ (_Olenus_).
Meifod Hills, Montgomeryshire. Silurian: _Trilobites_.
Mount Pleasant, Caermarthen. Silurian: _Trilobites_.
Newton Bushel. Devonian: _Trilobites_ (_Brontes_).
Rhiwlas, near Bala, North Wales. Lower Silurian: _Trilobites_.
Ringmer, near Lewes. In Galt: small _Crabs_, &c.
Sandown Bay, Isle of Wight. _Cyprides_, in Weald Clay.
Scarborough. Oolite: _Astacidæ_, in clay nodules.
Sheppey. London Clay: _Lobsters_ and _Crabs_.
Steyning, Sussex. In Chalk-marl: _Lobsters_, &c.
Tyrone, Ireland. Carboniferous and Silurian: _Trilobites_.
Wenlock, neighbourhood of. Upper Silurian: _Trilobites_.
Westbury, Gloucestershire. Lias: _Estheria_ and _Cyprides_.
Wilmington, near Marton, Salop. Silurian: _Trilobites_.
Wistanstow, Salop. Lower Silurian: _Trilobites_.
Worthing, Sussex. Neighbouring Chalk-pits. _Lobsters_, &c.
FOSSIL INSECTS, SCORPIONS, AND SPIDERS.
From the Crustaceans we pass by a natural transition to the other
Articulata, viz. the Arachnida (Scorpions and Spiders) and the Insecta,
in the last of which "the highest problem of animal mechanics is
solved, and the body and its appendages can be lifted from the ground
and propelled through the air" (_Owen_). The skeleton in these animals,
as in the Crustaceans, is chiefly external, and consists of a hard
shell or case (composed of a peculiar substance, termed _chitine_),
divided into segments, and furnished with articulated or jointed
hollow extremities. The head is distinct, and has a pair of compound
eyes, and of jointed antennæ. To the segments that form the thorax the
legs are attached, and these consist of three pieces in the hexapods
(insects with six feet), each supporting a pair of feet. The wings
in the flying insects are attached to the middle and third thoracic
segments. The legs, or articulated appendages, are hollow, as in the
Crustaceans, and contain the muscles and other soft parts. The generic
and other distinctions adopted by naturalists, to facilitate the study
of this most numerous division of the animal kingdom, are founded on
the structure and configuration of the antennæ and wings. The latter
consist of flat membranous expansions, supported by hollow tubes or
nervures; and in some orders consist of one pair, and in others of
two. In burrowing insects, as the Beetle, the front pair of wings
constitutes a hard case (_elytron_), which covers and protects the
membranous posterior pair, when the animal is in repose or walking. The
modifications of the wings furnish the characters by which the class is
divided into orders. Thus the _Coleoptera_ (sheathed-wings) comprise
the beetles and other burrowing insects, in which the membranous
wings are folded transversely beneath the elytra, or wing-cases. The
_Orthoptera_ (straight-wings), those with two pairs of wings, of which
the anterior encase the others, the posterior being membranous, and
folded longitudinal during repose; as the Earwig, Cockroach, Mantis,
and Locusts. _Neuroptera_ (nerved-wings), those with two pairs of
transparent reticulated wings, as the Libellula, or Dragon-fly, the
Ephemera, and the Termites. _Hymenoptera_ (membranous-wings), with
simply veined membranous wings, as the Gall-flies, the Bee, &c. The
Cicas, Aphis, and Coccus constitute the somewhat anomalous group
termed _Homoptera_ (equal-wings), in which the anterior pair of wings
are usually similar to the posterior in consistence, and shut up in a
roof-like manner. The _Heteroptera_ (different-wings) include the Nepa,
Notonecta, &c. and have the anterior wings coriaceous at the base,
membranous towards the point, and shutting up nearly horizontally,
partly lapping over one another. _Lepidoptera_ (scaly-wings) have
wings covered with scales, as the Butterfly and Moth. In the _Diptera_
(two-wings) the anterior pair of wings only are the instruments for
flying, and the hinder pair are reduced to mere clavate appendages, as
the Gnat and Fly. The Phryganeæ (Caddis flies) constitute the order of
_Trichoptera_ (hairy-wings), related to the Neuroptera, but resembling
the Lepidoptera in the distribution of the nervures of the wings, and
in many other characters. Lastly, there remain the Wingless Insects,
divisible into three orders, of which the Flea, the Parasites, and the
Podura are respectively the types. With these few remarks on those
durable parts of the structure of Insects which their fossil remains
generally present, we must quit this part of the subject, and enter
upon the examination of the relics which are the immediate objects of
our present inquiry.
From the enduring nature of the elytra, segments, and articulated
extremities of insects, the fossil remains of animals of this class
might naturally be expected to abound in lacustrine and fluviatile
deposits; this, however, is not the case, and except in a few favoured
localities, fossil insects are seldom met with, and good specimens
rank among the most rare and interesting of the organic remains of
the Secondary formations. In certain Tertiary beds, as at Œningen,
and Aix in Provence, insects of numerous species and genera have been
discovered; and the cream-coloured limestone of Solenhofen, among its
numerous other treasures, has yielded some fine examples of this class.
The strata in which remains of insects have been found in England[495]
are the Tertiary clays of the Bagshot series, the Hastings beds,
Purbeck marls and limestones, Kimmeridge Clay, Oxford Clay, Forest
Marble, Stonesfield Slate, Upper and Lower Lias, and the Coal Measures.
[495] Palæontologists are particularly indebted to the Rev. P. B.
Brodie, F.G.S. for his compendious and valuable "History of the Fossil
Insects in the Secondary Rocks of England" (8vo. 1845); and to J.
O. Westwood, Esq. the eminent Entomologist, for the very important
and interesting Observations on the Insect Remains, prefixed to the
above work. In an interesting paper on the Geology of the vicinity
of Ilminster, C. Moore, Esq. has noticed the numerous Insect remains
of the Upper Lias of that place. Prof. E. Forbes and W. R. Binfield,
Esq. have discovered Insects in the Hastings series; and Mr.
Binfield, besides having most successfully searched the Upper Lias of
Gloucestershire, has also detected some specimens in the Lias at Lyme.
Lastly, Mr. Morris has found Insects in the Upper Lias in Lincolnshire.
ARACHNIDA.
Fossil Scorpion. (_Bd._ pl. xlvi′.)--The discover of a fossil Scorpion
in coal-shale, associated with leaves, by Count Sternberg, and of
Spiders in the limestone of Solenhofen, by Count Münster, proves the
existence at a very remote period of both these insectivorous families
of _Arachnidans_, or spiders (_Bd._ p. 405). The fossil Scorpion was
found in a block of argillaceous shale, at Chomle, in Bohemia. It lies
imbedded amidst the carbonized remains of leaves, and a large trifid
carpolithe or seed-vessel (see _Bd._ pl. xlvi′.): by a fortunate
separation of the shale, the back or dorsal carapace is shown on one
surface; and the thorax, with five or six legs attached, and the
abdominal segments, are exposed on the other, together with a fragment
of the tail of another and larger Scorpion. The head and eyes, one
of the jaws with teeth, and a portion of the skin remain (_Bd._ pl.
xlvi. figs. 3, 4, 5, 6). The horny covering seems to have undergone no
change; it is still elastic and transient, and consists of two layers,
both retaining their texture, and structure, and exhibiting under the
microscope hexagonal cells divided by strong partitions.
Fossil Spiders.--With the numerous insects preserved in the gypseous
marls at Aix, of which we shall treat hereafter, Spiders are
occasionally found. A beautiful example, showing the under surface of
a small spider, with the papillæ of the spinning organs protruded by
pressure, from the cabinet of Mrs. Murchison, is figured, _Bd._ pl.
xlvi′. fig 12 In the beautiful lithographic stone of Solenhofen the
remains of spiders are not unfrequent.
INSECTA.
Fossil Neuroptera.--Of this order, the insects of which are
distinguished by their four finely reticulated membranous wings,
several fossil species have been found. Some of these are referable to
the family Libellulidæ;--insects so well known from their light and
elegant figure, their beautiful and variegated colours, their large
lustrous wings, and the velocity and gracefulness of their motions.
[Illustration: Lign. 179. Fossil Libellula, or Dragon-fly. _Solenhofen._
(_Drawn by Mr. Joseph Dinkel._)
In the cabinet of the late Marquess of Northampton.]
Fossil Libellulidæ. _Lign. 179._--Of the highly organized family of
carnivorous insects, the _Libellulidæ_, five or six specimens have been
discovered in the lithographic limestone of Solenhofen; a beautiful
specimen from that locality is represented, _Lign. 179_. In this
example both pairs of wings remain, but one wing is pressed down
beneath the abdomen: the nervures of the wings are admirably preserved.
A few examples of the remains of this family have been found in the
British strata. One species of _Libellula_ and one of _Æshna_ have
been found by the Rev. Mr. Brodie in the Purbeck beds of the Vale
of Wardour. Two species of _Libellula_,[496] two of _Æshna_, and
some other allied species have been obtained by Messrs. Strickland,
Buckman, Binfield, and Brodie from the Lias. The wing of the _Æshna
liassina_, discovered in the Lias, near Binton, in Warwickshire, by Mr.
Strickland, is two inches and ten and a half lines in length, and eight
and a half lines in its greatest breadth, being one-third larger than
the wing of the largest British species. See _Wond._ _Lign._ 119, and
p. 528.
[496] A very interesting specimen of fossil _Libellula_, discovered by
the Rev. Mr Brodie in the Upper Lias near Cheltenham, is figured in the
Quarterly Geol. Journal, for 1848, vol. v. pl. v.
Fossil Corydalis. _Lign. 181, fig. 2._--The wing of a remarkable
fossil Neuropterous insect was discovered by me in a nodule of
ironstone, from Coalbrook Dale, and mistaken for a leaf. The specimen
consists of one wing, which, as M. Audoin first ascertained, closely
resembles that of the living _Corydalis_ of Carolina; see _Lign. 181,
fig. 2_. The membranous structure and the distribution of the nervures
are distinctly preserved; on the portion figured the surface of the
wing lies in relief on the stone; and on the corresponding part of
the nodule, a sharp imprint remains,[497] I have named this fossil in
honour of the eminent French savant, M. Alex. Brongniart.
[497] This specimen is now in the collection of the British Museum.
Wings of _Corydalis_ have also been found in the Purbeck beds of the
Vale of Wardour, by the Rev. Mr. Brodie, who has also discovered
remains of _Phryganeidæ_ and a _Termes_ in the same group of strata.
Panorpa ? Liassica. _Lign. 180._--In the Lias, on the banks of the
Severn, at Wainlode Cliff, Gloucestershire, specimens of minute
neuropterous wings have been discovered. I subjoin accurate figures
of two specimens in the cabinet of the Geological Society; they are
represented twice the natural size; they resemble the wings of a recent
genus of Neuroptera, termed _Panorpa_; particularly _P. Germanica_. The
transverse lines are not fractures, but nervures, and are faithfully
copied from the originals.
To the above notice of British fossil neuropterous insects, I may add
that the wing of a large species (_Hemerobioides giganteus_) has been
discovered by Dr. Buckland in the Stonesfield slate.
[Illustration: Lign. 180. Wings of Neuropterous Insects. _Twice nat.
size._
(_Drawn by S. P. Woodward, Esq._)
_Lias._ _Wainlode Cliff._
Portions of the anterior wings of a species resembling Panorpa.]
[Illustration: Lign. 181. Fossil Wings of Insects.
(_Drawn by S. P. Woodward, Esq._)]
Fig. 1.--Elytron, or wing-case of Buprestis Bucklandi, _Great
Oolite_; _Stonesfield_.
2.--Wing of Corydalis Brongniarti. (G. A. M.) _Carboniferous_;
_Coalbrook Dale_.
Fossil Coleoptera.--The _elytra_ or wing-cases of coleopterous insects
have long since been noticed in the oolitic slate at Stonesfield, near
Oxford; a locality celebrated for the only mammalian relies hitherto
discovered in the Secondly strata of England. The Stonesfield elytra
are always found detached; in no instance, I believe, has any other
part of an insect been observed, except a single leg of a _Curculio_
(_Bd._ pl. xlvi′. _fig._ 10). The specimen figured _Lign. 181, fig.
1_, displays the usual characters of the largest species. These fossils
are of a reddish-brown colour, with a finely granulated surface; there
appear to be four or five species, all of which belong to _Buprestis_,
a family of beetles remarkable for their splendid metallic lustre.
Remains of _Coleoptera_ occur in the Tertiary clays near Corfe,
Dorset,[498] and in the Lias of Worcestershire and Gloucestershire; and
in the Danby oolitic coal-pits, in the eastern moorlands of Yorkshire,
the elytra of beetles have also been discovered, by Mr. R. C. Taylor
(_Bd._ vol. ii. p. 78).
[498] See Notice by the Rev. Mr. Brodie, Quart. Geol. Journ. vol. ix.
p. 51.
A most remarkable fossil of this kind is described by Dr. Buckland;
a unique specimen of _Buprestis_, from Japan, about an inch long,
_converted into chalcedony_, with the antennæ and portions of the legs
finely preserved. The surface of this insect is covered with clusters
of minute concentric rings of chalcedony; an appearance common in
silicified shells. Associated with this fossil, were fragments of
silicified wood, bored with tubular cavities, apparently by the larvæ
of insects of this family; and within these cavities was a quantity of
dust produced by the boring, also converted into chalcedony (_Bd._ vol.
ii. p. 78).
Of the Curculio, a genus of coleoptera distinguished by their splendid
elytra, of which the _Diamond Beetle_ is a familiar example, the
remains of two species have been discovered in the nodular ironstone
of Coalbrook Dale, by Mr. W. Anstice, and are figured and described by
Dr. Buckland (_Bd._ vol. ii. p. 76; and pl. xlvi′. figs. 1, 2). In one
of these specimens (_Curculioides Ansticii_), with the exception of the
rostrum and anterior part of the head, all the essential characters of
the insect are displayed; namely, the elytra, thorax, and six legs, the
hindmost of which exhibits the enlarged femur, or thigh, a character
peculiar to the _Curculionidæ_. The legs possess a tufted appearance,
which that eminent entomologist, Mr. Curtis, conceives may have been
caused by fungi, after the death of the animal, as often happens in
tropical climates. In the other example (_C. Prestvichii_), the insect
lies on its back, with the left side raised upwards, and exhibiting a
portion of the external surface of the left elytron; there are remains
of the antennæ, and indications of the proboscis and of the legs.
[Illustration: Lign. 182. Insectiferous Limestone. _Purbeck._
(_Magnified six diameters._)]
_a, b._ Membranous wings of the _Corydalis_ type.
_c._ Punctate-striate elytron of a Beetle.
_d._ Elytron of a Beetle, with a smooth surface.
_e._ Smooth elytron, the upper part impressed with
transverse lines.
The _Orthoptera_, _Homoptera_, and _Diptera_ are also represented
in the Lias of Gloucestershire, and in the Purbeck strata of the
Vale of Wardour, by numerous species, which have been enumerated,
and mostly discovered, by the Rev. Mr. Brodie.[499] This observer
has, indeed, been very successful in his researches in the latter
locality, for in the deposits of limestone and marl which yielded the
isopodous crustaceans, previously described (p. 521, _Lign. 171_), he
has discovered the remains of several orders of insects, and states
that, for abundance and variety of specimens, the beds may be said to
resemble the Tertiary marls of Aix and Œningen. These remains were
obtained from a quarry at Dinton, about twelve miles west of Salisbury.
They consist chiefly of _Coleoptera_, with the remains of _Neuroptera_,
_Trichoptera_, and _Homoptera_, and of several species of _Diptera_.
In the cream-coloured laminated Purbeck marls that axe exposed in
Durlstone Bay (about one mile from Swanage) insectiferous beds have
been found by the Rev. O. Fisher and Prof. E. Forbes, which are the
equivalents of those of the Vale of Wardour; and similar beds were met
with in the cutting of the railway through the Ridgway Hill, between
Dorchester and Weymouth.
[499] See Brodie’s Fossil Insects.
In a quarry on the road-side between the village of Stone and Hartwell,
Bucks, the Portland Oolite is covered by the Purbeck marls; in these
latter remains of Insects occur, together with scales and teeth of
small Fishes, and abundance of _Cyprides_.
All the British localities of fossil insects have now been alluded to;
but on the Continent, independently of the celebrated limestones of
Solenhofen, to which reference has been made, p. 550, there are several
tertiary deposits exceedingly rich in these interesting fossils.
[Sidenote: FOSSIL INSECTS.]
Fossil Insects of Aix, in Provence.--The town of Aix is situated in
the lowest part of a deep valley, the immediate flanks of which are
composed of a thick fresh-water formation, lying unconformably upon
strata of Jura limestone. The fresh-water series consists of white and
grey calcareous marls, calcareo-siliceous grits, and beds of gypsum;
and the quarries formed in the latter rock have long been celebrated
for the prodigious quantity of fish and plants which they contain.
M. Marcel de Serres first made known the great abundance of insects
in these gypseous marls, and has enumerated nearly seventy genera,
chiefly of the Coleoptera, Diptera, and Hemiptera; they are mostly
referable to European forms, and to existing genera. An interesting
Memoir on these strata, by Sir R. Murchison and Sir C. Lyell,[500]
first directed the attention of the English reader to these beautiful
fossils. In _Wond._ p. 261, an epitome of this valuable communication
is given, and five specimens of insects are here figured, which will
convey some idea of their forms and perfect state of preservation.
[500] Edinburgh New Philosophical Journal for October, 1829.
[Illustration: Lign. 183. Fossil Insects. _Tertiary._ _Aix in
Provence._]
Fig. 1.--Tettigonia spumaria.
2.--Mycetophila; the body distended by pressure.
3.--Lathrobium.
4.--Allied to Penthetria holosericea. The hinder legs are
broken off, and one of them reversed, so that the _tarsi_
nearly touch the thigh; the _palpi_ are long and perfect;
the antennæ are remarkably distinct.
5.--Liparus; resembling L. Punctatus.
Fossil Insects of Œningen.--In the immediate vicinity of Œningen, near
Constance, on the banks of the Rhine, there is the basin of an ancient
lake, filled up with marls and limestones, presenting a fine example
of a lacustrine formation, and abounding in fossil Fishes, Reptiles,
Plants, Shells, Crustaceans, and Insects.[501] These Insects are often
in an admirable state of preservation, and occur in the different
stages of larva, pupa, and imago. The pupa of a _Libellula_ shows the
mask, insertion of the legs, and the spiracula. Some belong to genera,
the species of which frequent marshy plants of the same kind as those
which are found associated with the insects; and it seems probable that
they fell into the lake from the plants which grew on its borders,
and became enveloped in the fine mud or sediment. Numerous species of
several genera of Ants also occur in these deposits of Œningen and at
Radoboj in Croatia.[502]
[501] See the Memoir by Sir R. I. Murchison on the lacustrine formation
at Œningen, near Constance, Geol. Trans, new series, vol. iii. p. 277.
[502] See Prof. O. Heer’s Memoir, translated in the Quart. Journ. Geol.
Soc. vol. vi. pt. ii. p. 61; and his History of Insects, ibid. p. 68.
[Sidenote: FOSSIL CADDIS-WORM.]
Fossil Larvæ of Phryganea. _Ly._ p. 185.--The _Caddis-worm_, so well
known to all the brethren of the angle, is the larva of the winged
insect termed _Phryganea_, and is abundant at the bottom of fresh-water
streams and lakes; the cases, like those of the marine _Sabella_
(p. 385, _fig._ 6), are always studded over with extraneous bodies,
cemented together by a glutinous secretion to the silken integument,
or case, which encloses the lava. Some species are coated with pieces
of stick or straw, others with minute shells, as planorbis, bithinia,
and the like; and when the larvæ have passed into the perfect state,
their cases, or _indusiæ_, remain. Many of the Tertiary fresh-water
limestones of Auvergne are almost wholly composed of the _indusiæ_
of Caddis-worms, cemented together by calcareo-siliceous matter into
stone, which is employed for building, and is called indusial limestone
(_Wond._ p. 273). These limestones are associated with marls abounding
in fresh-water shells and cyprides; the whole assemblage presenting
all the stratigraphical and zoological characters of a lacustrine
formation. "If," says Mr. Scrope,[503] "we consider that repeated
strata, of five or six feet in thickness, almost entirely composed of
these tubes, once extended over a district presenting a surface of many
hundred square miles, we may have some idea of the countless myriads
of minute beings which lived and died within the bosom of that ancient
lake."
[503] On the Geology of Central France, by G. Poulett Scrope, Esq. 4
to. 1827.
On Collecting Fossil Insects.--The localities in which the British
collector may reasonably expect to discover fossil remains of
Insects, are Stonesfield, where the elytra of beetles are by no
means scarce,--Coalbrook Dale, in which relics of this class are
sometimes, but very rarely, found in the ironstone nodules,--Bedford,
Warwickshire, and the Wainlode and Aust Cliffs, for Lower Lias
insects,--Dumbleton and Ilminster, for Upper Lias insects,--Dallards,
near Dinton, and Stone, near Aylesbury, and the exposures of similar
beds in Dorsetshire, for the Purbeck insects.
The white clays belonging to the Bagshot series of Bournemouth, Poole,
and Corfe, so rich in beautifully preserved leaves and other parts of
plants, should be carefully searched for insect remains, since these
clays at Creech, near Corfe, have already afforded a few specimens.[504]
[504] Quart. Geol. Journ. vol. ix. p. 51.
At page 549 a few other English localities yielding these delicate
and very interesting fossils are also indicated as having been lately
discovered by some of our most acute and active geologists.
Should the student visit the celebrated sites of these fossils in
France and Germany, namely, Aix, Œningen, Solenhofen, &c., he will have
but little difficulty in obtaining an interesting series, at a moderate
expense.
The marls and limestones in which insects occur are often of a
laminated character, and in general readily split asunder in the
direction favourable for the display of the insects. In some examples,
only the form of the animal is seen through a thin opaque pellicle of
calcareous earth, which may be removed by a penknife or graver, and the
wings, elytra, antennæ, legs, &c. will thus be disclosed. A very thin
coating of mastic varnish heightens the colours of such specimens, and
renders them more durable.
CHAPTER XIV.
FOSSIL ICHTHYOLOGY; COMPRISING THE SHARKS, RAYS AND OTHER PLACOID
FISHES.
[Illustration: Lign. 184. A group of Fossil Fishes. _Tertiary._ _Aix._
Lebias cephalotes (_Agassiz_).]
Ascending from the two grand subdivisions of the animal kingdom,
the _Mollusca_ and the _Articulata_, we advance to the Vertebrata,
animals distinguished from all those which have previously engaged
our attention, by the possession of a bony, jointed, hollow column of
support, or spine, formed of bones termed _vertebræ_ (_turn or whirl
bones_), and enclosing and protecting those strands or cords of the
nervous system called the spinal marrow; the former classes, being
destitute of such a structure, have the general name of _Invertebrata_.
In the beings whose mineralized remains form the subject of our present
investigation, the durable parts of the frame-work, or skeleton, are,
in most instances, situated internally, and their fossil relics consist
principally of the bones, or solid earthy portions of their structures,
either imbedded in the rocks in their natural relative position, or in
a state of dismemberment and dispersion. In most cases the teeth, and
in many the durable parts of their external integument, or skin, are
also preserved, in a greater or less degree of integrity.
In the lowest class of _vertebrata_, the Fishes, the skin is covered
with numerous pieces or scales, of a dense, durable substance, and
strengthened, in some families, by the addition of osseous plates; thus
constituting a flexible and almost impenetrable coat of armour, which
affords suitable protection to beings peculiarly exposed to external
injuries, from the nature of the regions they inhabit, and the state
of warfare with each other in which they are constantly engaged.
Confined to a fluid medium, they are provided with organs fitted for
aquatic respiration, called _branchiæ_, or gills, and with instruments
of progressive motion, termed _fins_, by which they are enabled to
propel themselves through the water with great velocity. The apparatus
for seizing, tearing, and crushing their prey presents numerous and
important modifications, corresponding to the habits and economy of the
different genera; their teeth offering as great variety of form and
structure as those of the higher orders of animals.
The cartilaginous or the osseous nature of the skeleton, and the
number and position of the fins, were the characters formerly employed
in the classification of Fishes; but Prof. Agassiz, conceiving the
structure of the skin to afford a natural index to the essential
modifications of organization and functions, has, with great sagacity,
adopted an arrangement founded upon the form and structure of the
scales; and he has divided the whole class into four orders, each
distinguished by essential differences in the dermal (skin) system.
To the geologist this method has proved of inestimable value; for it
is simple, easy of application, and, so far as our present knowledge
extends, may be relied upon as affording accurate conclusions as to
the nature and relations of the originals to which a few detached
fossil scales may have belonged. Another important aid has been derived
from the microscopical examination of the structure of the teeth; and
a splendid work on this subject by Professor Owen has opened a wide
field of palæontological investigation, which is yet but very partially
explored.[505]
[505] Odontography; or, a Treatise on the Comparative Anatomy of
the Teeth; their Physiological Relations, Mode of Development, and
Microscopic Structure; illustrated by upwards of 150 plates. By Prof.
R. Owen, F.R.S. &c. 4to. London. 1840-1845.
The living species of Fishes exceed eight thousand; and those found
in a fossil state, and determined by M. Agassiz, already amount to
upwards of one thousand five hundred; while several hundreds are
still undescribed; and the rapid progress of geological research is
continually adding to the number: upwards of six hundred British fossil
species are enumerated. In an initiatory work like the present, it will
be necessary to confine our remarks to an illustration of the mode in
which the investigation of the fossil remains of the animals of this
class should be conducted; and, by the elucidation of a few leading
principles, prepare the student for the perusal of works expressly
devoted to this branch of Palæontology.[506]
[506] The admirable and important work entitled "_Recherches sur
les Poissons Fossiles, par Louis Agassiz_," stands preeminent in
this department of science. It consists of five volumes, 4to. of
letter-press, and five volumes, folio, of coloured plates. It must be
consulted by all who would acquire a correct view of the present state
of fossil Ichthyology. _It is from this work that the commentary in the
text has been chiefly derived._
The fossil remains of fishes rank in the first class of the "Medals
of Creation," for they demonstrate the existence of numerous tribes
of highly organized beings in some of the most ancient fossiliferous
strata, and the continuance of the same type of organization, variously
modified, through the entire series of subsequent deposits to the
present time. Each geological formation contains peculiar groups of
fossil fishes, distinguished by distinct modifications of structure.
Thus, according to the data at present obtained, all the osseous fishes
anterior to the Chalk belong to genera which have no representatives
among existing species; and they are characterised by rhomboidal scales
covered with enamel.
The state of conservation in which the fossils of this class occur,
appears to have depended on the relative delicacy or firmness of the
original structures, and on the nature of the deposits in which the
fishes were imbedded. Thus the fossil fishes of the early formations,
which are characterised by their dense integument and enamelled
scales, often present the entire forms of the originals, and generally
considerable portions of the connected scales, with the fins and other
appendages: while the specimens of later deposits, which contain
a large proportion of species with delicate scales, more often
display the mineralized osseous skeleton, than the dermal structure.
Sedimentary strata composed of mud or fine detritus, of whatever age,
have been most favourable to the preservation of the entire forms;
hence we often find in the pulverulent clays and marls of the Tertiary
strata, in the Chalk of England and Westphalia, and in the fine
lithographic stone of Solenhofen, fishes perfect in form, and not only
individuals, but groups, with the scales, fins, head, teeth, and even
the capsule of the eye, in their natural positions. A small slab of
marl from Aix, in Provence, in the collection of Sir R. I. Murchison,
contains scores of small fishes, as perfect as if recently imbedded
in soft mud: a portion of this specimen is represented, _Lign. 184_;
and the beautiful fish figured in the frontispiece of vol. i. (pl. i.
_fig._ 3), from near Castellamare, will serve to illustrate the state
of perfection of some of the ichthyolites of the Jura limestone. In
the Chalk, many of the fishes are uncompressed, the body being as
perfect in form as if the original had been surrounded by soft plaster
of Paris while floating in the water. But in coarse limestones and
conglomerates,--in other words, in materials that have been subjected
to the action of the waves and torrents,--detached teeth, scales,
bones, &c. constitute the principal vestiges of this class of beings.
In illustration of this department of Paleontology, it will be
expedient to consider,--1stly, the characters afforded by the scales
and dermal appendages; 2dly, the teeth, or dental organs; 3dly, the
osseous and cartilaginous skeletons; and lastly, apply the data thus
obtained to the elucidation of some of the principal fossil genera and
species.
Scales of Fishes.--The dermal plates or scales are composed of two
substances, disposed in laminæ or plates; the one cartilaginous or
horny,--the other dense and osseous, possessing the structure of bone.
In most species the scales are imbricated, _i. e._ lie over each other
like the tiles of a roof; the margin of a front row partly covering the
series immediately behind. From this arrangement, the apparent shape
of the scales is very different from their true form; the processes of
attachment and the lateral angles being concealed. The scales that are
not imbricated are either very small, and imbedded in the substance of
the skin so as to be imperceptible to the naked eye, as in the shagreen
of Sharks; or are disposed in the form of bosses or scutcheons, as in
the Rays; sometimes bristling equally over the surface of the body,
as in the _Diodon_; and sometimes covering it like mosaic work; or
forming particular series on certain regions of the body, while the
other parts are garnished with different scales, as in the Sturgeon.
There are a few genera destitute of scales. In almost all fishes there
is a particular series disposed in a gently undulated line along each
side of the body, from the head to the tail, and constituting what
is termed the lateral line; these scales are tubular, and serve an
important purpose in the economy of these animals. Every one must be
aware that the body in most living fishes is constantly covered with
a kind of mucus, or slime, which serves to lubricate the skin and to
defend it from the action of the surrounding medium. This fluid is
secreted by a mucous canal or duct, which extends along the body, and
ramifies in all the bones of the head, jaws, &c.; and it is distributed
over the surface of the head by numerous pores in the bones, and over
the body by the tubes formed by the row of scales above described.
[Illustration: Lign. 185. Fossil Scales of Fishes. (_highly magn._)
_Chalk._ _Lewes._]
Fig. 1.--Scale or plate of the _shagreen_ of a Shark.
2.--Scale of Macropoma Mantelli; the exposed surface.
3.--Scale of Beryx Lewesiensis; the exposed surface.
4.--Scale of Osmeroides Mantelli; the entire form.
The four orders into which this grand class of vertebrata is divided by
M. Agassiz, are founded upon the peculiar structure of the scales;[507]
and are characterised as follow:--
[507] For illustrations of the scales of fishes, see _Wond._ p. 339,
_Lign. 68_, and p. 340, _Lign. 69_; _Foss. Brit. Mus._ p. 419; and
_Ly._ fig. 306, figs. 342-347, &c. And for their minute structure, see
Prof. Williamson’s important paper in the _Phil. Trans._ for 1851.
_Order I._ Placoid (_a broad plate_).--The skin covered irregularly
with enamelled plates, sometimes of a large size, but frequently in
small points, as the shagreen on the dermal integument of the _Sharks_
and the tubercles of the _Rays_. _Lign. 185, fig. 1_, a fossil
_placoidian_ scale from the skin of a shark, highly magnified.
_Order II._ Ganoid (_splendid_, from the brilliant surface of the
enamel).--The scales are of an angular form, and composed of plates of
horn or bone, covered with a thick layer of enamel; their structure
is identical with that of the teeth. The _Sturgeon_ is an example of
this order. _Lign. 196, figs. 1, 2, 3, 4_, are fossil scales of a
_ganoidian_ fish.
_Order III._ Ctenoid (_toothed, or comb-like_).--The scales are formed
of plates, which are toothed or pectinated on their posterior margin
or edge, like a comb. As the plates are superimposed on each other, so
that the lowermost always extend beyond the uppermost, their numerous
sharp points or teeth render the scales very harsh to the touch. The
_Perch_ belongs to this order. _Lign. 185, fig. 3_, represents a
fossil _ctenoidian_ scale.
_Order IV._ Cycloid (_circular_).--The scales are composed of simple
laminæ, or plates of bone or horn, without enamel, and have smooth
borders; but their external surface is often ornamented with markings.
The scales of the lateral line consist of funnels placed one within
the other; the contracted part of which, applied against the disk of
the scale, forms the tube through which the mucus flows. To this order
belong the Mullet, Salmon, and Carp. _Lign. 185, fig. 4_, is the
scale of a fossil _cycloidian_ fish.
Fins of Fishes.--As the progression of fishes through the water is
principally effected by the action of the tail, they have no limbs
commonly so called. The instruments for balancing the body, and for
assisting progression, are the fins, which are composed of numerous
rays that support a membranous expansion; and the number and situation
of the fins present various modifications in the different orders and
genera.
The fins are named according to the situation they occupy; for example,
_pectoral_, those on each side of the chest, and which correspond
to the anterior extremities of other vertebrated animals; _dorsal_,
on the back; _ventral_, on the belly; _caudal_, on the tail. (See
outlines of Fish, _Ligns. 186, 187, 195_.) The rays are of two kinds;
1st, the _Spinous_ rays; these consist of a single osseous piece,
usually dense and pointed, sometimes flexible and elastic, and divided
longitudinally (_Ligns. 188, 196_); 2d, _Soft_ or _articulated_ rays,
which are composed of numerous small articulations or joints, and
divide into branches at their extremities. Many species of fishes have
four fins; others six; some but two; and in certain genera they are
altogether wanting. In a fossil state the fins are often beautifully
preserved; even the soft rays in many of the Tertiary marls and in the
Chalk, are found entire, and attached to the body in their natural
situation. The large, strong, spinous rays of the dorsal fins of the
cartilaginous fishes, as the _Sharks_ and _Rays_, are generally found
detached, or connected only with a few vertebræ; but they are so
abundant in some of the Secondary deposits (and in numerous instances
they are the only vestiges of extinct species and genera), that they
possess great geological interest; they are distinguished by the term
_Ichthyodorulites_ (_fossil fish-weapons_), under which head they will
hereafter be described (see _Lign. 188_). The first ray in the dorsal
fin of some fishes is protected in front by a double row of enamelled
scales, and these often occur in a fossil state (see _Lign. 196,
fig. 5_).
Teeth of Fishes.--Of all the durable parts of animals teeth occur
in the mineral kingdom, the teeth of fishes present by far the most
numerous, varied, and striking modifications of form, structure,
composition, mode of arrangement, and attachment; and yet these dental
organs, separately considered, do not in many instances, either in
their structure or mode of implantation, afford characters by which the
natural affinities of the original can be satisfactorily ascertained;
and without the aid of other parts of the skeleton it is often
impossible to determine, from external characters only, whether an
unknown form of tooth belonged to an animal of the class of Fishes or
of Reptiles. Although the modifications of form are almost innumerable,
they are referable to four principal types; namely, the conical, the
flattened, the prismatic, and the cylindrical.[508]
[508] The "Odontography" of Professor Owen should be consulted by
those who would thoroughly comprehend this interesting department of
science. See also the _Article_ Teeth, by Prof. Owen in the Cyclopædia
of Anatomy and Physiology.
The conical teeth are extremely variable in size and form; some are
slender, almost invisible points, distributed like the pile of velvet
(_villous-teeth_), or set like the hairs of a brush (_brush-teeth_);
some are long and slender, or barbed at the point; others are obtuse;
and many are long and striated at the base, and closely resemble the
teeth of certain reptiles. The depressed teeth are equally diversified;
some have the grinding surface smooth; others, deeply grooved; in some
it is flat; in others convex. In form they are either lozenge-shaped,
elliptical, square, oblong, semilunar, &c. The cylindrical teeth are
hemispherical, or flattened; in some fishes they are short and thick;
in others slender and support an obtuse, conical crown. The prismatic
form is equally modified; from the compressed, sharp, lanceolate,
cutting teeth, to the strong, triangular, three-pointed dentary
organs.[509]
[509] For illustrations of the teeth of fishes, see plate iv. figs. 1,
2, 8, 10, and _Ligns._ 189, 191-194, 197, 198, 202, 205; _Foss. Brit.
Mus._ p. 449; and _Ly._ figs. 236, 308, 324, and 383.
The mode of arrangement and attachment of the teeth, is as diversified
as their forms. In some species all the teeth are of one type, and
disposed in somewhat of a serial order on both sides of the jaws; but
in a large proportion of fishes there are several kinds of teeth, which
are implanted not only in the jaws, properly so called, but on the
bones which form the cavity of the mouth, the arches of the palate,
tongue, &c.; and it is peculiar to this class of vertebrata to present
examples of teeth developed in the median line (_along the middle_) of
the mouth, as in certain species of Rays; or crossing the symphysis
(the _front line of union_ of the two sides) of the lower jaw, as in
_Myliobates_[510] (see _Lign. 194, fig. 2_). In some species the
teeth are implanted in sockets, to which they are attached only by the
soft parts, as in the rostral teeth of the _Saw-fish_; some have hollow
bases, supported upon bony prominences, which rise from the base of
the socket; as in several fossil teeth from the Chalk. "But by far the
most common mode of attachment is by a continuous ossification between
the dental pulp and the jaw,"[511] the teeth being thus anchylosed to
the bone. In the Sharks the osseous bases of the teeth are attached by
a ligamentous substance to the tough, dense crust, which covers the
cartilaginous jaws; the teeth of these fishes are therefore generally
found detached in a fossil state, in consequence of the decomposition
of this substance.
[510] Odontography, p. 5.
[511] Op. cit. p. 6.
The teeth are composed of a dense, osseous material, of a finely
tubular structure, termed _dentine_; which, in many species, forms on
the external surface of the tooth a layer of firmer texture, with a
glossy surface, resembling enamel. The essential character of their
organization is to have a pulp or medullary cavity, or cavities, filled
with a plexus of blood-vessels and nerves, from which the minute
tubes composing the dentine radiate.[512] The differences observable
in the size, mode of ramification, and distribution of the medullary
cavities or canals, and the _calcigerous tubes_,[513] as revealed by
microscopic exploration, constitute important distinctive characters;
particularly in the examination of the fossil teeth of extinct fishes.
In some teeth the dentine is traversed by equidistant, parallel,
medullary canals; in others, these channels frequently subdivide, and
their branches anastomose with each other. In some the medullary canals
form a reticulated, or net-like structure in the dentine, the meshes
of which are occupied by calcigerous tubes, and cells; often producing
a dendritical appearance, as in the tooth of a fossil fish named
_Dendrodus_. "In the highest type of structure, the dentine consists of
a simple medullary cavity or canal, and a single system of calcigerous
tubes, which radiate from the central or sub-central pulp-cavity, at
right angles to the periphery of the tooth" (_Owen_), as in the teeth of
the extinct Sauroid (_lizard-like_) fishes. A continued succession of
teeth takes place during the life of the fish, and we often find in
fossil specimens a series of successional teeth beneath the row in use;
as in the fragment of a jaw of _Lepidotus_, from Tilgate Forest _Lign.
107_.
[512] See Owen’s Odontography; and Tome’s Dental Physiology.
[513] _Calagerous tubes_; so named because they are composed of _calx_,
or lime.
Skeletons of Fishes.--The skeletons of the animals of this class differ
so remarkably in their relative degree of firmness and elasticity, in
consequence of peculiar modifications of their constituent substance,
as to form two grand divisions; one of which is termed the _osseous_, the
other the _cartilaginous_. The essential difference in the skeletons of
these two groups consists in the presence or absence of earthy matter
(_phosphate and carbonate of lime_) in the materials of which they are
constructed. In the cartilaginous fishes, the skeleton is cartilaginous
and transparent; but in some species, the skin has dense osseous
particles or plates on the skin, as in the _Rays_; and in others, the
head and body are protected by large osseous scutcheons, as in the
_Sturgeon_. There is also an intermediate group of fishes, termed the
_fibro-cartilaginous_, in which the skeleton contains lime, but in a
much less proportion than in the true osseous fishes. In some genera,
certain portions of the skeleton, as the bodies of the vertebra, are
cartilaginous, while the spinous processes, ribs, &c. are osseous;
these characters are of considerable importance in the investigation of
the fossil remains of fishes, as we shall hereafter have occasion to
demonstrate.
The skeleton consists of the _cranium_ or skull, which is composed
of numerous bones,--the jaws, and bones of the tongue,--the osseous
frame-work of the organs of respiration, consisting of the bones, rays,
and arches that support the gills, and the _opercula_, or covers which
close over the branchial apertures,--and of the vertebral column,
formed of numerous dorsal and caudal vertebræ, with the ribs and other
appendages; there are no proper cervical vertebræ, or spinal bones of
the neck.
The branchial arches are in general four or five on each side, and are
attached above to the cranium, and below to a chain of small bones, by
which they are connected with the _os hyoides_, or bone of the tongue.
The opercular bones, composing the cover or lid of the opening of the
gills, consist of three pieces on each side, and are distinguished by
the names, _opercular_, _pre-opercular_, and _sub-opercular_, according
to the situations which they respectively occupy.
The _vertebræ_ are double hollow cones,[514] not unlike an hour-glass
in form: the interval between two of these bones is filled up, in the
living state, by a gelatinous fluid. Along the upper part of each
vertebra, there is an annular cavity, which in the united vertebral
column forms a canal for the spinal marrow; the posterior dorsal and
caudal vertebæ have also a channel below, for the passage of the large
blood-vessels.
[514] There are certain exceptions; thus in the _Lepidosteus_ the
vertebral column is a series of ball-and-socket joints, the convexity
being anterior, as in the land Salamander, and in the fossil reptile
known as the Streptospondylus.
There are likewise bones analogous to some of those which enter into
the composition of the extremities, chest or thorax, and pelvis of
the higher vertebrata; but which it is not necessary for our present
purpose here to describe.
Of the organs of vision some fossil remains also occur. The sclerotic
coat, or capsule of the eye, being bony in fishes, is often preserved;
and in several chalk specimens I have found it occupying the orbit.
In addition to those durable parts of fishes, already mentioned, as
likely to be met with in a fossil state, the bones called _otolithes_
(ear-stones) must be enumerated. These calcareous bodies are found in
the membranous labyrinth of the organs of hearing; and, although more
or less developed in the ear-bulb of all animals, they are larger and
of more definite forms in the higher osseous and cartilaginous fishes.
The _otolithes_ are supposed to assist in communicating more vivid
impressions of sounds to the extremities of the auditory nerves; they
are stony in most aquatic animals, and friable or pulverulent in those
that live on land. Smooth, oblong otolithes are not uncommon in the
Crag deposits of Norfolk and Suffolk; and minute ear-bones are found in
the Barton Clay.
Tails of Fishes.--The tail, as we have previously mentioned, is the
chief instrument of progressive motion in these animals; it assumes two
principal modifications. In the greater number of the existing species
the vertebral column terminates in a triangular plate of bone (formed
by the fusion of the last few vertebræ), to which the caudal fin is
attached symmetrically; and its figure is either rounded, or divided
into two equal lobes or branches; these tails are termed _homocercal_,
i. e. even-tail. In the second modification the vertebral column
towards its extremity diverges from a straight line, rises up, and is
prolonged into the upper lobe of the tail; the caudal fin appearing
like a rudder, and its low’er lobe, being destitute of vertebræ, is
proportionably very feeble and small, as in the Shark and Dog-fish:
this form of tail is called _heterocercal_, i. e. unequal-tail (see
_Foss. Brit. Mus._ p. 421; and _Ly._ figs. 340, 341). In the embryonic
state the tail in all fishes is heterocercal, and it becomes homocercal
in the progress of development in those genera which have this type
of the caudal appendage. But few of the existing species have the
heterocercal tail, while it is found in all the fossil fishes that
occur in the ancient secondary strata; namely, the Magnesian limestone,
and antecedent deposits. The rounded and equal-bilobed, or homocercal,
tails, are seen in the fishes from the Chalk, _Wond._ pp. 347, &c.; and
in the Wealden Lepidotus, _Lign. 186_; and the unequal or heterocercal
tail is shown in the Amblypterus from the Carboniferous strata, _Lign.
187_.
[Illustration: Lign. 186. Lepidotus. _Wealden._ (1/6 _nat. size_.)
(Showing the Homocercal Tail.)]
[Illustration: Lign. 187. Amblypterus. _Carboniferous._ (1/4 _nat.
size_.)
_a._ The heterocercal tail.]
In the Annals of Nat. Hist, for 1848, p. 304, Prof. M’Coy has described
and figured an intermediate form of tail, which he regards as
characteristic of the _Diplopterus_ (of the Old Red Sandstone) and its
allies: this the Professor terms the _Diphycercal_ tail.
From this brief summary of the essential characters of those durable
parts of the organization of fishes, which most frequently occur in
a fossil state, we pass to the investigation of some illustrative
examples of this class of organic remains. But before describing any
entire specimens, it will be expedient to notice the separate fins,
and teeth, which abound in many deposits; in some instances occurring
in connexion with other parts of the skeleton, but more generally
detached, and constituting the only evidence of the existence of
numerous extinct species and genera. The greater part belong to
the first order--the Placoidians (_Poiss. Foss._ tom. iii.), and
to the families of _Sharks_ and _Rays_. The osseous dorsal rays of
cartilaginous fishes (named _Ichthyodorulites_ (_fossil-fish-weapons_)
by Dr. Buckland and Sir H. De la Beche) first demand our notice.
Ichthyodorulites. _Lign. 188._--This name is applied to the fossil
spines, or rays, of dorsal fins, of which numerous species occur in
the Secondary deposits; they belong, for the most part, to extinct
cartilaginous fishes of the _Cestracionidæ_ and _Hybodontidæ_ groups.
In the osseous tribes the dorsal spines have at their base two
articular processes, by which they are united to the osselets that
support them, as in the _Silurus_; but in the cartilaginous, they have
no articulations at the base, and terminate in an obtuse point, which
is implanted in the flesh; the posterior margin is grooved almost to
the upper extremity. They are of a fibrous, osseous texture. The common
Spinax, or Dog-fish (_Acanthias vulgaris_), has a spine of this kind in
the front of each dorsal fin. The rays of the Sharks are compressed,
and some have rows of teeth on the posterior margin; in the genus
_Cestracion_ (Port-Jackson Shark), these organs are strong, triangular,
straight, pointed, rounded in front, flat at the posterior face, and
widest at the base; in the Hays they are flattened or depressed.
These spines are generally capable of being elevated and depressed, and
not only serve the purpose of defence, but, in many instances, afford
support and protection to the soft rays of the fin; forming, as it
were, a moveable mast, by which the sail can be spread out or lowered
at pleasure.
In illustration of this subject, I would first direct attention to the
beautiful fossil, figured _Lign. 188, fig. 1_, which was discovered
in the Chalk near Lewes, and is figured, of the natural size, _Foss.
South D._ tab. xxxix. This ray, or spine, belongs to one of the
Cestraciont fishes (_Ptychodus_), whose teeth are so abundant in the
Chalk, and will presently be described. It is composed of fourteen
thick, flat, osseous rods, or strands, intimately united together, with
longitudinal furrows or sutures on the surface. The anterior margin is
embossed, and the projections form on the sides wide, rounded ribs, and
transverse depressions. Towards the base of the posterior part, there
are large osseous fibres inserted vertically and obliquely, which
appear to have been processes of attachment. The rods, or plates, are
parallel With the posterior margin, and each terminates in a rounded
extremity, or boss, on the front edge of the spine. This ray is wider
at its base than at the superior part the anterior margin is oblique,
and the posterior straight. The surface, where entire, is covered with
a dense osseous substance, which is finely engrailed.[515]
[515] This specimen is figured in _Poiss. Foss._; but it is represented
too short, from the two portions being drawn as if they were connected,
without any interval between them, as in _Lign. 188._ It is in the
British Museum. See _Petrifactions_, p. 450.
[Illustration: Lign. 188. Dorsal Rays of Sharks. _Sussex._]
Fig. 1.--Ptychodus spectabilis. 1/5 _nat._ _Chalk._ _Lewes._
1_a_.--Portion of a Dorsal Ray, with oblique serrated sutures.
_Chalk._ _Lewes._
2.--Ptychodus Gibberulus. 1/5 _nat._ _Chalk._ _Lewes._
3.--Hybodus subcarinatus. _nat._ _Weald._ _Tilgate Forest._
In 1850 I discovered in the Plastic Clay of Castle Hill Newhaven, a
dorsal fin of Ptychodus, with eight vertebræ. A nearly entire fin-ray
of this species, three feet in length, has recently been discovered by
Mr. Charles Potter, of Lewes, in the Chalk near that town. The remains
of another ray, of equal proportions, were found near it; and these
dorsal spines might have belonged to the same individual, for there are
no reasons to forbid the supposition that the Ptychodus had two dorsal
fin-rays. The length of these spines necessarily indicates a very large
fish.
A smaller species of Ichthyodorulite, also found in the Lewes Chalk, is
distinguished from _P. spectabilis_ by its osseous plates contracting
towards their extremities, and terminating more suddenly on the front
margin, producing gibbosities less acute and more distant than in _P.
spectabilis_; this species is named _P. gibberulus_: see _Lign. 188,
fig. 2_.[516]
[516] This fossil is figured of the natural size, Foss. South D. pl.
xl. _fig._ 3.
The bony plates of these fins are occasionally found lying in irregular
groups in the Chalk, as if the fin had partially decomposed and the
plates separated. In one example, the rays are split asunder by a
piece of bone, apparently a portion of a long pointed tooth, firmly
impacted between them; as if the fish had been seized by some enemy,
and had escaped, with the tooth of its adversary in its fin. Very fine
specimens have been found at Charing, Kent, by W. Harris, Esq. F.G.S.
In the fragment of an Ichthyodorulite from the Lewes Chalk, a
remarkable structure is displayed; the osseous plates are united
laterally by smooth, longitudinal lines, as in those above described;
but they are also traversed by numerous oblique, finely-serrated
sutures. _Lign. 188, fig. 1a_.
The Chalk contains rays of other species of _Ptychodus_, as well as of
some allied genera. Of these, the most remarkable are smooth, arched,
pointed spines, having a shallow posterior groove, with an enamelled
surface, marked with fine longitudinal striæ, and frequent, parallel,
oblique lines. These, according to Sir P. Egerton, belong to a true
_Cestracion_ (see p. 584): they were first figured and described by me
(_Foss. South D._ tab. xxxiii. _fig._ 5) as belonging to the _Acanthias
major_, and were subsequently assigned to the genus _Spinax_ by Prof.
Agassiz (_Poiss. Foss._ iii. p. 62).
It may be necessary to remark, that the fins first described have been
referred to the fishes which yielded the large grooved teeth so common
in the Chalk (see _Pl. VI fig. 2_) in consequence of their affinity
to existing species, which have similar fins and teeth; and from the
circumstance that the Sharks of the genus _Lamna_, whose teeth also
abound in the Chalk, have no dorsal rays of this kind; still the
proof of identity remains to be discovered. In one specimen only have
I observed indications of any other part of the skeleton; it is a
spine of _Acanthias major_, the base of which rests on several dorsal
vertebræ (_Foss. South D._ tab. xxxiii.).
Hybodus subcarinatus. _Lign. 188, fig. 3._--The fishes of another
extinct genus of Sharks, termed _Hybodus_, from the gibbous form of
the teeth, were also provided with dorsal spines, which may be readily
distinguished from the preceding. These Ichthyodorulites are generally
long, slightly arched, and terminate in a point at the extremity; the
base, which was implanted in the flesh, is deeply grooved, and much
prolonged, being sometimes equal to one-third of the entire length.
The surface is marked with strong longitudinal ridges, parallel with
the anterior margin which is rounded and laterally compressed. The
posterior edge, which is more or less flat, has, towards the base two
rows of sharp arched teeth, which gradually approach ’each other, and
blend into one line on the upper part of the ray There are numerous
species of this genus in the Oolite and Lias. I have found one species
in the Chalk and a few in the Wealden. The small Ray figured _Lign.
188 fig. 3_, is from Tilgate Forest, and displays the usual characters
of these fossils. From specimens discovered in the Lias, associated
with the teeth, it appears that the _Hybodus_ had two dorsal fins, each
furnished with rays, as in the recent Dog-fish.
The microscopic structure of these rays is stated by M. Agassiz to
resemble that of the teeth: in some there is a pulp cavity, which
occupies the centre of the spine, and is surrounded by dentine, in
which the calcigerous tubes radiate direct to the surface; the external
enamel is a layer of dentine, in which the medullary canals are wanting.
In the strata below the Lias there are numerous Ichthyodorulites, some
of a large size, belonging chiefly to the Cestracion family, and of
extinct species, not observed in more recent deposits. Thus there are
several species of dorsal rays (named Onchus, from their hooked form,)
that are wide at the base, and bent backwards, with the posterior
margin destitute of teeth, in the Carboniferous, Devonian, and Silurian
formations; also immense compressed spines, having small teeth on the
posterior margin, and the surface covered with longitudinal striæ,
and finely toothed, transversely; hence termed Ctenacanthus, or
pectinated-spine (_Murch. Sil. Syst._ p. 596).
The fossil spine, named Orthacanthus (_Poiss. Foss._ vol. iii. pl.
xlv.), and found in the Coal of Manchester, has been discovered in
connexion with the body of the fish to which it belonged in the
Carboniferous deposits of Ruppersdorf in Bohemia (_Geol. Journal_, vol.
v. part ii. p. 23).