FOSSIL ECHINIDÆ, OR SEA-URCHINS.
[Illustration: Lign. 100. Turban Echinus, with its spines; 1/2 _nat._
(_Hemicidaris crenularis, Agassiz._)
_Jura limestone._]
The fossils we have now to examine are among the most familiar of the
objects commonly known as petrifactions; for as the enveloping cases
of the Echini possess considerable durability, they have served as
moulds into which silex, calc-spar, limestone, pyrites, and other
mineral substances, when in solution, or in a semi-fluid state, have
percolated, and formed sharp and enduring casts, which exhibit the
forms of the plates, and the disposition of the pores, striæ, &c. of
the original structures.
The common Echinus of our sea-coasts (_Echinus sphæra_), known by the
name of Sea-egg, Sea-urchin, or Sea-hedgehog, presents the typical
characters of this order of Radiata, which differs from the Crinoids
and Star-fishes in the absence of arms.
The calcareous envelope of the Echinus, or shell, as it is popularly
called, is composed of many calcareous, polygonal plates, closely
fitted to each other, and arranged in regular and elegant patterns;
appearing in the globular and spherical kinds like the lines of the
meridian on a globe. The plates are disposed in ten vertical series,
united by serrated sutures, and form sections, into which the envelope
or case very commonly separates upon the decay of the investing
integuments. Of the ten bands five are large and five small. The large
bands (_areæ_) are each composed of a double row of plates, ornamented
with tubercles supporting large spines (_Lign. 100_). The five narrower
bands consist of much smaller plates, and the spines are minute, or
altogether wanting; hence these bands appear like avenues through the
spinous tracks, and have therefore been fancifully termed ambulacra
or walks. They are the equivalents of the grooves or furrows of the
Star-fishes (_Lign. 108, a_.); and are traversed by numerous pores,
for the exsertion of tubular feet, or tentacula. Besides these rows
of minute openings, there are two principal apertures, the mouth and
the outlet or vent; and also a few large pores, commonly five, on the
summit, for the exclusion of the ova, and the free admission of water.
The form and relative position of the parts above described, afford
characters by which the order is divided into genera. The mouth, which
is situated on the lower part, is in some species furnished with five
sharp angular teeth, attached to a calcareous framework that admits
of being protruded; this apparatus, when entire, commonly bears the
name of "_Diogenes' lantern_." The eminences on the surface of the
plates vary in size from mere granules, or papillæ, to large mamillated
tubercles; they serve for the attachment of movable spines, which also
present great variety of form and ornament. The spines have a cup-like
cavity at the base which fits on the papillæ, and in many species are
only supported by the capsular envelopment of the common integument;
but in others, the large spines are attached by a ligament which passes
from the centre of the socket, and is received in a perforation of the
papilla of each tubercle, in the same manner as the _ligamentum teres_
of the human thigh-bone. Transverse slices of the spines exhibit the
internal structure, and are beautiful objects under the microscope.
There are also minute appendages to the integument, called
_pedicellariæ_, or pinchers, of a very remarkable character, whose
functions are not known. They are slender columnar bodies, each crowned
with three calcareous teeth-like spines, beautifully sculptured, and
which in some species are long and slender, in others short and obtuse.
I have not observed any traces of these bodies, even in the best
preserved echinites, but as they are as durable as the spines and case,
they may exist in a fossil state.
[Sidenote: TURBAN ECHINITES.]
This general view of the structure of the recent Echinites will enable
the student to understand the nature of the fossil remains; for the
parts above described, are found more or less perfectly preserved,
either in their natural arrangement, or separated and dispersed in
the rock. The habits of these animals, of burrowing in the sand, were
favourable to their preservation in a mineralized state; and in some
of the oolitic limestones, hundreds of beautiful examples of Turban
Echinites (_Cidares_), having their spines spread out on the face of
the rock, are found lying in the positions they evidently occupied
when living. The quarries near Calne and Chippenham, in Wilts, are
celebrated for such fossils.
It was my intention to give figures of the genera into which the
numerous fossil species have been divided by modern authors: but
I found the attempt hopeless, from the changes in nomenclature and
arrangement which are constantly taking place. The monographs on the
fossil Echinidæ, by Prof. E. Forbes, now in course of publication by
the Government School of Mines, will, when completed, place this branch
of Palæontology on a satisfactory basis.
For the convenience of study, the fossil Echini may be separated into
three principal groups; viz. the Cidaritidæ, or Turban Echinites; the
Clypeasteridæ, or Buckler-shaped Echinites; and the Spatangidæ, or
Heart-shaped Echinites.
[Sidenote: CIDARITIDÆ.]
Cidaritidæ. _Turban Echinites._--In these sea-urchins the vent is
situated on the summit of the shell, and is surrounded by five minute
apertures for the exclusion of the ova. The mouth, or oral aperture,
is placed directly opposite, in the middle of the base, or inferior
surface. The mouth is large, and furnished with a powerful apparatus,
armed with teeth; which is sometimes found fossil (see _fig._ 1, 3,
_Lign. 101_). The structure of the animal, in consequence of the
symmetrical position of the two chief outlets of the shells, is
strictly radiated; that is, all the parts proceed from, or are arranged
around one common centre. The tubercles are larger and fewer than in
the other tribes of this family; they support long and powerful spines,
which vary much in form and ornament in different species (see _Lign.
102_). The larger tubercles and spines are beset with smaller ones,
disposed in regular series.
The Turban Echinites are the most ancient types of the order, some
forms appearing in the Carboniferous deposit. The species are very
numerous. The echinites of this group are subdivided into four tribes:
1. _Cidares_, properly so called.--In these the tubercles are
perforated; the ambulacra narrow, and beset with granular tubercles,
and the two lines of pores are close together. The pseudopodia can be
protruded to a great length, even beyond the spines, so as to reach
objects the latter could not touch.
2. _Echini_, which differ from the above in the tubercles being
imperforate, and the ambulacral area wide: the spines and tubes are of
a moderate size.
3. _Diademæ._--The tubercles are perforated, and the ambulacra wide and
studded with large tubercles.
4. _Saleniæ._--The tubercles are imperforate, and relatively large; the
chief distinction is a solid ovarian disk on the summit, composed of
several large flat plates, anchylosed together.
The case of the Turban Echinites is composed of twenty vertical series
of plates, the ambulacra, or porous grooves, forming continuous bands
from the summit to the mouth, which is armed with five angular teeth.
This tribe comprises many of the most elegant fossil species; those
which, from their shape and highly ornamented surface, have received
the popular name of Fairy's night-caps. The genus _Cidaris_, which
is characterized by perforated spinous tubercles, affords the most
beautiful examples, and these are occasionally found with the spines
in contact; a circumstance less rare than might be supposed, when the
nature of the attachment of these organs is considered; for, upon the
decomposition of the integument, and the ligaments which connect the
spines with the tubercles in a living state, these appendages quickly
fall off, even in recent specimens.
The interesting fossil figured _Lign. 100_ (_ante_, p. 311), is a
choice example of a Cidarite with the spines attached. This species
(_Hemicidaris crenularis_, _Agassiz_) is said to be characteristic
of the Upper Jura limestone of Switzerland, and was supposed to be
identical with Mr. Parkinson's _Cidaris parpillata var._ (_Pict.
Atlas_, pl. lvi. _fig._ 6), from Calne, in Wiltshire; but spines like
those of _Lign. 100_, do not occur in the English oolite. These spines
are not homogeneous throughout; the central part appears to have
been of a less dense tissue than the outer coat, as is shown in the
fractured spine in the figure. This structure does not exist in the
spines of the depressed Turban Echinites, but is stated by M. Agassiz
to prevail in all the species of the genus _Hemicidaris_, of which the
fossil figured in _Lign. 100_ is the type.
[Illustration: Lign. 101. Cidarites from the Oolite and Chalk.
Fig. 1.--Hemicidaris intermedia; view in profile, showing the
teeth projecting: 1/3 _nat._ _Calne._
2.--The same seen from above, displaying the outlet, and surrounding
pores.
3.--The same, view of the base, displaying the mouth, surrounded
by five angular bipartite teeth. The ambulacral
tubercles at the base are omitted in the figure.
4.--Diadema rotulare (_Agass._): viewed in profile: not.
5.--A siliceous cast of another species of _Diadema_. _South
Downs._
6.--View of the base of Diadema rotulare. This species
occurs in the so-called Neocomian Formation of France.
]
Cidaris (_Hemicidaris?_) intermedia, _Lign. 101, fig. 1, 2, 3._--The
shell of this echinite, from the Oolite at Calne, so closely resembles
that of _H. crenularis_, above described. that without the spines it
could not be distinguished.[298] It is of a depressed form, and has
very long subcylindrical spines; a specimen is figured _Lign. 102_,
_fig. 9_. It is this cidarite which occurs in such immense numbers in
the Oolite at Calne, Chippenham, Faringdon, &c. Slabs of limestone
are occasionally extracted from the quarries at Calne, with more than
thirty of these echinites surrounded by their spines. I have one
specimen, attached to a block of limestone, with fifty spines; but it
is difficult to detach a perfect spine.
[298] Mr. Woodward. I am indebted to this gentleman for many valuable
remarks on fossil Echinoderms.
[Sidenote: CIDARIS BLUMENBACHII.]
Cidaris Blumenbachii. _Lign. 127, fig. 3._--This is another beautiful
characteristic Turban Echinite of the Oolite. The tubercles are very
large and prominent, and the spines remarkably neat, being covered with
longitudinal granulated striæ; they are of an elongated cucumerine
form, and homogeneous in structure (_Lign. 127, fig. 5_). They occur
by hundreds in some of the layers of friable stone in the quarries
around Calne.
Many species of Turban Echinites abound in the White Chalk, especially
near Gravesend, Northfleet, Purfleet, Charlton, and other places
in Kent; the softness of the cretaceous strata in those localities
rendering the removal of the chalk an easy task. Splendid specimens,
with the spines and tubercles almost as fresh as if recent, have
rewarded the patience and skill of collectors. The British Museum
contains many fine examples, especially a specimen of _Cidaris
clavigera_ from Charlton, with sixteen spines, and the dental apparatus
_in situ_.[299]
[299] Several coloured figures of Cidarites and spines are given in
Pict, Atlas, pl. liii. and lvi. A fine series of Chalk specimens are
figured in Dixon's Fossils, tab. xxv.; many from the choice cabinet of
Henry Catt, Esq. of Brighton.
The collection of Chalk Cidarites with their spines, formed by W. H.
Taylor, Esq. F.G.S., of Winterslow-place, Brixton, is the most splendid
assemblage of these fossils I have seen.
Diadema. _Lign. 101, fig. 4, 6._--The shell in this genus is of
a more depressed form than in Cidaris; there are two rows of large
tubercles, which are crenulated and perforated, on the ambulacra as
well as on the interambulacral spaces. The spines are slender and
annulated. Mr. Woodward remarks that the common Chalk species referred
by authors to this genus, belong to the sub-genus _Cyphosoma_ of M.
Agassiz, in which the tubercles are imperforate. There are nearly fifty
fossil species known, and they range from the Lias to the Chalk. The
recent analogues inhabit the seas of warm regions.
Echinus.--The shells of the genus Echinus resemble those of Cidaris in
their general structure, but the tubercles are imperforate. More than
twenty fossil species are described, from the Oolite and Chalk.
Salenia.--In the greensand pits near Faringdon, in Berkshire, which
abound in fossil sponges and other poriferæ (_ante_, p. 228), there are
immense numbers of a small elegant Turban Echinite, which belongs to
the genus thus named by the eminent zoologist. Dr. J. E. Gray, of the
British Museum. The collector will easily recognize these sea-urchins
by the plated summit. The shell has five ovarian and five interovarian
plates, and an eleventh or odd one. The tubercles are crenulated. The
common species at Faringdon is _S. petalifera_, of Desmarest. Two
species of this genus, viz. _S. scutigera_ and _S. stellulata_, from
near Warminster, are figured in _Pict. Atlas_, pl. liii. _fig._ 12, 13.
_Cidarites of New Zealand._--Detached plates and spines of sea-urchins,
belonging to the family Cidaritidæ, have been discovered by Mr.
Walter Mantell, in the Ototara limestone of New Zealand; which is a
fawn-coloured stone, composed of foraminiferæ, like the Chalk, and
containing terebratulæ, corals, and teeth of sharks.[300]
[300] Geol. Journal, vol. vi. p. 319.
[Sidenote: SPINES OF CIDARITES.]
Spines of Cidarites. _Lign. 102._--Allusion has been made to the
immense numbers of the spines of two or three kinds of Cidarites that
occur in the oolitic limestones of certain localities. The spines of
other species and genera abound in the Chalk, Greensand, &c.; occurring
detached and intermingled with corals, shells, and the usual fossils of
those deposits. There is great variety in the form, size, and sculpture
of these organs. In the subjoined _Lign. 102_, a few distinct kinds are
represented.
[Illustration: Lign. 102. Fossil Spikes of Cidarites.
Fig. 1.--Spine of Acrocidaris nobilis (_Agassiz_).
2.-- ---- Cidaris cucumifera.
3.-- ---- ----- stemmacantha.
4.-- ---- ----- meandrina.
5.-- ---- ----- spinosa.
6.-- ---- ----- clunifera.
7.-- ---- ----- sceptrifera.
8.-- ---- ----- Parkinsoni.
9.-- ---- Hemicidaris intermedia.
]
Flint Casts of Turban Echinites. _Lign. 103._--The siliceous casts of
the shells of the Turban Echinites are interesting objects, for they
are often beautiful models of the interior. A specimen of this kind is
represented in _Lign. 101, fig. 5_. Casts of the larger Cidarites are
often seen on the ploughed lands of the South Downs, in beds of gravel,
and among the shingle on the sea-shore of chalk districts; appearing
as flattened spherical bodies, with a circular protuberance at each
pole, and vertical rows of nodular projections. Impressions of the
external surface of the cases are also frequent on chalk-flints, and
exhibit exquisite casts, in intaglio, of the mamillated tubercles, and
ambulacral grooves and pores.
[Illustration: Lign. 103. Echinital Remains in Flint. _Chalk. Lewes._
(_One-third the natural size._)
Fig. 1.--Cast of an Ananchyte, showing the form of the plates.
2.--Imprint of a segment of a Cidarite on a pebble.
_a._--One of the impressions of a spinous tubercle: _nat._
3.--Portion of an Ananchyte, having the cavity of the shell
covered at the bottom with flint, and lined above with
crystals of carbonate of lime.
]
A fragment of a flint, impressed by a portion of a Cidarite, is
represented _Lign. 103, fig. 2_. The perforations around the imprint
indicate tubular cavities in the flinty formed by the spines, and show
that these processes were attached to the shell when the latter was
enveloped by the fluid silex; the case and the spines having since
perished. But in the Chalk, exquisite specimens of Cidarites occur with
the case perfect, and filled with flint: examples of this kind are
often attached to a nodule by the slender column of silex that fills up
the aperture of the shell. The mineralized condition of the originally
friable calcareous cases of Cidarites and other Echini, is worthy of
attention: for whether the shell or spines be imbedded in chalk, flint,
or pyrites, if the structure and form remain, the constituent substance
is invariably opaque crystallized carbonate of lime, having an oblique
fracture.
As this conversion of a crustaceous envelope into calc-spar is
constant, it has probably resulted from the peculiar nature of the
original animal structures; but the cause of such transmutation is
unknown.
[Sidenote: CIDARITIDÆ OF THE PALÆOZOIC ROCKS.]
Cidaritidæ of the Palæozoic Rocks.--Three genera of this family,
comprising several species, have been discovered in the carboniferous
limestone of Northumberland and Ireland. One genus is undistinguishable
from Cidaris; and the species are placed under that name in Mr.
Morris's Cat. Brit. Foss.
These fossils have been figured and described by Prof. John Phillips,
and Col. Portlock. Prof. M'Coy, with his accustomed penetration and
sagacity, has ascertained, that notwithstanding the general resemblance
between the Cidarites of the secondary and those of the palæozoic
formations, the latter are constructed on an entirely different plan.
In the turban echini of the secondary, tertiary, and modern seas,
the interambulacral plates always consist of _two_ rows; but in the
palæozoic Cidarites there are _three_ or some greater odd number of
these plates. This eminent naturalist, therefore, places the earliest
type of Echinidæ at present known in the order _Perischoechinidæ_.
The case is spheroidal, formed of more than twenty rows of plates;
five ambulacra composed of two rows of pentagonal plates each; rows of
interambulacral plates, three, five, or more, terminating dorsally in
five large pentagonal ovarian plates.
As in the more recent forms, these Cidarites are separable into
two groups or families; one in which the spiniferous tubercles are
imperforate as in the Echinus (_Palæchinidæ_ of M'Coy); the other
with numerous small secondary tubercles and a few large primary
ones, perforated for the ligament of the spine as in Cidaris (the
_Archæocidaridæ_ of M'Coy).[301]
[301] Prof. Sedgwick's "British Palæozoic Fossils," p. 124.
Clypeastridæ.--The shell in this family of sea-urchins, is oblong or
rounded; the mouth is of an angular form, and situated in the middle
of the base or inferior face; it is furnished with well-developed
dental organs. The outlet is distant from the summit. The tubercles
are mere granulations, and the spines proportionally small. This group
is subdivided into two tribes: the Galeritidæ (_helmet-like_), and the
Clypeideæ (_buckler-like_).
Galerites albo-galerus. _Lign. 104._--The tribe of which this genus is
the type has the shell inflated, orbicular, oblong, or pentangular.
The ambulacra are simple, never petaloid; the poriferous zones extend
uninterruptedly from the summit to the mouth.
In the species figured _Lign. 104, fig. 1_, the shell is of a
conical form, in some varieties subpentagonal; narrowest at the hinder
part. The mouth is of a decagonal shape, and armed with teeth: it is
situated in the centre of the base (_Lign. 104, fig, 1a_); the outlet
is near the posterior margin of the base. The surface of the shell
is covered with granulations irregularly distributed. This species,
which received the name of _albo-galerus_, from its fancied resemblance
to the white conical caps of the priests of Jupiter, occurs in great
numbers and perfection in the Kentish chalk; it is less common in that
of Sussex. Siliceous casts of the shell are constantly found among the
drift and gravel, and on the ploughed lands of chalk districts; they
are popularly termed "_sugar-loaves_." The specimens obtained from the
chalk, when filled with flint, yield exquisite casts, if the shell be
dissolved in dilute hydrochloric acid; by this means the form of the
plates, and casts of the minutest ambulacral pores are obtained.
[Sidenote: ECHINITES FROM THE CHALK.]
[Illustration: Lign. 104. Echinites from the Chalk. _Lewes._
Fig. 1.--Galerites albo-galerus: _nat._
1_a_.--Base of the same, with the five teeth.
2.--Ananchytes ovatus: 1/2 _nat._
2_a_.--Base of the same.
]
[Illustration: Lign. 105. Holectypus (_Galerites_) inflatus.
_Kimmeridge Clay, Switzerland._
The left-hand figure shows the summit; the middle figure a profile; and
the right hand, a view of the base, with the mouth in the centre, and
the outlet towards the margin. (_M. Agassiz_).]
Holectypus (_Galerites_) inflatus. _Lign. 105._--In certain kinds of
Galerites, the shell is strengthened internally by five strong ribs
or projections, which of course leave corresponding deep furrows or
channels on the flint casts moulded in them; such fossils are not
numerous on the ploughed lands of the South Downs. These echinites
are placed by M. Desor in the genus of which an example is figured in
_Lign. 105_.
The shell is hemispherical, and circular; the base flat; the tubercles
are disposed in series; the inside of the case is supported by ribs.
[Sidenote: DISCOIDEA CASTANEA.]
Discoidea (_Galerites_) castanea. _Lign. 106._--The Galeritidæ, which
have a polygonal mouth, with the tubercles disposed in vertical rows
from the summit to the centre of the base, as in the Cidarites, instead
of being uniformly spread over the surface, as in _G. albo-galerus_,
are placed in the genus _Discoidea_, by M. Agassiz.
A species, in which the mouth is pentagonal, and the outlet on the
margin, occurs in the Chalk-marl of Sussex, Dorset, and the Alps; a
specimen is figured in _Lign. 106_.
[Illustration: Lign. 106. Discoidea (_Galeritcs_) castanea.
_Chalk-marl, Dorset._
Fig. 1.--Profile. The pores and plates of the ambulacra only are
inserted in this figure.
2.--View from above.
3.--The base, showing the central pentangular mouth, and
the vent in the margin.
]
The Clypeideæ differ from the tribe of echinites last described, in the
ambulacra being petaloid, that is, of a leaf-like shape, and disposed
in a stellated figure on the upper part of the shell. The ambulacra do
not extend to the mouth. The shell is generally of a depressed form;
and the petaloid ambulacra in many species appear like an elegant star,
richly fretted, spread over the shell. There are numerous species of
this type, both recent and fossil; many of the latter, being of a large
size, are beautiful objects in a cabinet of petrifactions.
Clypeus sinuatus (_Pict. Atlas_, pl. liv. _fig._ 1).--Of this genus,
which is the type of the tribe, a large species, _C. sinuatus_, is
very common in the Oolite of Wilts, Gloucestershire, and Oxfordshire,
and must be familiar to collectors. The shell is circular, and much
depressed; and has five petaloid ambulacra: the odd interambulacral
area forms a deep furrow in which the outlet is situated: the mouth is
median, pentagonal, and surrounded by a strong margin. The coloured
figures in _Pict. Atlas_, pl. liv. will enable the student to recognize
these fossils without difficulty. Splendid specimens of an allied
form (_Clypeaster_) occur in the tertiary limestone of Malta, (_Pict.
Atlas_, pl. lvi. fig. 7,) and are not uncommon in collections.
Nucleolites (_Wond._ p. 328).--There is a small type belonging to
this family, of which several species are so abundant in the Oolite,
Greensand, and Chalk-marl, that a brief notice of their characters may
be useful. The shell is oblong and inflated, rounded in front and flat
behind. The pores are united by grooves; the outlet is in a deep furrow
on the superior face; the mouth is sub-central. One species occurs in
the Tertiary strata, and there is a recent species inhabits the seas of
Australia. There are coloured figures of Nucleolites in _Pict. Atlas_,
pl. liv. fig. 5, pl. lv. figs. 6, 8.
Spatangidæ.--In this tribe of echinites, the case is oblong or
cordiform. The mouth, elongated transversely and destitute of proper
jaws, is situated in front of the centre of the base, near the
anterior border of the periphery. The outlet is towards the posterior
margin. The tubercles and spines are very small. Four subdivisions are
established, namely,--
1. _Ananchytes_.--A thick and oval shell; the ambulacra simple and
converging towards the summit; the mouth transverse; the outlet is
situated on the inferior face. (_Lign. 104, fig. 2._)
2. _Spatangus._--This name is now restricted to those Spatangidæ in
which the ambulacra are petaloid, the external row of pores slightly
elongated, and the inner rows round.
3. _Micraster._--By this term are now distinguished the Spatangidæ
which have the ambulacra depressed, and the shell cordiform. The
pores of the even ambulacra are united by a furrow. The vent is on
the posterior face. The common chalk Spatangus belongs to this genus.
(_Lign. 107._)
4. _Holaster._--The shells are heart-shaped; the ambulacra simple, and
converging towards the summit; the mouth is transverse; the outlet is
within the posterior face.
[Sidenote: ANANCHYTES OVATUS.]
Ananchytes ovatus. _Lign. 104. fig. 2._--These sea urchins are among
the most characteristic of the fossils of the Upper Chalk, and are
peculiar to the Cretaceous formation. They are readily distinguished by
their elevated helmet-like form, and by the transverse mouth and oblong
outlet situated on the inferior face of the flat base, and towards the
margin. (_Lign. 104. fig. 2^a._)
The vernacular names of "_Shepherd's Crown_," and "_Fairy Loaf_,"
indicate the form of these abundant fossils. The shell is oval in its
longest diameter; flat, or nearly so, below; and rounded, conical,
and somewhat laterally compressed towards the summit. The ambulacra
are five, between double lines of pores; the tubercles are minute and
scattered; the substance of the shell is of great thickness. More than
twenty species of the genus are known.
The helmet Echinites, like the preceding, have given rise to
innumerable siliceous casts, which are found associated with those
of other forms in the Drift, on the ploughed lands, and among the
shingle on the sea-shore; they are often placed as ornaments on the
mantel-shelves of the cottagers. A flint cast of an Ananchyte, in which
the plates were partially separated, is represented _Lign. 103, fig.
1_. The shells are sometimes filled with pyrites; and occasionally
are found partially empty, with crystals of calc-spar symmetrically
arranged on the inside of the shell, parallel with the rows of
ambulacral pores. _Lign. 103, fig. 3_, is a remarkable example, in
which flint occupies the base of the shell, while the upper surface is
lined with crystals of calcareous spar.
[Illustration: Lign. 107. Micraster (_Spatangus_) cor-anguinum.
_Chalk. Lewes._
Fig. 1.--View from above, showing the petaloid ambulacra.
2.--View of the base, with the mouth.
]
Micraster cor-anguinum (_Snake-heart_). _Lign. 107._--Of this genus
there are many species in the Chalk. This type of Spatangidæ are
more or less oval, elongated, and heart-shaped, wider before than
behind, with a sulcus, or furrow, in front. The shell is fragile,
and composed of large polygonal plates; the tubercles small and
irregularly distributed; the spines are short. The mouth is transverse,
situated anteriorly, and protected by a strong projection of the odd
interambulacrum, which is named the lip. The vent is terminal, and
placed above the margin. There are but four ambulacra, and these
are incomplete, comparatively of small extent, and situated in deep
furrows. A large and new species of Micraster (_M. cor-bovis_, of Prof.
E. Forbes), from the Sussex Chalk, is figured in Dixon's _Fossils_, pl.
xxiv. figs. 3, 4, p. 342.
[Sidenote: TOXASTER COMPLANATUS.]
[Illustration: Lign. 108. Toxaster complanatus.
_Greensand. Switzerland._
Fig. 1.--Profile.
2.--View of the summit, showing the _Vent_ at the side; _e_.
3.--View of the base, displaying the situation of the mouth,
and the union of the five _ambulacra_; their pores are not
introduced in figs. 2 and 3.
_a._ The narrow porous divisions of the shell, termed
_Ambulacra_.
_b._ Interambulacral spaces.
_c._ _Areæ_, or spaces covered by the wide plates.
_d._ The _Mouth_.
_e._ The _Vent_, or _Outlet_.
]
Toxaster (_Spatangus_) complanatus. _Lign. 108._--In this form of
Spatangus (constituting the genus _Toxaster Agass_.), the ambulacra
are not depressed or furrowed, as in the preceding echinites, nor
petaloid (_leaf-shaped_), as in those which M. Agassiz denominates true
Spatangi, but converge to a point on the summit, as is shown in _fig._
2; the external rows of pores are elongated horizontally, and form a
kind of furrow. The odd ambulacrum is in a deep groove. The mouth is
transverse, _fig._ 3, _d_; and at the anterior part of the inferior
face there is a depression, which results from the convergence of the
ambulacral areæ towards that point. The vent is in the posterior face.
This species is from the _Neocomian_ strata of France; I introduce it
to illustrate the characters of several other echinites, which the
French geologists suppose to be confined to the so-called Neocomian
formation; but which also occur in the Upper Greensand of Blackdown.
Holaster is another genus of Spatangidæ established by M. Agassiz, for
those echinites that are heart-shaped, with simple ambulacra converging
towards the summit. The mouth is elongated transversely; the outlet is
on the posterior face. A specimen first described in my _Foss. S. D._
(_pl._ xvii. _fig._ 9, 21), as Spatangus planus, is common in the Lower
Chalk, and Chalk-marl, and abundant in the Firestone Malm-rock.
* * * * *
Our limits will not allow of a more extended notice of the fossil
_Echinidæ_. The student should consult the Memoirs on the genera,
now in course of publication at the Government School of Mines, by
Professor Edward Forbes; the plates are exquisite, as works of art, and
the descriptions all that can be desired.
Mr. Dixon's work contains three excellent plates of cretaceous
Echinites. Several chalk species are figured in my Foss. South Down.
The numerous coloured figures of fossil sea-urchins in the Pictorial
Atlas of Organic Remains, have already been mentioned.
Geological Distribution of Echinites.--No vestiges of this order
of radiata have been discovered in the Silurian deposits: the
earliest known occurrence of any type is in the Carboniferous
formation. The most ancient Echinidæ, according to the present state
of our knowledge, are the Cidares, in the modified form previously
noticed,--_Archæocidaridæ_ (_ante_, p. 322).
In the Trias another type appears, _Hemicidaris_, which holds an
intermediate place between the Cidarites properly so called and the
Diadema.
In the Oolite, and Jurassic formations, numerous forms are for the
first time met with, constituting the genera Echinus, _Clypeus_,
_Disaster_, _Holectypus_, Diadema, Nucleolites, &c.
The Cretaceous seas swarmed with echini belonging to genera of which
no traces have been found in earlier rocks; viz. Holaster, _Salenia_,
_Micraster_, Salerites, Discoidea, _Ananchytes_, Cassidulus, &c.
In the Tertiary formations, Spatangus, Scutella, Clypeaster, and other
new genera appear, and many of the ancient ones are absent; or at least
have not been observed. Of the genera printed above in italics, no
living species are known.
[Sidenote: ON COLLECTING FOSSIL ECHINODERMATA.]
On collecting and developing fossil Echinodermata.--In the previous
remarks on the fossil remains of radiated animals, we have pointed
out those remains that are the most important and instructive, and
should be sought for by the student. Thus, in collecting Crinoids, the
receptacle or body should be the principal object of research, and if
only detached plates can be extracted from the rock, their relative
position should be carefully noted, and the specimens glued to a card
or board, in their natural order; and some of the ossicula of the
column, and of the arms, or tentacula, be placed with them.
Mr. Miller dissected specimens of every genus, and has figured
the separate plates or bones that enter into the composition of
the receptacle,[302] and arms. Traces of the tentacula, and their
subdivisions, must be sought for, and if discovered, should be removed
with the stone to which they are attached, and the block be afterwards
reduced in size by a mason's saw, and not by the blows of a hammer,
which might displace the ossicula.
[302] Natural History of the Crinoidea.
If imbedded in Lias shale, or other fragile material, a thick slab
should be removed, for greater safety in conveyance; this, when reduced
to a convenient size and thinness, may be imbedded in a tray with
plaster of Paris, or glued to a piece of thin, well-seasoned mahogany,
or deal. The specimens of the _Pear Encrinite_ of Bradford, and of the
Pentacrinites from Lyme Regis, in the British Museum,[303] were prepared
in this manner.
[303] Petrifactions, p. 78.
The crinoideal remains in Chalk belong but to few genera; they merely
require the usual manipulation of cretaceous fossils. The collector,
however, should remember that the ossicula and plates of the receptacle
(as for instance of the _Marsupites_), are but slightly adherent to
each other, and the chalk must not be wholly removed, or these parts
will become detached.
The receptacles of the Apiocrinites of the chalk are rarely found
with more than a few joints of the column attached; and I believe no
vestiges of the arms have been observed: these parts are therefore
desiderata, and should be diligently sought for: the radicles of these
crinoideans are long, articulated, and branching, and without due
caution may be mistaken for the arms, or for another species. The first
remark will also apply to the Marsupite; any specimens with but a few
ossicula of the arms are very precious. I may observe that there is yet
much to learn as to the number of species and genera, and the peculiar
characters of the Crinoidea of the chalk, and that any addition to our
knowledge on this subject will be valuable.
The _Asteridæ_ are so simple in form and structure, that it is
unnecessary to offer any suggestions for their development; of course
they must not be removed from the stone.
Among the detached ossicula dispersed through the chalk, the student
will remember that the large _madrepore-like_ tubercle of the
Star-fishes, (_ante_, p. 304,) may often occur. It may easily be
mistaken for an encrinital body, or for a coral, but an accurate
inspection will show that it is not composed of anchylosed plates, like
the receptacle of an Apiocrinite, but has surfaces for attachment to
other ossicula; while the ends, which in a crinoideal column would have
radiated surfaces, are rounded and entire.
In collecting Echinites, much caution is required in dissecting
specimens surrounded by spines. If imbedded in hard limestone, or in
laminated clay, it is scarcely possible to preserve the spines in
connexion with the shell; but it often happens that the Cidarites of
the Oolite are attached by the base to the solid limestone, and the
case with the spines is imbedded in sandy, friable aggregate, not
difficult of removal. A specimen in my cabinet, with upwards of fifty
spines attached to the shell, was obtained under such circumstances.[304]
[304] Now in the British Museum.
The Chalk Echinites will be found to possess spines more frequently
than is commonly supposed, if care be taken to explore the surrounding
chalk before it be removed. T have often procured Cidarites with
spines, when there were no apparent vestiges of these appendages, by
carefully scraping away the surrounding mass until the extremity of a
spine appeared, and then tracing it up to its connexion with the shell;
another point was discovered by further removal, and that was developed
in the same manner; and at length a Cidaris with several spines was
obtained. The chalk around the mouth should always be cautiously
removed in the dentated species, in the hope of preserving the teeth,
as in the specimens, _Lign. 101, fig. 1_, and _Lign. 104, fig. 1_.
As the shells of Chalk Echinites, when hollow, are often lined with
crystals (see _Lign. 103, fig. 3_), it is worth while to break
all indifferent specimens of the common species, with the chance of
obtaining an example of this kind.
The chalk must not be _scraped off_ from the crust or shell of the
Echinites, or the minute granulations and papillæ will be injured or
removed; it should be flaked off with a blunt point.
In friable arenaceous strata, as in some of the Maestricht and Tertiary
deposits, the Echinites may be extricated in as perfect a condition
as if fresh from the sea; it is, indeed, probable, from the habit of
these animals of burrowing in mud and sand, that in many instances they
were entombed alive by the sediment in which their fossil remains are
imbedded.
Beautiful Cidarites and their spines may be collected in the Oolitic
strata at Calne, Chippenham, Bath, &c.; and in the coralline Oolite
near Faringdon; and of Saleniæ, in the Greensand gravel-pits near that
town.
The Upper Greensand near Warminster, and at Chute Farm, near
Heytesbury, abounds in small Cidaritidæ and other echinites. The large
sinuated Clypeus is found in great perfection in the Oolite at Malton,
Cheltenham, Gloucester, &c.
The cretaceous echinites are to be met with in most localities of
the white chalk. The chalk-pits in Kent, especially at Gravesend,
Northfleet, Chatham, &c. are rich in Cidarites, and their spines.
The Galerites, and Ananchytes, are also very fine and numerous; and
the softness of the chalk renders their extrication from the stone a
delightful task for the young geologist.
Specimens of the common kinds of fossil Echinoderms may be obtained at
moderate prices of the dealers named in the Appendix.
There is a matchless suite of fossil Echinidæ in the British Museum,
which has been arranged and named by Mr. S. P. Woodward, and is now
the most instructive and interesting collection extant. It contains
examples of the following genera, viz., Ananchytes, Echinocorys,
Echinolampas, Holaster, Galerites, Cidaris, Diadema, Acrosalenia,
Glypticus, Disaster, Pygurus, Clypeaster, Scutella, Salmasis,
Echinocyanus, &c. There is also a good series of echinital spines.