THE UPPER SILURIAN PERIOD.
Having now treated of the Lower Silurian period at considerable
length, it will not be necessary to discuss the succeeding group
of the _Upper Silurian_ in the same detail--the more so, as with a
general change of _species_ the Upper Silurian animals belong for
the most part to the same great types as those which distinguish
the Lower Silurian. As compared, also, as regards the total bulk of
strata concerned, the thickness of the Upper Silurian is generally
very much below that of the Lower Silurian, indicating that they
represent a proportionately shorter period of time. In considering
the general succession of the Upper Silurian beds, we shall,
as before, select Wales and America as being two regions where
these deposits are typically developed.
In Wales and its borders the general succession of the Upper
Silurian rocks may be taken to be as follows, in ascending order
(fig. 57):--
(1) The base of the Upper Silurian series is constituted by a
series of arenaceous beds, to which the name of "May Hill Sandstone"
was applied by Sedgwick. These are succeeded by a series of
greenish-grey or pale-grey slates ("Tarannon Shales"), sometimes
of great thickness; and these two groups of beds together form
what may be termed the "_May Hill Group_" (Upper Llandovery of
Murchison). Though not very extensively developed in Britain, this
zone is one very well marked by its fossils; and it corresponds
with the "Clinton Group" of North America, in which similar fossils
occur. In South Wales this group is clearly unconformable to the
highest member of the subjacent Lower Silurian (the Llandovery
group); and there is reason to believe that a similar, though
less conspicuous, physical break occurs very generally between
the base of the Upper and the summit of the Lower Silurian.
(2) The _Wenlock Group_ succeeds the May Hill group, and constitutes
the middle member of the Upper Silurian. At its base it may have
an irregular limestone ("Woolhope Limestone"), and its summit may
be formed by a similar but thicker calcareous deposit ("Wenlock
Limestone"); but the bulk of the group is made up of the argillaceous
and shaly strata known as the "Wenlock Shale." In North Wales
the Wenlock group is, represented by a great accumulation of
flaggy and gritty strata (the "Denbighshire Flags and Grits"),
and similar beds (the "Coniston Flags" and "Coniston Grits")
take the same place in the north of England.
(3) The _Ludlow Group_ is the highest member of the Upper Silurian,
and consists typically of a lower arenaceous and shaly series (the
"Lower Ludlow Rock") a middle calcareous member (the "Aymestry
Limestone"), and an upper shaly and sandy series (the "Upper
Ludlow Rock" and "Downton Sandstone"). At the summit, or close
to the summit, of the Upper Ludlow, is a singular stratum only a
few inches thick (varying from an inch to a foot), which contains
numerous remains of crustaceans and fishes, and is well known
under the name of the "bone-bed." Finally, the Upper Ludlow rock
graduates invariably into a series of red sandy deposits, which,
when of a flaggy character, are known locally as the "Tile-stones."
These beds are probably to be regarded as the highest member
of the Upper Silurian; but they are sometimes looked upon as
passage-beds into the Old Red Sandstone, or as the base of this
formation. It is, in fact, apparently impossible to draw any
actual line of demarcation between the Upper Silurian and the
overlying deposits of the Devonian or Old Red Sandstone series.
Both in Britain and in America the Lower Devonian beds repose
with perfect conformity upon the highest Silurian beds, and the
two formations appear to pass into one another by a gradual and
imperceptible transition.
The Upper Silurian strata of Britain vary from perhaps 3000 or
4000 feet in thickness up to 8000 or 10,000 feet. In North America
the corresponding series, though also variable, is generally of
much smaller thickness, and may be under 1000 feet. The general
succession of the Upper Silurian deposits of North America is
as follows:--
(1) _Medina Sandstone_.--This constitutes the base of the Upper
Silurian, and consists of sandy strata, singularly devoid of life,
and passing below in some localities into a conglomerate ("Oneida
Conglomerate"), which is stated to contain pebbles derived from
the older beds, and which would thus indicate an unconformity
between the Upper and Lower Silurian.
(2) _Clinton Group_.--Above the Medina sandstone are beds of
sandstone and shale, sometimes with calcareous bands, which
constitute what is known as the "Clinton Group." The Medina and
Clinton groups are undoubtedly the equivalent of the "May Hill
Group" of Britain, as shown by the identity of their fossils.
[Illustration: Fig. 57. GENERALIZED SECTION OF THE UPPER SILURIAN
STRATA OF WALES AND SHROPSHIRE.]
(3) _Niagara Group_.--This group consists typically of a series of
argillaceous beds ("Niagara Shale") capped by limestones ("Niagara
Limestone"); and the name of the group is derived from the fact
that it is over limestones of this age that the Niagara river
is precipitated to form the great Falls. In places the Niagara
group is wholly calcareous, and it is continued upwards into a
series of marls and sandstones, with beds of salt and masses
of gypsum (the "Salina Group"), or into a series of magnesian
limestones ("Guelph Limestones"). The Niagara group, as a whole,
corresponds unequivocally with the Wenlock group of Britain.
(4) _Lower Helderberg Group_.--The Upper Silurian period in North
America was terminated by the deposition of a series of calcareous
beds, which derive the name of "Lower Helderberg" from the Helderberg
mountains, south of Albany, and which are divided into several zones,
capable of recognition by their fossils, and known by local names
(Tentaculite Limestone, Water-lime, Lower Pentamerus Limestone,
Delthyris Shaly Limestone, and Upper Pentamerus Limestone). As
a whole, this series may be regarded as the equivalent of the
Ludlow group of Britain, though it is difficult to establish any
precise parallelism. The summit of the Lower Heiderberg group
is constituted by a coarse-grained sandstone (the "Oriskany
Sandstone"), replete with organic remains, which have to a large
extent a Silurian _facies_. Opinions differ as to whether this
sandstone is to be regarded as the highest bed of the Upper Silurian
or the base of the Devonian. We thus see that in America, as
in Britain, no other line than an artificial one can be drawn
between the Upper Silurian and the overlying Devonian.
As regards the _life_ of the Upper Silurian period, we have,
as before, a number of so-called "Fucoids," the true vegetable
nature of which is in many instances beyond doubt. In addition
to these, however, we meet for the first time, in deposits of
this age, with the remains of genuine land-plants, though our
knowledge of these is still too scanty to enable us to construct
any detailed picture of the terrestrial vegetation of the period.
Some of these remains indicate the existence of the remarkable genus
_Lepidodendron_--a genus which played a part of great importance
in the forests of the Devonian and Carboniferous periods, and
which may be regarded as a gigantic and extinct type of the
Club-mosses (_Lycopodiaceoe_). Near the summit of the Ludlow
formation in Britain there have also been found beds charged
with numerous small globular bodies, which Dr Hooker has shown
to be the seed-vessels or "sporangia" of Club-mosses. Principal
Dawson further states that he has seen in the same formation
fragments of wood with the structure of the singular Devonian
Conifer known as _Prototaxites_. Lastly, the same distinguished
observer has described from the Upper Silurian of North America
the remains of the singular land-plants belonging to the genus
_Psilophyton_, which will be referred to at greater length hereafter.
The marine life of the Upper Silurian is in the main constituted
by types of animals similar to those characterising the Lower
Silurian, though for the most part belonging to different species.
The _Protozoans_ are represented principally by _Stromatopora_ and
_Ischadites_, along with a number of undoubted sponges (such as
_Amphispongia, Astroeospongia, Astylospongia_, and _Paloeomanon_).
Amongst the _Coelenterates_, we find the old group of _Graptolites_
now verging on extinction. Individuals still remain numerous,
but the variety of generic and specific types has now become
greatly reduced. All the branching and complex forms of the Arenig,
the twin-Graptolites and _Dicranograpti_ of the Llandeilo, and
the double-celled _Diplograpti_ and _Climacograpti_ of the Bala
group, have now disappeared. In their place we have the singular
_Retiolites_, with its curiously-reticulated skeleton; and several
species of the single-celled genus _Monograptus_, of which a
characteristic species (_M. Priodon_) is here figured. If we
remove from this group the plant-like _Dictyonemoe_, which are
still present, and which survive into the Devonian, no known
species of _Graptolite_ has hitherto been detected in strata
higher in geological position than the Ludlow. This, therefore,
presents us with the first instance we have as yet met with of
the total disappearance and extinction of a great and important
series of organic forms.
[Illustration: Fig. 58.--A, _Monograptus priodon_, slightly enlarged.
B, Fragment of the same viewed from behind. C, Fragment of the
same viewed in front, showing the mouths of the cellules. D,
Cross-section of the same. From the Wenlock Group (Coniston Flags
of the North of England). (Original.)]
The _Corals_ are very numerously represented in the Upper Silurian
rocks some of the limestones (such as the Wenlock Limestone)
being often largely composed of the skeletons of these animals.
Almost all the known forms of this period belong to the two great
divisions of the Rugose and Tabulate corals, the former being
represented by species of _Zaphrentis, Omphyma, Cystiphyllum,
Strombodes, Acervularia, Cyathophyllum_, &c.; whilst the latter
belong principally to the genera _Favosites, Choetetes, Halysites,
Syringopora, Heliolites_, and _Plasmopora_. Amongst the _Rugosa_, the
first appearance of the great and important genus _Cyathophyllum_,
so characteristic of the Palæozoic period, is to be noted; and
amongst the _Tabulata_ we have similarly the first appearance,
in force at any rate, of the widely-spread genus _Favosites_--the
"Honeycomb-corals." The "Chain-corals" (_Halysites_), figured
below (fig. 59), are also very common examples of the Tabulate
corals during this period, though they occur likewise in the
Lower Silurian.
[Illustration: Fig. 59.--a, _Halysites catenularia_, small variety,
of the natural size; b, Fragment of a large variety of the same,
of the natural size; c, Fragment of limestone with the tubes
of _Halysites agglomerata_, of the natural size; d, Vertical
section of two tubes of the same, showing the tabulæ, enlarged.
Niagara Limestone (Wenlock), Canada. (Original.)]
[Illustration: Fig. 60.--Upper Silurian Star-fishes. 1, _Palasterina
primoeva_, Lower Ludlow; 2, _Paloeaster Ruthveni_, Lower Ludlow;
3, _Paloeocoma Colvini_, Lower Ludlow. (After Salter.)]
[Illustration: Fig. 61.--A, _Protaster Sedgwickii_, showing the
disc and bases of the arms; B, Portion of an arm, greatly enlarged.
Lower Ludlow. (After Salter.)]
Amongst the _Echinodermata_, all those orders which have hard parts
capable of ready preservation are more or less largely represented.
We have no trace of the Holothurians or Sea-cucumbers; but this
is not surprising, as the record of the past is throughout almost
silent as to the former existence of these soft-bodied creatures,
the scattered plates and spicules in their skin offering a very
uncertain chance of preservation in the fossil condition. The
Sea-urchins (_Echinoids_) are said to be represented by examples
of the old genus _Paloechinus_. The Star-fishes (_Asteroids_) and
the Brittle-stars (_Ophiuroids_) are, comparatively speaking,
largely represented; the former by species of _Palasterina_ (fig.
60), _Paloeaster_ (fig. 60), _Paloeocoma_ (fig. 60), _Petraster,
Glyptaster_, and _Lepidaster_--and the latter by species of
_Protaster_ (fig. 61), _Paloeodiscus, Acroura_, and _Eucladia_.
The singular _Cystideans_, or "Globe Crinoids," with their globular
or ovate, tesselated bodies (fig. 46, A, C, D,), are also not
uncommon in the Upper Silurian; and if they do not become finally
extinct here, they certainly survive the close of this period by
but a very brief time. By far the most important, however, of
the Upper Silurian Echinodenns, are the Sea-lilies or _Crinoids_.
The limestones of this period are often largely composed of the
fragmentary columns and detached plates of these creatures, and
some of them (such as the Wenlock Limestone of Dudley) have yielded
perhaps the most exquisitely-preserved examples of this group
with which we are as yet acquainted. However varied in their
forms, these beautiful organisms consist of a globular, ovate,
or pear-shaped body (the "calyx"), supported upon a longer or
shorter jointed stem (or "column"). The body is covered externally
with an armour of closely-fitting calcareous plates (fig. 62),
and its upper surface is protected by similar but smaller plates
more loosely connected by a leathery integument. From the upper
surface of the body, round its margin, springs a series of longer
or shorter flexible processes, composed of innumerable calcareous
joints or pieces, movably united with one another. The arms are
typically five in number; but they generally subdivide at least
once, sometimes twice, and they are furnished with similar but
more slender lateral branches or "pinnules," thus giving rise
to a crown of delicate feathery plumes. The "column" is the stem
by which the animal is attached permanently to the bottom of the
sea; and it is composed of numerous separate plates, so jointed
together that whilst the amount of movement between any two pieces
must be very limited, the entire column acquires more or less
flexibility, allowing the organism as a whole to wave backwards and
forwards on its stalk. Into the exquisite _minutioe_ of structure
by which the innumerable parts entering into the composition
of a single Crinoid are adapted for their proper purposes in
the economy of the animal, it is impossible to enter here. No
period, as before said, has yielded examples of greater beauty
than the Upper Silurian, the principal genera represented being
_Cyathocrinus, Platycrinus, Marsupiocrinus, Taxocrinus,
Eucalyptocrinus, Ichthyocrinus, Mariacrinus, Periechocrinus,
Glyptocrinus, Crotalocrinus_, and _Edriocrinus_.
[Illustration: Fig 62.--Upper Silurian Crinoids. a, Calyx and
arms of _Eucalyptocrinus polydactylus_, Wenlock Limestone; b,
_Ichthyocrinus loevis_, Niagara Limestone, America; c, _Taxocrinus
tuberculatus_, Wenlock Limestone. (After M'Coy and Hall.)]
[Illustration: Fig. 63.--_Planolites vulgaris_, the filled-up
burrows of a marine worm. Upper Silurian (Clinton Group), Canada.
(Original.)]
The tracks and burrows of _Annelides_ are as abundant in the
Upper Silurian strata as in older deposits, and have just as
commonly been regarded as plants. The most abundant forms are the
cylindrical, twisted bodies (Planolites), which are so frequently
found on the surfaces of sandy beds, and which have been described
as the stems of sea-weeds. These fossils (fig. 63), however,
can be nothing more, in most cases, than the filled-up burrows
of marine worms resembling the living Lob-worms. There are also
various remains which belong to the group of the tube-inhabiting
Annelides (_Tubicola_). Of this nature are the tubes of _Serpulites_
and _Cornultites_, and the little spiral discs of _Spirorbis
Lewisii_.
[Illustration: Fig. 64.--Upper Silurian Trilobites. a, _Cheirurus
bimucronatus_, Wenlock and Caradoc; b, _Phacops longicaudatus_,
Wenlock, Britain, and America; c, _Phacops Downingioe_, Wenlock
and Ludlow; d, _Harpes ungula_, Upper Silurian, Bohemia. (After
Salter and Barrande.)]
Amongst the _Articulates_, we still meet only with the remains of
_Crustaceans_. Besides the little bivalved _Ostracoda_--which here
are occasionally found of the size of beans--and various _Phyllopods_
of different kinds, we have an abundance of _Trilobites_. These
last-mentioned ancient types, however, are now beginning to show
signs of decadence; and though still individually numerous, there
is a great diminution in the number of generic types. Many of
the old genera, which flourished so abundantly in Lower Silurian
seas, have now died out; and the group is represented chiefly
by species of _Cheirurus, Encrinurus, Harpes, Proetus, Lichas,
Acidaspis, Illoenus, Calymene, Homalonotus_, and _Phacops_--the
last of these, one of the highest and most beautiful of the groups
of Trilobites, attaining here its maximum of development. In the
annexed illustration (fig. 64) some of the characteristic Upper
Silurian Trilobites are represented--all, however, belonging
to genera which have their commencement in the Lower Silurian
period. In addition to the above, the Ludlow rocks of Britain
and the Lower Helderberg beds of North America have yielded the
remains of certain singular Crustaceans belonging to the extinct
order of the _Eurypterida_. Some of these wonderful forms are
not remarkable for their size; but others, such as _Pterygotus
Anglicus_ (fig. 65), attain a length of six feet or more, and
may fairly be considered as the giants of their class. The
Eurypterids are most nearly allied to the existing King-crabs
(_Limuli_), and have the anterior end of the body covered with
a great head-shield, carrying two pairs of eyes, the one simple
and the other compound. The feelers are converted into pincers,
whilst the last pair of limbs have their bases covered with spiny
teeth so as to act as jaws, and are flattened and widened out
towards their extremities so as to officiate as swimming-paddles.
The hinder extremity of the body is composed of thirteen rings,
which have no legs attached to them; and the last segment of
the tail is either a flattened plate or a narrow, sword-shaped
spine. Fragments of the skeleton are easily recognised by the
peculiar scale-like markings with which the surface is adorned,
and which look not at all unlike the scales of a fish. The most
famous locality for these great Crustaceans is Lesmahagow, in
Lanarkshire, where many different species have been found. The
true King-crabs (_Limuli_) of existing seas also appear to have
been represented by at least one form (_Neolimulus_) in the Upper
Silurian.
[Illustration: Fig. 65.--_Pterygotus Anglicus_, viewed from the
under side, reduced in size, and restored. c c, The feelers
(antennæ), terminating in nipping-claws; o o, Eyes; m m,
Three pairs of jointed limbs, with pointed extremities; n n,
Swimming-paddles, the bases of which are spiny and act as jaws.
Upper Silurian, Lanarkshire. (After Henry Woodward.)]
Coming to the _Mollusca_, we note the occurrence of the same
great groups as in the Lower Silurian. Amongst the Sea-mosses
(_Polyzoa_), we have the ancient Lace-corals (_Fenestella_ and
_Retepora_), with the nearly-allied _Glauconome_, and species
of _Ptilodictya_ (fig. 66); whilst many forms often referred
here may probably have to be transferred to the Corals, just as
some so-called Corals will ultimately be removed to the present
group.
[Illustration: Fig. 66.--Upper Silurian Polyzoa. 1, Fan-shaped
frond of _Rhinopora verrucosa_; 1a, Portion of the surface of
the same, enlarged; 2 and 2a, _Phoenopora ensiformis_, of the
natural size and enlarged; 3 and 3a, _Helopora fragilis_, of
the natural size and enlarged; 4 and 4a, _Ptilodictya raripora_,
of the natural size and enlarged. The specimens are all from the
Clinton Formation (May Hill Group) of Canada. (Original.)]
[Illustration: Fig. 67.--_Spirifera hysterica_. The right-hand
figure shows the interior of the dorsal valve with the calcareous
spires for the support of the arms.]
[Illustration: Fig. 68.--Upper Silurian Brachiopods. a a',
_Leptocoelia plano-convexa_, Clinton Group, America; b b',
_Rhynchonella neglecta_, Clinton Group, America; c, _Rhynchonella
cuneata_, Niagara Group, America, and Wenlock Group, Britain;
d d', _Orthis elelgantula_, Llandeilo to Ludlow, America and
Europe; e e', _Atrypa hemispherica_, Clinton Group, America, and
Llandovery and May Hill Groups, Britain; f f', _Atrypa congesta_,
Clinton Group, America; g g', _Orthis Davidsoni_, Clinton Group,
America. (After Hall, Billings, and the Author.)]
The Brachiopods continued to flourish during the Upper Silurian
Period in immense numbers and under a greatly increased variety
of forms. The three prominent Lower Silurian genera _Orthis,
Strophomena_, and _Leptoena_ are still well represented, though
they have lost their former preeminence. Amongst the numerous
types which have now come upon the scene for the first time,
or which have now a special development, are _Spirifera_ and
_Pentamerus_. In the first of these (fig. 69. b, c), one of
the valves of the shell (the dorsal) is furnished in its interior
with a pair of great calcareous spires, which served for the
support of the long and fringed fleshy processes or "arms" which
were attached to the sides of the mouth.[16] In the genus
_Pentamerus_ (fig. 70) the shell is curiously subdivided in its
interior by calcareous plates. The _Pentameri_ commenced their
existence at the very close of the Lower Silurian (Llandovery),
and survived to the close of the Upper Silurian; but they are
specially characteristic of the May Hill and Wenlock groups,
both in Britain and in other regions. One species, _Pentamerus
galeatus_, is common to Sweden, Britain, and America. Amongst
the remaining Upper Silurian Brachiopods are the extraordinary
_Trimerellids_; the old and at the same time modern _Linguloe,
Discinoe_, and _Cranioe_; together with many species of _Atrypa_
(fig. 68, e), _Leptocoelia_ (fig. 68, a), _Rhynchonella_
(fig. 68, b, c), _Meristella_ (fig. 69, a, e, f), _Athyris,
Retzia, Chonetes_, &c.
[Footnote 16: In all the Lamp-shells the mouth is provided with
two long fleshy organs, which carry delicate filaments on their
sides, and which are usually coiled into a spiral. These organs
are known as the "arms," and it is from their presence that the
name of "_Brachiopoda_" is derived (Gr. _brachion_, arm; _podes_,
feet). In some cases the arms are merely coiled away within the
shell, without any support; but in other cases they are carried
upon a more or less elaborate shelly loop, often spoken of as the
"carriage-spring apparatus." In the _Spirifers_, and in other ancient
genera, this apparatus is coiled up into a complicated spiral (fig.
67). It is these "arms," with or without the supporting loops or
spires, which serve as one of the special characters distinguishing
the _Brachiopods_ from the true Bivalves (_Lamellibranchiata_).]
[Illustration: Fig. 69.-a a', Meristella intermedia_, Niagara
Group, America; b, _Spirifera Niagarensis_, Niagara Group, America;
c c', _Spirifera crispa_, May Hill to Ludlow, Britain, and Niagara
Group, America; d, _Strophomena (Streptorhynchus) subplana_,
Niagara Group, America; e, _Meristella naviformis_, Niagara Group,
America; f, _Meristella cylindrica_, Niagara Group, America.
(After Hall, Billings, and the Author.)]
[Illustration: Fig. 70.--_Pentamerus Knightii_. Wenlock and Ludlow.
The right-hand figure shows the internal partitions of the shell.]
[Illustration: Fig. 71.--Upper Silurian Bivalves. A, _Cardiola
interrupta_, Wenlock and Ludlow; B, _Pterinea subfalcata_, Wenlock;
C, _Cardiola fibrosa_, Ludlow. (After Salter and M'Coy.)]
[Illustration: Fig. 72.--Upper Silurian Gasteropods. a, _Platyceras
ventricosum_, Lower Helderberg, America; b, _Euomphalus discors_,
Wenlock, Britain; c, _Holopella obsoleta_ Ludlow, Britain; d,
_Platyschisma helicites_, Upper Ludlow, Britain; e, _Holopella
gracilior_, Wenlock, Britain; f, _Platyceras multisinuatum_, Lower
Helderberg, America; g, _Holopea subconica_, Lower Helderberg,
America; h, h', _Platyostoma Niagarense_, Niagara Group, America.
(After Hall, M'Coy, and Salter.)]
[Illustration: Fig 73.--_Tentaculites ornatus_. Upper Silurian
of Europe and North America.]
The higher groups of the _Mollusca_ are also largely represented
in the Upper Silurian. Apart from some singular types, such as
the huge and thick-shelled _Megalomi_ of the American Wenlock
formation, the Bivalves (_Lamellibranchiata_) present little of
special interest; for though sufficiently numerous, they are rarely
well preserved, and their true affinities are often uncertain.
Amongst the most characteristic genera of this period may be
mentioned _Cardiola_ (fig. 71, A and C) and _Pterinea_ (fig. 71,
B), though the latter survives to a much later date. The Univalves
(_Gasteropoda_) are very numerous, and a few characteristic forms
are here figured (fig. 72). Of these, no genus is perhaps more
characteristic than _Euomphalus_ (fig. 72, b), with its flat
discoidal shell, coiled up into an oblique spiral, and deeply
hollowed out on one side; but examples of this group are both
of older and of more modern date. Another very extensive genus,
especially in America, is Platyceras (fig. 72, a and f),
with its thin fragile shell--often hardly coiled up at all--its
minute spire, and its widely-expanded, often sinuated mouth. The
British _Acroculioe_ should probably be placed here, and the
group has with reason been regarded as allied to the Violet-snails
(_Ianthina_) of the open Atlantic. The species of _Platyostoma_
(fig. 72, h) also belong to the same family; and the entire
group is continued throughout the Devonian into the Carboniferous.
Amongst other well-known Upper Silurian Gasteropods are species
of the genera _Holopea_ (fig. 72, g), _Holopella_ (fig. 72.
e), _Platyschisma_ (fig. 72, d), _Cyclonema, Pleurotomaria,
Murchisonia, Trochonema_, &c. The oceanic Univalves (_Heteropods_)
are represented mainly by species of _Bellerophon_; and the Winged
Snails, or _Pteropods_, can still boast of the gigantic _Thecoe_
and _Conularioe_, which characterise yet older deposits. The
commonest genus of _Pteropoda_, however, is _Tentaculites_ (fig.
73), which clearly belongs here, though it has commonly been
regarded as the tube of an Annelide. The shell in this group
is a conical tube, usually adorned with prominent transverse
rings, and often with finer transverse or longitudinal striæ as
well; and many beds of the Upper Silurian exhibit myriads of
such tubes scattered promiscuously over their surfaces.
The last and highest group of the _Mollusca_--that of the
_Cephalopoda_--is still represented only by _Tetrabranchiate_
forms; but the abundance and variety of these is almost beyond
belief. Many hundreds of different species are known, chiefly
belonging to the straight _Orthoceratites_, but the slightly-curved
_Cyrtoceras_ is only little less common. There are also numerous
forms of the genera _Phragmoceras, Ascoceras, Gyroteras, Lituites_,
and _Nautilus_. Here, also, are the first-known species of the
genus _Goniatites_--a group which attains considerable importance
in later deposits, and which is to be regarded as the precursor
of the _Ammonites_ of the Secondary period.
[Illustration: Fig. 74.--Head-shield of _Pteraspis Banksii_, Ludlow
rocks. (After Murchison.)]
[Illustration: Fig. 75.--A, Spine of _Onchus tenuistriatus_;
B, Shagreen-scales of _Thelodus_. Both from the "bone-bed" of
the Upper Ludlow rocks. (After Murchison.)]
Finally, we find ourselves for the first time called upon to
consider the remains of undoubted vertebrate animals, in the
form of _Fishes_. The oldest of these remains, so far as yet
known, are found in the Lower Ludlow rocks, and they consist of
the bony head-shields or bucklers of certain singular armoured
fishes belonging to the group of the _Ganoids_, represented at
the present day by the Sturgeons, the Gar-pikes of North America,
and a few other less familiar forms. The principal Upper Silurian
genus of these is _Pteraspis_, and the annexed illustration (fig.
74) will give some idea of the extraordinary form of the shield
covering the head in these ancient fishes. The remarkable stratum
near the top of the Ludlow formation known as the "bone-bed" has
also yielded the remains of shark-like fishes. Some of these,
for which the name of _Onchus_ has been proposed, are in the form
of compressed, slightly-curved spines (fig. 75, A), which would
appear to be of the nature of the strong defensive spines implanted
in front of certain of the fins in many living fishes. Besides
these, have been found fragments of prickly skin or shagreen
(_Sphagodus_), along with minute cushion-shaped bodies (_Thelodus_,
fig. 75, B), which are doubtless the bony scales of some fish
resembling the modern Dog-fishes. As the above mentioned remains
belong to two distinct, and at the same time highly-organised,
groups of the fishes, it is hardly likely that we are really
presented here with the first examples of this great class. On
the contrary, whether the so-called "Conodonts" should prove
to be the teeth of fishes or not, we are justified in expecting
that unequivocal remains of this group of animals will still be
found in the Lower Silurian. It is interesting, also, to note
that the first appearance of fishes--the lowest class of vertebrate
animals--so far as known to us at present, does not take place
until after all the great sub-kingdoms of invertebrates have
been long in existence; and there is no reason for thinking that
future discoveries will materially affect the _relative_ order
of succession thus indicated.
LITERATURE.
From the vast and daily-increasing mass of Silurian literature, it
is impossible to do more than select a small number of works which
have a classical and historical interest to the English-speaking
geologist, or which embody researches on special groups of Silurian
animals--anything like an enumeration of all the works and papers
on this subject being wholly out of the question. Apart, therefore,
from numerous and in many cases extremely important memoirs,
by various well-known observers, both at home and abroad, the
following are some of the more weighty works to which the student
may refer in investigating the physical characters and succession
of the Silurian strata and their fossil contents:--
(1) 'Siluria.' Sir Roderick Murchison.
(2) 'Geology of Russia in Europe.' Murchison (with M. de Verneuil
and Count von Keyserling).
(3) 'Bassin Silurien de Bohême Centrale.' Barrande.
(4) 'Introduction to the Catalogue of British Palæozoic Fossils in
the Woodwardian Museum of Cambridge.' Sedgwick.
(5) 'Die Urwelt Russlands.' Eichwald.
(6) 'Report on the Geology of Londonderry, Tyrone,' &c. Portlock.
(7) "Geology of North Wales"--'Mem. Geol. Survey of Great Britain,'
vol. iii. Ramsay.
(8) 'Geology of Canada,' 1863. Sir W. E. Logan; and the 'Reports of
Progress of the Geological Survey' since 1863.
(9) 'Memoirs of the Geological Survey of Great Britain,'
(10) 'Reports of the Geological Surveys of the States of New York,
Illinois, Ohio, Iowa, Michigan, Vermont, Wisconsin, Minnesota,'
&c. By Emmons, Hall, Worthen, Meek, Newberry, Orton, Winchell,
Dale Owen, &c.
(11) 'Thesaurus Siluricus.' Bigsby.
(12) 'British Palæozoic Fossils.' M'Coy.
(13) 'Synopsis of the Silurian Fossils of Ireland,' M'Coy.
(14) "Appendix to the Geology of North Wales"--'Mem. Geol. Survey,'
vol. iii. Salter.
(15) 'Catalogue of the Cambrian and Silurian Fossils in the
Woodwardian Museum of Cambridge.' Salter.
(16) 'Characteristic British Fossils.' Baily.
(17) 'Catalogue of British Fossils.' Morris.
(18) 'Palæozoic Fossils of Canada.' Billings.
(19) 'Decades of the Geological Survey of Canada.' Billings,
Salter, Rupert Jones.
(20) 'Decades of the Geological Survey of Great Britain.' Salter,
Edward, Forbes.
(21) 'Palæontology of New York,' vols. i.-iii. Hall.
(22) 'Palæontology of Illinois.' Meek and Worthen.
(23) 'Palæontology of Ohio.' Meek, Hall, Whitfield, Nicholson.
(24) 'Silurian Fauna of West Tennessee' (Silurische Fauna des
Westlichen Tennessee). Ferdinand Roemer.
(25) 'Reports on the State Cabinet of New York.' Hall.
(26) 'Lethæa Geognostica.' Bronn.
(27) 'Index Palæontologicus.' Bronn.
(28) 'Lethæa Rossica.' Eichwald.
(29) 'Lethæa Suecica.' Hisinger.
(30) 'Palæontologica Suecica.' Angelin.
(31) 'Petrefacta Germaniæ.' Goldfuss.
(32) 'Versteinerungen der Grauwacken-Formation in Sachsen.' Geinitz.
(33) 'Organisation of Trilobites' (Ray Society). Burmeister.
(34) 'Monograph of the British Trilobites' (Palæontographical
Society). Salter.
(35) 'Monograph of the British Merostomata' (Palæontographical Society).
Henry Woodward.
(36) 'Monograph of British Brachiopoda' (Palæontographical Society).
Thomas Davidson.
(37) 'Graptolites of the Quebec Group.' James Hall.
(38) 'Monograph of the British Graptolitidæ.' Nicholson.
(39) 'Monographs on the Trilobites. Pteropods, Cephalopods,
Graptolites,' &c. Extracted from the 'Système Silurien du Centre
de la Bohême.' Barrande.
(40) 'Polypiers Fossiles des Terrains Paleozoiques,' and 'Monograph
of the British Corals' (Palæontographical Society). Milne
Edwards and Jules Haime.