THE JURASSIC PERIOD.
Resting upon the Trias, with perfect conformity, and with an almost
undeterminable junction, we have the great series of deposits
which are known as the _Oolitic Rocks_, from the common occurrence
in them of oolitic limestones, or as the _Jurassic Rocks_, from
their being largely developed in the mountain-range of the Jura,
on the western borders of Switzerland. Sediments of this series
occupy extensive areas in Great Britain, on the continent of
Europe, and in India. In North America, limestones and marls
of this age have been detected in "the Black Hills, the Laramie
range, and other eastern ridges of the Rocky Mountains; also
over the Pacific slope, in the Uintah, Wahsatch, and Humboldt
Mountains, and in the Sierra Nevada" (Dana); but in these regions
their extent is still unknown, and their precise subdivisions
have not been determined. Strata belonging to the Jurassic period
are also known to occur in South America, in Australia, and in
the Arctic zone. When fully developed, the Jurassic series is
capable of subdivision into a number of minor groups, of which
some are clearly distinguished by their mineral characters, whilst
others are separated with equal certainty by the differences of
the fossils that they contain. It will be sufficient for our
present purpose, without entering into the more minute subdivisions
of the series, to give here a very brief and general account
of the main sub-groups of the Jurassic rocks, as developed in
Britain--the arrangement of the Jura-formation of the continent
of Europe agreeing in the main with that of England.
I. THE LIAS.--The base of the Jurassic series of Britain is formed
by the great calcareo-argillaceous deposit of the "Lias," which
usually rests conformably and almost inseparably upon the Rhætic
beds (the so-called "White Lias"), and passes up, generally
conformably, into the calcareous sandstones of the Inferior Oolite.
The Lias is divisible into the three principal groups of the Lower,
Middle, and Upper Lias, as under, and these in turn contain many
well-marked "zones;" so that the Lias has some claims to be
considered as an independent formation, equivalent to all the
remaining Oolitic rocks. The _Lower Lias_ (_Terrain Sinemurien_ of
D'Orbigny) sometimes attains a thickness of as much as 600 feet,
and consists of a great series of bluish or greyish laminated
clays, alternating with thin bands of blue or grey limestone--the
whole, when seen in quarries or cliffs from a little distance,
assuming a characteristically striped and banded appearance. By
means of particular species of _Ammonites_, taken along with
other fossils which are confined to particular zones, the Lower
Lias may be subdivided into several well-marked horizons. The
_Middle Lias_, or _Marlstone Series_ (_Terrain Liasien_ of
D'Orbigny), may reach a thickness of 200 feet, and consists of
sands, arenaceous marls, and argillaceous limestones, sometimes
with ferruginous beds. The _Upper Lias_ (_Terrain Toarcien_ of
D'Orbigny) attains a thickness of 300 feet, and consists principally
of shales below, passing upwards into arenaceous strata.
II. THE LOWER OOLITES.--Above the Lias comes a complex series of
partly arenaceous and argillaceous, but principally calcareous
strata, of which the following are the more important groups:
a, The _Inferior Oolite_ (_Terrain Bajocien_ of D'Orbigny),
consisting of more than 200 feet of oolitic limestones, sometimes
more or less sandy; b, The _Fuller's Earth_, a series of shales,
clays, and marls, about 120 feet in thickness; c, The _Great
Oolite_ or _Bath Oolite_ (_Terrain Bathonien_ of D'Orbigny),
consisting principally of oolitic limestones, and attaining a
thickness of about 130 feet. The well-known "Stonesfield Slates"
belong to this horizon; and the locally developed "Bradford Clay,"
"Corn brash," and "Forest-marble" may be regarded as constituting
the summit of this group.
III. THE MIDDLE OOLITES.--The central portion of the Jurassic
series of Britain is formed by a great argillaceous deposit,
capped by calcareous strata, as follows: a, The _Oxford Clay_
(_Terrain Callovien_ and _Terrain Oxfordien_ of D'Orbigny),
consisting of dark-coloured laminated clays, sometimes reaching
a thickness of 700 feet, and in places having its lower portion
developed into a hard calcareous sandstone ("Kelloway Rock");
b, The Coral-Rag (_Terrain Corallien_ of D'Orbigny, "Nerinean
Limestone" of the Jura, "Diceras Limestone" of the Alps), consisting,
when typically developed, of a central mass of oolitic limestone,
underlaid and surmounted by calcareous grits.
IV. THE UPPER OOLITES.--a, The base of the Upper Oolites of
Britain is constituted by a great thickness (600 feet or more)
of laminated, sometimes carbonaceous or bituminous clays, which
are known as the _Kimmeridge Clay_ (_Terrain Kimméridgien_ of
D'Orbigny); b, The _Portland Beds_ (_Terrain Portlandien_ of
D'Orbigny) succeed the Kimmeridge clay, and consist inferiorly of
sandy beds surmounted by oolitic limestones ("Portland Stone"),
the whole series attaining a thickness of 150 feet or more, and
containing marine fossils; c, The _Purbeck_ Beds are apparently
peculiar to Great Britain, where they form the summit of the entire
Oolitic series, attaining a total thickness of from 150 to 200
feet. The Purbeck beds consist of arenaceous, argillaceous, and
calcareous strata, which can be shown by their fossils to consist
of a most remarkable alternation of fresh-water, brackish-water,
and purely marine sediments, together with old land-surfaces,
or vegetable soils, which contain the upright stems of trees,
and are locally known as "Dirt-beds."
One of the most important of the Jurassic deposits of the continent
of Europe, which is believed to be on the horizon of the Coral-rag
or of the lower part of the Upper Oolites, is the "_Solenhofen
Slate_" of Bavaria, an exceedingly fine-grained limestone, which
is largely used in lithography, and is celebrated for the number
and beauty of its organic remains, and especially for those of
Vertebrate animals.
The subjoined sketch-section (fig. 159) exhibits in a diagrammatic
form the general succession of the Jurassic rocks of Britain.
Regarded as a whole, the Jurassic formation is essentially marine;
and though remains of drifted plants, and of insects and other
air-breathing animals, are not uncommon, the fossils of the formation
are in the main marine. In the Purbeck series of Britain,
anticipatory of the great river-deposit of the Wealden, there are
fresh-water, brackish-water, and even terrestrial strata, indicating
that the floor of the Oolitic ocean was undergoing upheaval, and
that the marine conditions which had formerly prevailed were nearly
at an end. In places also, as in Yorkshire and Sutherlandshire,
are found actual beds of coal: but the great bulk of the formation
is an indubitable sea-deposit; and its limestones, oolitic as
they commonly are, nevertheless are composed largely of the
comminuted skeletons of marine animals. Owing to the enormous
number and variety of the organic remains which have been yielded
by the richly fossiliferous strata of the Oolitic series, it will
not be possible here to do more than to give an outline-sketch
of the principal forms of life which characterise the Jurassic
period as a whole. It is to be remembered, however, that every
minor group of the Jurassic formation has its own peculiar fossils,
and that by the labours of such eminent observers as Quenstedt,
Oppel, D'Orbigny, Wright, De la Beche, Tate, and others, the
entire series of Jurassic sediments admits of a more complete
and more elaborate subdivision into zones characterised by special
life-forms than has as yet been found practicable in the case
of any other rock-series.
[Illustration: Fig. 159. GENERALIZED SECTION OF THE JURASSIC ROCKS
OF ENGLAND.]
[Illustration: Fig. 160.--_Mantellia_ (_Cycadeoidea_) _megalophylla_,
a Cycad from the Purbeck "dirt-bed." Upper Oolites, England.]
The _plants_ of the Jurassic period consist principally of Ferns,
Cycads, and Conifers--agreeing in this respect, therefore, with
those of the preceding Triassic formation. The _Ferns_ are very
abundant, and belong partly to old and partly to new genera. The
_Cycads_ are also very abundant, and, on the whole, constitute the
most marked feature of the Jurassic vegetation, many genera of this
group being known (_Pterophyllum, Otozamites, Zamites, Crossozamia,
Williamsonia, Bucklandia,_ &c.) The so-called "dirt-bed" of the
Purbeck series consists of an ancient soil, in which stand erect
the trunks of Conifers and the silicified stools of Cycads of
the genus _Mantellia_ (fig.160). The _Coniferoe_ of the Jurassic
are represented by various forms more or less nearly allied to
the existing _Araucarioe_; and these are known not only by their
stems or branches, but also in some cases by their cones. We
meet, also, with the remains of undoubted Endogenous plants,
the most important of which are the fruits of forms allied to
the existing Screw-pines (_Pandaneoe_), such as _Podocarya_ and
_Kaidacarpum_. So far, however, no remains of Palms have been
found; nor are we acquainted with any Jurassic plants which could
be certainly referred to the great "Angiospermous" group of the
Exogens, including the majority of our ordinary plants and trees.
Amongst animals, the _Protozoans_ are well represented in the
Jurassic deposits by numerous _Foraminifers_ and _Sponges_; as
are the _Coelenterates_ by numerous _Corals_. Remains of these
last-mentioned organisms are extremely abundant in some of the
limestones of the formation, such as the "Coral-rag" and the
Great Oolite; and the former of these may fairly be considered
as an ancient "reef." The _Rugose Corals_ have not hitherto been
detected in the Jurassic rocks; and the "_Tabulate Corals_,"
so-called, are represented only by examples of the modern genus
_Millepora_. With this exception, all the Jurassic Corals belong
to the great group which predominates in recent seas (_Zoantharia
sclerodermata_); and the majority belong to the important
reef-building family of the "Star-corals" (_Astroeidoe_). The
form here figured (_Thecosmilia annularis_, fig. 161) is one
of the characteristic species of the Coral-rag.
[Illustration: Fig. 161.--_Thecosmilia annularis_, Coral-rag,
England.]
[Illustration: Fig. 162.--_Pentacrinus fasciculos_, Lias. The
left-hand figure shows a few or the joints of the column; the
middle figure shows the arms, and the summit of the column with
its side-arms; and the right-hand figure shows the articulating
surface of one of the column-joints.]
The _Echinoderms_ are very numerous and abundant fossils in the
Jurassic series, and are represented by Sea-lilies, Sea-urchins,
Star-fishes, and Brittle-stars. The _Crinoids_ are still common,
and some of the limestones of the series are largely composed
of the _débris_ of these organisms. Most of the Jurassic forms
resemble those with which we are already familiar, in having
the body permanently attached to some foreign object by means
of a longer or shorter jointed stalk or "column." One of the
most characteristic Jurassic genera of these "stalked" Crinoids
(though not exclusively confined to this period) is _Pentacrinus_
(fig. 162). In this genus, the column is five-sided, with whorls
of "side-arms;" and the arms are long, slender, and branched.
The genus is represented at the present day by the beautiful
"Medusa-head Pentacrinite" (_Pentacrinus caput-medusoe_). Another
characteristic Oolitic genus is _Apiocrinus_, comprising the
so-called "Pear Encrinites." In this group the column is long
and rounded, with a dilated base, and having its uppermost joints
expanded so as to form, with the cup itself, a pear-shaped mass,
from the summit of which spring the comparatively short arms.
Besides the "stalked" Crinoids, the Jurassic rocks have yielded
the remains of the higher group of the "free" Crinoids, such as
_Saccosoma_. These forms resemble the existing "Feather-stars"
(_Comatula_) in being attached when young to some foreign body by
means of a jointed stem, from which they detach themselves when
fully grown to lead an independent existence. In this later stage
of their life, therefore, they closely resemble the Brittle-stars
in appearance. True Star-fishes (_Asteroids_) and Brittle-stars
(_Ophiuroids_) are abundant in the Jurassic rocks, and the
Sea-urchins (_Echinoids_) are so numerous and so well preserved
as to constitute quite a marked feature of some beds of the series.
All the Oolitic urchins agree with the modern _Echinoids_ in
having the shell composed of no more than twenty rows of plates.
Many different genera are known, and a characteristic species
of the Middle Oolites (_Hemicidaris crenularis_, fig. 163) is
here figured.
[Illustration: Fig. 163.--_Hemicidaris crenularis_, showing the
great tubercles on which the spines were supported. Middle Oolites.]
Passing over the _Annelides_, which, though not uncommon, are
of little special interest, we come to the _Articulates_, which
also require little notice. Amongst the _Crustaceans_, whilst
the little Water-fleas (_Ostracoda_) are still abundant, the
most marked feature is the predominance which is now assumed by
the _Decapods_--the highest of the known groups of the class.
True Crabs (_Brachyura_) are by no means unknown; but the principal
Oolitic Decapods belonged to the "Long-tailed" group (_Macrura_),
of which the existing Lobsters, Prawns, and Shrimps are members.
The fine-grained lithographic slates of Solenhofen are especially
famous as a depot for the remains of these Crustaceans, and a
characteristic species from this locality (_Eryon arctiformis_,
fig. 164) is here represented. Amongst the air-breathing
_Articulates_, we meet in the Oolitic rocks with the remains of
Spiders (_Arachnida_), Centipedes (_Myriapoda_), and numerous
true Insects (_Insecta_). In connection with the last-mentioned
of these groups, it is of interest to note the occurrence of
the oldest known fossil Butterfly--the _Paloeontina Oolitica_
of the Stonesfield slate--the relationships of which appear to
be with some of the living Butterflies of Tropical America.
[Illustration: Fig. 164.--_Eryon arctiformis_, a "Long-tailed
Decapod," from the Middle Oolites (Solenhofen Slate).]
Coming to the _Mollusca_, the _Polyzoans_, numerous and beautiful
as they are, must be at once dismissed; but the _Brachiopods_
deserve a moment's attention. The Jurassic Lamp-shells (fig.
165) do not fill by any means such a predominant place in the
marine fauna of the period, as in many Palæozoic deposits, but
they are still individually numerous. The two ancient genera
_Leptoena_ (fig. 165, a) and _Spirifera_ (fig. 165, b), dating
the one from the Lower and the other from the Upper Silurian,
appear here for the last time upon the scene, but they have not
hitherto been recognised in deposits later than the Lias. The
great majority of the Jurassic _Brachiopods_, however, belong to
the genera _Terebratula_ (fig. 165, c, e, f) and _Rhynchonella_
(fig. 165. d), both of which are represented by living forms
at the present day. The _Terebratuloe_, in particular, are very
abundant, and the species are often confined to special horizons
in the series.
[Illustration: Fig. 165.--Jurassic Brachiopod. a. _Leptoena
Liassica_, enlarged, the small cross below the figure indicating
the true size of the shell--Lias; b, _Spirifera rostrata_, Lias;
c, _Terebratula quadrifida_, Lias; d, d', _Rhynchonella varians_,
Fulter's Earth and Kelloway Rock; e, _Terebratula sphoeroidalis_,
Inferior Oolite; f, _Terebratula digona_, Bradford Clay,
Forest-marble, and Great Oolite. (After Davidson).]
[Illustration: Fig. 166.--_Ostrea Marshii_. Middle and Lower
Oolites.]
[Illustration: Fig. 167.--_Gryphoea incurva_. Lias.]
Remains of _Bivalves_ (_Lamellibranchiata_) are very numerous in
the Jurassic deposits, and in many cases highly characteristic.
In the marine beds of the Oolites, which constitute by far the
greater portion of the whole formation, the Bivalyes are of course
marine, and belong to such genera as _Trigonia, Lima, Pholadomya,
Cardinia, Avicula, Hippopodium_, &c.; but in the Purbeck beds, at
the summit of the series, we find bands of Oysters alternating
with strata containing fresh-water or brackish-water Bivalves,
such as _Cyrenoe_ and _Corbuloe_. The predominant Bivalves of
the Jurassic, however, are the _Oysters_, which occur under many
forms, and often in vast numbers, particular species being commonly
restricted to particular horizons. Thus of the true Oysters,
_Ostrea distorta_ is characteristic of the Purbeck series, where
it forms a bed twelve feet in thickness, known locally as the
"Cinder-bed;" _Ostrea expansa_ abounds in the Portland beds;
_Ostrea deltoidea_ is characteristic of the Kimmeridge clay;
_Ostrea gregaria_ predominates in the Coral-rag; _Ostrea acuminata_
characterises the small group of the Fuller's Earth; whilst the
plaited _Ostrea Marshii_ (fig. 166) is a common shell in the
Lower and Middle Oolites. Besides the more typical Oysters, the
Oolitic rocks abound in examples of the singularly unsymmetrical
forms belonging to the genera _Exogyra_ and _Gryphoea_ (fig.
167). In the former of these are included Oysters with the beaks
"reversed"--that is to say, turned towards the hinder part of
the shell; whilst in the latter are Oysters in which the lower
valve of the shell is much the largest, and has a large incurved
beak, whilst the upper valve is small and concave. One of the
most characteristic _Exogyroe_ is the _E. Virgula_ of the Oxford
Clay, and of the same horizon on the Continent; and the _Gryphoea
incurva_ (fig. 167) is equally abundant in, and characteristic
of, the formation of the Lias. Lastly, we may notice the
extraordinary shells belonging to the genus _Diceras_ (fig. 168),
which are exclusively confined to the Middle Oolites. In this
formation in the Alps they occur in such abundance as to give
rise to the name of "Calcaire à Dicerates," applied to beds of
the same age as the Coral-rag of Britain. The genus _Diceras_
belongs to the same family as the "Thorny Clams" (Chama) of the
present day--the shell being composed of nearly equally-sized
valves, the beaks of which are extremely prominent and twisted
into a spiral. The shell was attached to some foreign body by
the beak of one of its valves.
[Illustration: Fig. 168.--_Diceras arietina_. Middle Oolite.]
[Illustration: Fig. 169.--_Nerinoea Goodhallii_, one-fourth of the
natural size. The left-hand figure shows the appearance presented
by the shell when vertically divided. Coral-rag, England.]
Amongst the Jurassic Univalves (_Gasteropoda_) there are many
examples of the ancient and long-lived _Pleurotomaria_; but on
the whole the Univalves begin to have a modern aspect. The
round-mouthed ("holostomatous"), vegetable-eating Sea-snails,
such as the Limpets (_Patellidoe_), the Nerites (_Nerita_), the
_Turritelloe, Chemnitzioe_, &c., still hold a predominant place.
The two most noticeable genera of this group are _Cerithium_
and _Nerinoea_--the former of these attaining great importance
in the Tertiary and Recent seas, whilst the latter (fig. 169)
is highly characteristic of the Jurassic series, though not
exclusively confined to it. One of the limestones of the Jura,
believed to be of the age of the Coral-rag (Middle Oolite) of
Britain, abounds to such an extent in the turreted shells of
_Nerinoea_ as to have gained the name of "Calcaire à Nérinées."
In addition to forms such as the preceding, we now for the first
time meet, in any force, with the Carnivorous Univalves, in which
the mouth of the shell is notched or produced into a canal, giving
rise to the technical name of "siphonostomatous" applied to the
shell. Some of the carnivorous forms belong to extinct types,
such as the _Purpuroidea_ of the Great Oolite; but others are
referable to well-known existing genera. Thus we meet here with
species of the familiar groups of the Whelks (_Buccinum_), the
Spindle-shells (_Fusus_), the Spider-shells (_Pteroceras_), _Murex,
Rostellaria_, and others which are not at present known to occur
in any earlier formation.
Amongst the Wing-shells (_Pteropoda_), it is sufficient to mark
the final appearance in the Lias of the ancient genus _Conularia_.
[Illustration: Fig. 170.--_Ammonites Humphresianus_. Inferior
Oolite.]
[Illustration: Fig. 171.--_Ammonites bifrons_. Lias.]
Lastly, the order of the _Cephalopoda_, in both its Tetrabranchiate
and Dibranchiate sections, undergoes a vast development in the
Jurassic period. The old and comparatively simple genus _Nautilus_
is still well represented, one species being very similar to the
living Pearly Nautilus (_N. Pompilius_); but the _Orthocerata_
and _Goniatites_ of the Trias have finally disappeared; and the
great majority of the Tetrabranchiate forms are referable to
the comprehensive genus _Ammonites_, with its many sub-genera
and its hundreds of recorded species. The shell in _Ammonites_
is in the form of a flat spiral, all the coils of which are in
contact (figs. 170 and 171). The innermost whorls of the shell
are more or less concealed; and the body-chamber is elongated
and narrow, rather than expanded towards the mouth. The tube or
siphuncle which runs through the air-chambers is placed on the
dorsal or _convex_ side of the shell; but the principal character
which distinguishes _Ammonites_ from _Goniatites_ and _Ceratites_ is
the wonderfully complex manner in which the _septa_, or partitions
between the air-chambers, are folded and undulated. To such an
extent does this take place, that the edges of the septa, when
exposed by the removal of the shell-substance, present in an
exaggerated manner the appearance exhibited by an elaborately-dressed
shirt-frill when viewed edgewise. The species of _Ammonites_ range
from the Carboniferous to the Chalk; but they have not been found
in deposits older than the Secondary, in any region except India;
and they are therefore to be regarded as essentially Mesozoic
fossils. Within these limits, each formation is characterised by
particular species, the number of individuals being often very
great, and the size which is sometimes attained being nothing short
of gigantic. In the Lias, particular species of _Ammonites_ may
succeed one another regularly, each having a more or less definite
horizon, which it does not transgress. It is thus possible to
distinguish a certain number of zones, each characterised by
a particular Ammonite, together with other associated fossils.
Some of these zones are very persistent and extend over very
wide areas, thus affording valuable aid to the geologist in his
determination of rocks. It is to be remembered, however, that
there are other species which are not thus restricted in their
vertical range, even in the same formations in which definite
zones occur.
[Illustartion: Fig. 172.--_Beloteuthis subcostata_ Jurassic (Lias).]
The Cuttle-fishes or _Dibranchiate Cephalopods_ constitute a
feature in the life of the Jurassic period little less conspicuous
and striking than that afforded by the multitudinous and varied
chambered shells of the _Ammonitidoe_. The remains by which these
animals are recognised are necessarily less perfect, as a rule,
than those of the latter, as no external shell is present (except
in rare and more modern groups), and the internal skeleton is
not necessarily calcareous. Nevertheless, we have an ample record
of the Cuttle-fishes of the Jurassic period, in the shape of
the fossilised jaws or beak, the ink-bag, and, most commonly
of all, the horny or calcareous structure which is embedded in
the soft tissues, and is variously known as the "pen" or "bone."
The beaks of Cuttle-fishes, though not abundant, are sufficiently
plentiful to have earned for themselves the general title of
"Rhyncholites;" and in their form and function they resemble
the horny, parrot-like beak of the existing Cephalopods. The
ink-bag or leathery sac in which the Cuttle-fishes store up the
black pigment with which they obscure the water when attacked,
owes its preservation to the fact that the colouring-matter which
it contains is finely-divided carbon, and therefore nearly
indestructible except by heat. Many of these ink-bags have been
found in the Lias; and the colouring-matter is sometimes so well
preserved that it has been, as an experiment, employed in painting
as a fossil "sepia." The "pens" of the Cuttle-fishes are not
commonly preserved, owing to their horny consistence, but they
are not unknown. The form here figured (_Beloteuthis subcostata_,
fig. 172) belonged to an old type essentially similar to our modern
Calamaries, the skeleton of which consists of a horny shaft and
two lateral wings, somewhat like a feather in general shape. When,
on the other hand, the internal skeleton is calcareous, then it is
very easily preserved in a fossil condition; and the abundance of
remains of this nature in the Secondary rocks, combined with their
apparent total absence in Palæozoic strata, is a strong presumption
in favour of the view that the order of the Cuttle-fishes did
not come into existence till the commencement of the Mesozoic
period. The great majority of the skeletons of this kind which are
found in the Jurassic rocks belong to the great extinct family
of the "Belemnites" (_Belemnitidoa_), which, so far as known, is
entirely confined to rocks of Secondary age. From its pointed,
generally cylindro-conical form, the skeleton of the Belemnite is
popularly known as a "thunderbolt". (fig. 173, C). In its perfect
condition--in which it is, however, rarely obtainable--the skeleton
consists of a chambered conical shell (the "phragmacone"), the
partitions between the chambers of which are pierced by a marginal
tube or "siphuncle." This conical shell--curiously similar in its
structure to the _external_ shell of the Nautilus--is extended
forwards into a horny "pen," and is sunk in a corresponding conical
pit (fig. 173, B), excavated in the substance of a nearly cylindrical
fibrous body or "guard," which projects backwards for a longer or
shorter distance, and is the part most usually found in a fossil
condition. Many different kinds of _Belemnites_ are known, and
their guards literally swarm in many parts of the Jurassic series,
whilst some specimens attain very considerable dimensions. Not
only is the internal skeleton known, but specimens of _Belemnites_
and the nearly allied _Belemnoteuthis_ have been found in some
of the fine-grained sediments of the Jurassic formation, from
which much has been learnt even as to the anatomy of the soft
parts of the animal. Thus we know that the Belemnites were in
many respects comparable with the existing Calamaries or Squids,
the body being furnished with lateral fins, and the head carrying
a circle of ten "arms," two of which were longer than the others
(fig. 173, A). The suckers on the arms were provided, further,
with horny hooks; there was a large ink-sac; and the mouth was
armed with horny mandibles resembling in shape the beak of a
parrot.
[Illustration: Fig. 173.--A, Restoration of the animal of the
Belemnite; B, Diagram showing the complete skeleton of a Belemnite,
consisting of the chambered phragmacone (a), the guard (b), and
the horny pen (c); C, Specimen of _Belemnites canaliculatus_,
from the Inferior Oolite. (After Phillips.)]
[Illustration: Fig. 174.--_Tetragonolepis (restored), and scales
of the same. Lias.]
Coming next to the _Vertebrates_, we find that the Jurassic _Fishes_
are still represented by _Ganoids_ and _Placoids_. The Ganoids,
however, unlike the old forms, now for the most part possess
nearly or quite symmetrical ("homocercal") tails. A characteristic
genus is _Tetragonolepis_ (fig. 174), with its deep compressed
body, its rhomboidal, closely-fitting scales, and its single
long dorsal fin. Amongst the _Placoids_ the teeth of true Sharks
(_Notidanus_) occur for the first time; but by far the greater
number of remains referable to this group are still the fin-spines
and teeth of "Cestracionts," resembling the living Port-Jackson
Shark. Some of these teeth are pointed (_Hybodus_); but others
are rounded, and are adapted for crushing shell-fish. Of these
latter, the commonest are the teeth of _Acrodus_ (fig. 175), of
which the hinder ones are of an elongated form, with a rounded
surface, covered with fine transverse striæ proceeding from a
central longitudinal line. From their general form and striation,
and their dark colour, these teeth are commonly called "fossil
leeches" by the quarrymen.
[Illustration: Fig. 175.--Tooth of _Acrodus nobilis_. Lias.]
The Amphibian group of the _Labyrinthodonts_, which was so
extensively developed in the Trias, appears to have become extinct,
no representative of the order having hitherto been detected in
rocks of Jurassic age.
[Illustration: Fig. 176.--_Ichthyosaurus communis. Lias.]
Much more important than the Fishes of the Jurassic series are
the _Reptiles_, which are both very numerous, and belong to a
great variety of types, some of these being very extraordinary
in their anatomical structure. The predominant group is that
of the "Enaliosaurs" or "Sea-lizards," divided into two great
orders, represented respectively by the _Ichthyosaurus_ and the
_Plesiosaurus_.
The _Ichthyosauri_ or "Fish-Lizards" are exclusively Mesozoic
in their distribution, ranging from the Lias to the Chalk, but
abounding especially in the former. They were huge Reptiles, of
a fish-like form, with a hardly conspicuous neck (fig. 176),
and probably possessing a simply smooth or wrinkled skin, since
no traces of scales or bony integumentary plates have ever been
discovered. The tail was long, and was probably furnished at its
extremity with a powerful expansion of the skin, constituting a
tail-fin similar to that possessed by the Whales. The limbs are
also like those of Whales in the essentials of their structure,
and in their being adapted to act as swimming-paddles. Unlike
the Whales, however, the Ichthyosaurs possessed the hind-limbs
as well as the fore-limbs, both pairs having the bones flattened
out and the fingers completely enclosed in the skin, the arm
and leg being at the same time greatly shortened. The limbs are
thus converted into efficient "flippers," adapting the animal
for an active existence in the sea. The different joints of the
backbone (vertebræ) also show the same adaptation to an aquatic
mode of life, being hollowed out at both ends, like the biconcave
vertebræ of Fishes. The spinal column in this way was endowed
with the flexibility necessary for an animal intended to pass
the greater part of its time in water. Though the _Ichthyosaurs_
are undoubtedly marine animals, there is, however, reason to
believe that they occasionally came on shore, as they possess
a strong bony arch, supporting the fore-limbs, such as would
permit of partial, if laborious, terrestrial progression. The
head is of enormous size, with greatly prolonged jaws, holding
numerous powerful conical teeth lodged in a common groove. The
nature of the dental apparatus is such as to leave no doubt as
to the rapacious and predatory habits of the Ichthyosaurs--an
inference which is further borne out by the examination of their
petrified droppings, which are known to geologists as "coprolites,"
and which contain numerous fragments of the bones and scales
of the Ganoid fishes which inhabited the same seas. The orbits
are of huge size; and as the eyeball was protected, like that
of birds, by a ring of bony plates in its outer coat, we even
know that the pupils of the eyes were of correspondingly large
dimensions. As these bony plates have the function of protecting
the eye from injury under sudden changes of pressure in the
surrounding medium, it has been inferred, with great probability,
that the Ichthyosaurs were in the habit of diving to considerable
depths in the sea. Some of the larger specimens of _Ichthyosaurus_
which have been discovered in the Lias indicate an animal of
from 20 to nearly 40 feet in length; and many species are known
to have existed, whilst fragmentary remains of their skeletons
are very abundant in some localities. We may therefore safely
conclude that these colossal Reptiles were amongst the most
formidable of the many tyrants of the Jurassic seas.
[Illustration: Fig. 177.--_Plesiosaurus dolichodeirus_, restored.
Lias.]
The _Plesiosaurus_ (fig. 177) is another famous Oolitic Reptile,
and, like the preceding, must have lived mainly or exclusively in
the sea. It agrees with the Ichthyosaur in some important features
of its organisation, especially in the fact that both pairs of
limbs are converted into "flippers" or swimming-paddles, whilst
the skin seems to have been equally destitute of any scaly or bony
investiture. Unlike the _Ichthyosaur_, however, the Plesiosaur
had the paddles placed far back, the tail being extremely short,
and the neck greatly lengthened out, and composed of from twenty
to forty vertebræ. The bodies of the vertebræ, also, are not
deeply biconcave, but are flat, or only slightly cupped. The
head is of relatively small size, with smaller orbits than those
of the _Ichthyosaur_, and with a snout less elongated. The jaws,
however, were armed with numerous conical teeth, inserted in
distinct sockets. As regards the habits of the Plesiosaur, Dr
Conybeare arrives at the following conclusions: "That it was
aquatic is evident from the form of its paddles; that it was
marine is almost equally so from the remains with which it is
universally associated; that it may have occasionally visited
the shore, the resemblance of its extremities to those of the
Turtles may lead us to conjecture: its movements, however, must
have been very awkward on land; and its long neck must have impeded
its progress through the water, presenting a strong contrast to
the organisation which so admirably fits the _Ichthyosaurus_
to cut through the waves." As its respiratory organs were such
that it must of necessity have required to obtain air frequently,
we may conclude "that it swam upon or near the surface, arching
back its long neck like a swan, and occasionally darting it down
at the fish which happened to float within its reach. It may
perhaps have lurked in shoal water along the coast, concealed
amongst the sea-weed; and raising its nostrils to a level with
the surface from a considerable depth, may have found a secure
retreat from the assaults of powerful enemies; while the length
and flexibility of its neck may have compensated for the want
of strength in its jaws, and its incapacity for swift-motion
through the water."
About twenty species of _Plesiosaurus_ are known, ranging from
the Lias to the Chalk, and specimens have been found indicating
a length of from eighteen to twenty feet. The nearly related
"_Pliosaurs_," however, with their huge heads and short necks,
must have occasionally reached a length of at least forty feet--the
skull in some species being eight, and the paddles six or seven
feet long, whilst the teeth are a foot in length.
[Illustration: Fig. 178.--_Pterodactylus crassirostis_. From the
Lithographic Slates of Solenhofen (Middle Oolite). The figure is
"restored," and it seems certain that the restoration is incorrect
in the comparatively unimportant particular, that the hand should
consist of no more than four fingers, three short and one long,
instead of five, as represented.]
Another extraordinary group of Jurassic Reptiles is that of the
"Winged Lizards" or _Pterosauria_. These are often spoken of
collectively as "Pterodactyles," from _Pterodactylus_, the type-genus
of the group. As now restricted, however, the genus _Pterodactylus_
is more Cretaceous than Jurassic, and it is associated in the
Oolitic rocks with the closely allied genera _Dimorphodon_ and
_Rhamphorhynchus_. In all three of these genera we have the same
general structural organisation, involving a marvellous combination
of characters, which we are in the habit of regarding as peculiar
to Birds on the one hand, to Reptiles on another hand, and to the
Flying Mammals or Bats in a third direction. The "Pterosaurs"
are "Flying" Reptiles, in the true sense of the term, since they
were indubitably possessed of the power of active locomotion
in the air, after the manner of Birds. The so-called "Flying"
Reptiles of the present day, such as the little _Draco volans_
of the East Indies and Indian Archipelago, possess, on the other
hand, no power of genuine flight, being merely able to sustain
themselves in the air through the extensive leaps which they take
from tree to tree, the wing-like expansions of the skin simply
exercising the mechanical function of a parachute. The apparatus
of flight in the "Pterosaurs" is of the most remarkable character,
and most resembles the "wing" of a Bat, though very different in
some important particulars. The "wing" of the Pterosaurs is like
that of Bats, namely, in consisting of a thin leathery expansion of
the skin which is attached to the sides of the body, and stretches
between the fore and hind limbs, being mainly supported by an
enormous elongation of certain of the digits of the hand. In
the Bats, it is the four outer fingers which are thus lengthened
out; but in the Pterosaurs, the wing-membrane is borne by a single
immensely-extended finger (fig. 178). No trace of the actual
wing-membrane itself has, of course, been found fossilised; but
we could determine that the "Pterodactyles" possessed the power
of flight, quite apart from the extraordinary conformation of
the hand. The proofs of this are to be found partly in the fact
that the breast-bone was furnished with an elevated ridge or
keel, serving for the attachment of the great muscles of flight,
and still more in the fact that the bones were hollow and were
filled with air--a peculiarity wholly confined amongst living
animals to Birds only. The skull of the Pterosaurs is long, light,
and singularly bird-like in appearance--a resemblance which is
further increased by the comparative length of the neck and the
size of the vertebræ of this region (fig. 178). The jaws, however,
unlike those of any existing Bird, were, with one exception to be
noticed hereafter, furnished with conical teeth sunk in distinct
sockets; and there was always a longer or shorter tail composed
of distinct vertebræ; whereas in all existing Birds the tail
is abbreviated, and the terminal vertebræ are amalgamated to
form a single bone, which generally supports the great feathers
of the tail.
Modern naturalists have been pretty generally agreed that the
_Pterosaurs_ should be regarded as a peculiar group of the Reptiles;
though they have been and are still regarded by high authorities,
like Professor Seeley, as being really referable to the Birds, or
as forming a class by themselves. The chief points which separate
them from Birds, as a class, are the character of the apparatus
of flight, the entirely different structure of the fore-limb, the
absence of feathers, the composition of the tail out of distinct
vertebræ, and the general presence of conical teeth sunk in distinct
sockets in the jaws. The gap between the Pterosaurs and the Birds
has, however, been greatly lessened of late by the discovery
of fossil animals (_Ichthyornis_ and _Hesperornis_) with the
skeleton proper to Birds combined with the presence of teeth
in the jaws, and by the still more recent discovery of other
fossil animals (_Pteranodon_) with a Pterosaurian skeleton, but
without teeth; whilst the undoubtedly feathered _Archoeopteryx_
possessed a long tail composed of separate vertebræ. Upon the
whole, therefore, the relationships of the Pterosaurs cannot
be regarded as absolutely settled. It seems certain, however,
that they did not possess feathers--this implying that they were
cold-blooded animals; and their affinities with Reptiles in this,
as in other characters, are too strong to be overlooked.
[Illustration: Fig. 179--_Rhamphorhynchus Bucklandi_, restored.
Bath Oolite, England. (After the late Professor Phillips.)]
The _Pterosaurs_ are wholly Mesozoic, ranging from the Lias to
the Chalk inclusive; and the fine-grained Lithographic Slate of
Solenhofen has proved to be singularly rich in their remains.
The genus _Pterodactylus_ itself has the jaws toothed to the
extremities with equal-sized conical teeth, and its species range
from the Middle Oolites to the Cretaceous series, in connection
with which they will be again noticed, together with the toothless
genus _Pteranodon_. The genus _Dimorphodon_ is Liassic, and is
characterised by having the front teeth long and pointed, whilst
the hinder teeth are small and lancet-shaped. Lastly, the singular
genus _Rhamphorhynchus_, also from the Lower Oolites, is
distinguished by the fact that there are teeth present in the
hinder portions of both jaws; but the front portions are toothless,
and may have constituted a horny beak. Like most of the other
Jurassic Pterosaurs, _Rhamphorhynchus_ (fig. 179) does not seem
to have been much bigger than a pigeon, in this respect falling
far below the giant "Dragons" of the Cretaceous period. It differed
from its relatives, not only in the armature of the mouth, but
also in the fact that the tail was of considerable length. With
regard to its habits and mode of life, Professor Phillips remarks
that, "gifted with ample means of flight, able at least to perch
on rocks and scuffle along the shore, perhaps competent to dive,
though not so well as a Palmiped bird, many fishes must have
yielded to the cruel beak and sharp teeth of Rhamphorhynchus.
If we ask to which of the many families of Birds the analogy of
structure and probable way of life would lead us to assimilate
Rhamphorhynchus, the answer must point to the swimming races with
long wings, clawed feet, hooked beak, and habits or violence and
voracity; and for preference, the shortness of the legs, and other
circumstances, may be held to claim for the Stonesfield fossil a
more than fanciful similitude to the groups of Cormorants, and
other marine divers, which constitute an effective part of the
picturesque army of robbers of the sea."
Another extraordinary and interesting group of the Mesozoic Reptiles
is constituted by the _Deinosauria_, comprising a series of mostly
gigantic forms, which range from the Trias to the Chalk. All the
"Deinosaurs" are possessed of the two pairs of limbs proper to
Vertebrate animals, and these organs are in the main adapted for
walking on the dry land. Thus, whilst the Mesozoic seas swarmed
with the huge Ichthyosaurs and Plesiosaurs, and whilst the air
was tenanted by the Dragon-like Pterosaurs, the land-surfaces of
the Secondary period were peopled by numerous forms of Deinosaurs,
some of them of even more gigantic dimensions than their marine
brethren. The limbs of the _Deinosaurs_ are, as just said, adapted
for progression on the land; but in some cases, at any rate, the
hind-limbs were much longer and stronger than the fore-limbs;
and there seems to be no reason to doubt that many of these forms
possessed the power of walking, temporarily or permanently, on their
hind-legs, thus presenting a singular resemblance to Birds. Some
very curious and striking points connected with the structure of
the skeleton have also been shown to connect these strange Reptiles
with the true Birds; and such high authorities as Professors Huxley
and Cope are of opinion that the Deinosaurs are distinctly related
to this class, being in some respects intermediate between the
proper Reptiles and the great wingless Birds, like the Ostrich
and Cassowary. On the other hand, Professor Owen has shown that the
Deinosaurs possess some weighty points of relationship with the
so-called "Pachydermatous" Quadrupeds, such as the Rhinoceros and
Hippopotamus. The most important Jurassic genera of _Deinosauria_
are _Megalosaurus_ and _Cetiosaurus_, both of which extend their
range into the Cretaceous period, in which flourished, as we
shall see, some other well-known members of this order.
[Illustration: Fig. 180.--Skull of _Megalosaurus_, on a scale
one-tenth of nature. Restored. (After Professor Phillips.)]
_Megalosaurus_ attained gigantic dimensions, its thigh and shank
bones measuring each about three feet in length, and its total
length, including the tail, being estimated at from forty to
fifty feet. As the head of the thigh-bone is set on nearly at
right angles with the shaft, whilst all the long bones of the
skeleton are hollowed out internally for the reception of the
marrow, there can be no doubt as to the terrestrial habits of
the animal. The skull (fig. 180) was of large size, four or five
feet in length, and the jaws were armed with a series of powerful
pointed teeth. The teeth are conical in shape, but are strongly
compressed towards their summits, their lateral edges being finely
serrated. In their form and their saw-like edges, they resemble
the teeth of the "Sabre-toothed Tiger" (_Machairodus_), and they
render it certain that the Megalosaur was in the highest degree
destructive and carnivorous in its habits. So far as is known, the
skin was not furnished with any armour of scales or bony plates;
and the fore-limbs are so disproportionately small as compared
with the hind-limbs, that this huge Reptile--like the equally
huge Iguanodon--may be conjectured to have commonly supported
itself on its hind-legs only.
The _Cetiosaur_ attained dimensions even greater than those of
the Megalosaur, one of the largest thigh-bones measuring over
five feet in length and a foot in diameter in the middle, and
the total length of the animal being probably not less than fifty
feet. It was originally regarded as a gigantic Crocodile, but
it has been shown to be a true Deinosaur. Having obtained a
magnificent series of remains of this reptile, Professor Phillips
has been able to determine many very interesting points as to
the anatomy and habits of this colossal animal, the total length
of which he estimates as being probably not less than sixty or
seventy feet. As to its mode of life, this accomplished writer
remarks:--
"Probably when 'standing at ease' not less than ten feet in height,
and of a bulk in proportion, this creature was unmatched in magnitude
and physical strength by any of the largest inhabitants of the
Mesozoic land or sea. Did it live in the sea, in fresh waters,
or on the land? This question cannot be answered, as in the case
of Ichthyosaurus, by appeal to the accompanying organic remains;
for some of the bones lie in marine deposits, others in situations
marked by estuarine conditions, and, out of the Oxfordshire district,
in Sussex, in fluviatile accumulations. Was it fitted to live
exclusively in water? Such an idea was at one time entertained,
in consequence of the biconcave character of the caudal vertebræ,
and it is often suggested by the mere magnitude of the creature,
which would seem to have an easier life while floating in water,
than when painfully lifting its huge bulk, and moving with slow
steps along the ground. But neither of these arguments is valid. The
ancient earth was trodden by larger quadrupeds than our elephant;
and the biconcave character of vertebræ, which is not uniform
along the column in Cetiosaurus, is perhaps as much a character
of a geological period as of a mechanical function of life. Good
evidence of continual life in water is yielded in the case of
Ichthyosaurus and other Enaliosaurs, by the articulating surfaces
of their limb-bones, for these, all of them, to the last phalanx,
have that slight and indefinite adjustment of the bones, with much
intervening cartilage, which fits the leg to be both a flexible
and forcible instrument of natation, much superior to the ordinary
oar-blade of the boatman. On the contrary, in Cetiosaur, as well as
in Megalosaur and Iguanodon, all the articulations are definite,
and made so as to correspond to determinate movements in particular
directions, and these are such as to be suited for walking. In
particular, the femur, by its head projecting freely from the
acetabulum, seems to claim a movement of free stepping more parallel
to the line of the body, and more approaching to the vertical than
the sprawling gait of the crocodile. The large claws concur in this
indication of terrestrial habits. But, on the other hand, these
characters are not contrary to the belief that the animal may have
been amphibious; and the great vertical height of the anterior
part of the tail seems to support this explanation, but it does
not go further.... We have therefore a marsh-loving or river-side
animal, dwelling amidst filicine, cycadaceous, and coniferous
shrubs and trees full of insects and small mammalia. What was
its usual diet? If _ex ungue leonem_, surely _ex dente cibum_.
We have indeed but one tooth, and that small and incomplete. It
resembles more the tooth of Iguanodon than that of any other
reptile; for this reason it seems probable that the animal was
nourished by similar vegetable food which abounded in the vicinity,
and was not obliged to contend with Megalosaurus for a scanty
supply of more stimulating diet."
All the groups of Jurassic Reptiles which we have hitherto been
considering are wholly unrepresented at the present day, and
do not even pass upwards into the Tertiary period. It may be
mentioned, however, that the Oolitic deposits have also yielded
the remains of Reptiles belonging to three of the existing orders
of the class-namely, the Lizards (_Lacertilia_), the Turtles
(_Chelonia_), and the Crocodiles (_Crocodilia_). The Lizards
occur both in the marine strata of the Middle Oolites and also
in the fresh-water beds of the Purbeck series; and they are of
such a nature that their affinities with the typical Lacertilians
of the present day cannot be disputed. The Chelonians, up to
this point only known by the doubtful evidence of footprints
in the Permian and Triassic sandstones, are here represented by
unquestionable remains, indicating the existence of marine Turtles
(the _Chelone planiceps_ of the Portland Stone). No remains of
Serpents (_Ophidians_) have as yet been detected in the Jurassic;
but strata of this age have yielded the remains of numerous
_Crocodilians_, which probably inhabited the sea. The most important
member of this group is _Teleosaurus_, which attained a length of
over thirty feet, and is in some respects allied to the living
Gavials of India.
[Illustration: Fig. 181.--_Archoeopteryx macrura_, showing tail
and tail-feathers, with detached bones. Reduced. From the
Lithographic Slate of Solenhofen.]
[Illustration: Fig. 182.--Restoration of _Archoeopteryx macrura_.
(After Owen.)]
The great class of the Birds, as we have seen, is represented
in rocks earlier than the Oolites simply by the not absolutely
certain evidence of the three-toed footprints of the Connecticut
Trias. In the Lithographic Slate of Solenhofen (Middle Oolite),
there has been discovered, however, the at present unique skeleton
of a Bird well known under the name of the _Archoeopteryx macrura_
(figs. 181, 182). The only known specimen--now in the British
Museum--unfortunately does not exhibit the skull; but the
fine-grained matrix has preserved a number of the other bones
of the skeleton, along with the impressions of the tail and wing
feathers. From these remains we know that _Archoeopteryx_ differed
in some remarkable peculiarities of its structure from all existing
members of the class of Birds. This extraordinary Bird (fig.
182) appears to have been about as big as a Rook--the tail being
long and extremely slender, and composed of separate vertebræ,
each of which supports a single pair of quill-feathers. In the
flying Birds of the present day, as before mentioned, the terminal
vertebræ of the tail are amalgamated to form a single bone
("ploughshare-bone"), which supports a cluster of tail-feathers;
and the tail itself is short. In the embryos of existing Birds
the tail is long, and is made up of separate vertebræ, and the
same character is observed in many existing Reptiles. The tail
of _Archoeopteryx_, therefore, is to be regarded as the permanent
retention of an embryonic type of structure, or as an approximation
to the characters of the Reptiles. Another remarkable point in
connection with _Archoeopteryx_, in which it differs from all
known Birds, is, that the wing was furnished with two free claws.
From the presence of feathers, _Archoeopteryx_ may be inferred to
have been hot-blooded; and this character, taken along with the
structure of the skeleton of the wing, may be held as sufficient
to justify its being considered as belonging to the class of
Birds. In the structure of the tail, however, it is singularly
Reptilian; and there is reason to believe that its jaws were
furnished with teeth sunk in distinct sockets, as is the case
in no existing Bird. This conclusion, at any rate, is rendered
highly probable by the recent discovery of "Toothed Birds"
(_Odonturnithes_) in the Cretaceous rocks of North America.
[Illustration: Fig. 183.--Lower jaw of _Amphitherium_
(_Thylacotherium_) _Prevostii_. Stonesfield Slate (Great Oolite.)]
[Illustration: Fig. 184. Oolitic Mammals.--1, Lower jaw and teeth
of _Phascolotherium_, Stonesfield Slate; 2, Lower jaw and teeth
of _Amphitherium_, Stonesfield Slate; 3, Lower jaw and teeth of
_Triconodon_, Purbeck beds; 4, Lower jaw and teeth of _Plagiaulax_,
Purbeck beds. All the figures are of the natural size.]
The _Mammals_ of the Jurassic period are known to us by a number
of small forms which occur in the "Stonesfield Slate" (Great
Oolite) and in the Purbeck beds (Upper Oolite). The remains of
these are almost exclusively separated halves of the lower jaw,
and they indicate the existence during the Oolitic period in
Europe of a number of small "Pouched animals" (_Marsupials_).
In the horizon of the Stonesfield Slate four genera of these
little Quadrupeds have been described--viz., _Amphilestes,
Amphitherium, Phascolotherium_, and _Stereognathus_. In
_Amphitherium_ (fig. 183), the molar teeth are furnished with
small pointed eminences or "cusps;" and the animal was doubtless
insectivorous. By Professor Owen, the highest living authority on
the subject, _Amphitherium_ is believed to be a small Marsupial,
most nearly allied to the living Banded Ant-eater (_Myrmecobius_)
of Australia (fig. 158). _Amphilestes_ and _Phascolotherium_
(fig. 184) are also believed by the same distinguished anatomist
and palæontologist to have been insect-eating Marsupials, and
the latter is supposed to find its nearest living ally in the
Opossums (_Didelphys_) of America. Lastly, the _Stereognathus_ of
the Stonesfield Slate is in a dubious position. It may have been
a Marsupial; but, upon the whole, Professor Owen is inclined to
believe that it must have been a hoofed and herbivorous Quadruped
belonging to the series of the higher Mammals (_Placentalia_).
In the Middle Purbeck beds, near to the close of the Oolitic
period, we have also evidence of the existence of a number of
small Mammals, all of which are probably Marsupials. Fourteen
species are known, all of small size, the largest being no bigger
than a Polecat or Hedgehog. The genera to which these little
quadrupeds have been referred are _Plagiaulax, Spalacotherium,
Triconodon_, and _Galestes_. The first of these (fig. 184, 4)
is believed by Professor Owen to have been carnivorous in its
habits; but other authorities maintain that it was most nearly
allied to the living Kangaroo-rats (_Hypsiprymnus_) of Australia,
and that it was essentially herbivorous. The remaining three
genera appear to have been certainly insectivorous, and find
their nearest living representatives in the Australian Phalangers
and the American Opossums.
Finally, it is interesting to notice in how many respects the
Jurassic fauna of Western Europe approached to that now inhabiting
Australia. At the present day, Australia is almost wholly tenanted
by Marsupials; upon its land-surface flourish _Araucarioe_ and
Cycadaceous plants, and in its seas swims the Port-Jackson Shark
(_Cestracion Philippi_); whilst the Molluscan genus _Trigonia_
is nowadays exclusively confined to the Australian coasts. In
England, at the time of the deposition of the Jurassic rocks,
we must have had a fauna and flora very closely resembling what
we now see in Australia. The small Marsupials, _Amphitherium,
Phascolotherium_, and others, prove that the Mammals were the same
in order; cones of Araucarian pines, with tree-ferns and fronds
of Cycads, occur throughout the Oolitic series; spine-bearing
fishes, like the Port-Jackson Shark, are abundantly represented
by genera such as _Acrodus_ and _Strophodus_; and lastly, the
genus _Trigonia_, now exclusively Australian, is represented
in the Oolites by species which differ little from those now
existing. Moreover, the discovery during recent years of the
singular Mud-fish, the _Ceratodus Fosteri_ in the rivers of
Queensland, has added another and a very striking point of
resemblance to those already mentioned; since this genus of Fishes,
though preeminently Triassic, nevertheless extended its range
into the Jurassic. Upon the whole, therefore, there is reason
to conclude that Australia has undergone since the close of the
Jurassic period fewer changes and vicissitudes than any other
known region of the globe; and that this wonderful continent
has therefore succeeded in preserving a greater number of the
characteristic life-features of the Oolites than any other country
with which we are acquainted.
LITERATURE.
The following list comprises some of the more important sources of
information as to the rocks and fossils of the Jurassic series:--
(1) 'Geology of Oxford and the Thames Valley.' Phillips.
(2) 'Geology of Yorkshire,' vol. ii. Phillips.
(3) 'Memoirs of the Geological Survey of Great Britain.'
(4) 'Geology of Cheltenham.' Murchison, 2d ed. Buckman.
(5) 'Introduction to the Monograph of the Oolitic Asteriadæ'
(Palæontographical Society). Wright.
(6) "Zone of Avicula contorta and the Lower Lias of the South of
England"--'Quart. Journ. Geol. Soc.,' vol. xvi., 1860. Wright.
(7) "Oolites of Northamptonshire"--'Quart. Journ. Geol. Soc.,'
vols. Xxvi. and xxix. Sharp.
(8) 'Manual of Geology.' Dana.
(9) 'Der Jura.' Quenstedt.
(10) 'Das Flötzgebirge Württembergs.' Quenstedt.
(11) 'Jura Formation.' Oppel.
(12) 'Paléontologie du Département de la Moselle.' Terquem.
(13) 'Cours élémentaire de Paléontologie.' D'Orbigny.
(14) 'Paléontologie Française.' D'Orbigny.
(15) 'Fossil Echinodermata of the Oolitic Formation'
(Palæontographical Society). Wright.
(16) 'Brachiopoda of the Oolitic Formation' (Palæontographical
Society). Davidson.
(17) 'Mollusca of the Great Oolite' (Palæontographical Society).
Morris and Lycett.
(18) 'Monograph of the Fossil Trigoniæ' (Palæontographical Society).
Lycett.
(19) 'Corals of the Oolitic Formation' (Palæontographical Society).
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