With regard to the life of the Triassic period, we have to notice a difference as concerns the different members of the group similar to that which has been already mentioned in connection with the Permian formation. The arenaceous deposits of the series, namely, resemble those of the Permian, not only in being commonly red or variegated in their colour, but also in their conspicuous paucity of organic remains. They for the most part are either wholly unfossiliferous, or they contain the remains of plants or the bones of reptiles, such as may easily have been drifted from some neighbouring shore. The few fossils which may be considered as properly belonging to these deposits are chiefly Crustaceans (Estheria) or Fishes, which may well have lived in the waters of estuaries or vast inland seas. We may therefore conclude, with considerable probability, that the barren sandy and marly accumulations of the Bunter Sandstein and Lower Keuper were not laid down in an open sea, but are probably brackish-water deposits, formed in estuaries or land-locked bodies of salt water. This at any rate would appear to be the case as regards these members of the series as developed in Britain and in their typical areas on the continent of Europe; and the origin of most of the North American Trias would appear to be much the same. Whether this view be correct or not, it is certain that the beds in question were laid down in shallow water, and in the immediate vicinity of land, as shown by the numerous drifted plants which they contain and the common occurrence in them of the footprints of air-breathing animals (Birds, Reptiles, and Amphibians). On the other hand, the middle and highest members of the Trias are largely calcareous, and are replete with the remains of undoubted marine animals. There cannot, therefore, be the smallest doubt but that the Muschelkalk and the Rhætic or Kössen beds were slowly accumulated in an open sea, of at least a moderate depth; and they have preserved for us a very considerable selection from the marine fauna of the Triassic period.

The plants of the Trias are, on the whole, as distinctively Mesozoic in their aspect as those of the Permian are Palæozoic. In spite, therefore, of the great difficulty which is experienced in effecting a satisfactory stratigraphical separation between the Permian and the Trias, we have in this fact a proof that the two formations were divided by an interval of time sufficient to allow of enormous changes in the terrestrial vegetation of the world. The Lepidodendroids, Asterophyllites, and Annulariœ, of the Coal and Permian formations, have now apparently wholly disappeared: and the Triassic flora consists mainly of Ferns, Cycads, and Conifers, of which only the two last need special notice. The Cycads (fig. 140) are true exogenous plants, which in general form and habit of growth present considerable Fig. 140
Fig. 140.—Zamia spiralis, a living Cycad. Australia. resemblance to young Palms, but which in reality are most nearly related to the Pines and Firs (Coniferœ). The trunk is unbranched, often much shortened, and bears a crown of feathery pinnate fronds. The leaves are usually "circinate"—they unroll in expanding, like the fronds of ferns. The seeds are not protected by a seed-vessel, but are borne upon the edge of altered leaves, or are carried on the scales of a cone. All the living species of Cycads are natives of warm countries, such as South America, the West Indies, Japan, Australia, Southern Asia, and South Africa. The remains of Cycads, as we have seen, are not known to occur in the Coal formation, or only to a very limited extent towards its close; nor are they known with certainty as occurring in Permian deposits. In the Triassic period, however, the remains of Cycads belonging to such genera as Pterophyllum (fig. 141, b), Zamites, and Podozamites (fig. 141, c), are sufficiently abundant to constitute quite a marked feature in the vegetation; and they continue to be abundantly represented throughout the whole Mesozoic series. The name "Age of Cycads," as applied to the Secondary epoch, is therefore, from a botanical point of view, an extremely appropriate one. The Conifers of the Trias are not uncommon, the principal form being Veltzia (fig. 141, a), which possesses some peculiar characters, but would appear to be most nearly related to the recent Cypresses.

As regards the Invertebrate animals of the Trias, our knowledge is still principally derived from the calcareous beds which constitute the centre of the system (the Muschelkalk) on the continent of Europe, and from the St Cassain and Rhætic beds still higher in the series; whilst some of the Triassic strata Fig. 141
Fig. 141.—Triassic Conifers and Cycads. a, Voltzia (Schizoneura) heterophylla, portion of a branch, Europe and America; b, Part of the frond of Pterophyllum Jœgeri, Europe; c, Part of the frond of Podozamites lanceolatus, America. of California and Nevada have likewise yielded numerous remains of marine Invertebrates. The Protozoans are represented by Foraminifera and Sponges, and the Cœlenterates by a small number of Corals; but these require no special notice. It may be mentioned, however, that the great Palæozoic group of the Rugose corals has no known representative here, its place being taken by corals of Secondary type (such as Montlivaltia, Synastœa, &c.)

The Echinoderms are represented principally by Crinoids, the remains of which are extremely abundant in some of the limestones. The best-known species is the famous "Lily-Encrinite" (Encrinus liliiformis, fig. 142), which is characteristic of the Muschelkalk. In this beautiful species, the flower-like head is supported upon a rounded stem, the joints Fig. 142
Fig. 142.—Head and upper part of the column of Encrinus liliiformis. The lower figure shows the articulating surface of one of the joints of the column. Muschelkalk, Germany. of which are elaborately articulated with one another; and the fringed arms are composed each of a double series of alternating calcareous pieces. The Palæozoic Urchins, with their supernumerary rows of plates, the Cystideans, and the Pentremites have finally disappeared; but both Star-fishes and Brittle-stars continue to be represented. One of the latter—namely, the Fig. 143
Fig. 143.—Aspidura loricata, a Triassic Ophiuroid. Muschelkalk, Germany. Aspidura loricata of Goldfuss (fig. 143)—is highly characteristic of the Muschelkalk.

The remains of Articulate Animals are not very abundant in the Trias, if we except the bivalved cases of the little Water-fleas (Ostracoda), which are occasionally very plentiful. There are also many species of the horny, concentrically-striated valves of the Estheriœ (see fig. 122, b), which might easily be taken for small Bivalve Molluscs. The "Long-tailed" Decapods of the type of the Lobster, are not without examples but they become much more numerous in the succeeding Jurassic period. Remains of insects have also been discovered.

Amongst the Mollusca we have to note the disappearance, amongst the lower groups, of many characteristic Palæozoic types. Amongst the Polyzoans, the characteristic "Lace-corals," Fenestella, Retepora,[22] Synocladia, Polypora, &c., have become apparently extinct. The same is true of many of the ancient types of Brachiopods, and conspicuously so of the great family of the Productidœ, which played such an important part in the seas of the Carboniferous and Permian periods.

[Footnote 22: The genus Retefora is really a recent one, represented by living forms; and the so-called Reteporœ of the Palæozoic rocks should properly receive another name (Phyllopora), as being of a different nature. The name Retepora has been here retained for these old forms simply in accordance with general usage.]

Bivalves (Lamellibranchiata) and Univalves (Gasteropoda) are well represented in the marine beds of the Trias, and some of the former are particularly characteristic either of the formation as a whole or of minor subdivisions of it. A few of these characteristic species are figured in the accompanying illustration (fig. 144). Bivalve shells of the genera Daonella (fig. 144, a) and Halobia (Monotis) are very Fig. 144
Fig. 144. Triassic Lamellibranchs. a, Daonella (Halobia) Lommelli; b, Pecten Valoniensis; c, Myophoria lineata; d. Cardium Rhœticum; e. Avicula contorta; f. Avicula socialis. abundant, and are found in the Triassic strata of almost all regions. These groups belong to the family of the Pearl-oysters (Aviculidœ), and are singular from the striking resemblance borne by some of their included forms to the Strophomenœ amongst the Lamp-shells, though, of course, no real relation exists between the two. The little Pearl-oyster, Avicula socialis (fig. 144, f), is found throughout the greater part of the Triassic series, and is especially abundant in the Muschelkalk. The genus Myophoria (fig. 144, c), belonging to the Trigoniadœ, and related therefore to the Permian Schizodus, is characteristically Triassic, many species of the genus being known in deposits of this age. Lastly, the so-called "Rhætic" or "Kössen" beds are characterised by the occurrence in them of the Scallop, Pecten Valoniensis (fig. 144, b); the small Cockle, Cardium Rhœticum (fig. 144, d); and the curiously-twisted Pearl-oyster, Avicula contorta (fig. 144, e)—this last Bivalve being so abundant that the strata in question are often spoken of as the "Avicula contorta beds."

Passing over the groups of the Heteropods and Pteropods, we have to notice the Cephalopoda, which are represented in the Trias not only by the chambered shells of Tetrabranchiates, but also, for the first time, by the internal skeletons of Dibranchiate forms. The Trias, therefore, marks the first recognised appearance of true Cuttle-fishes. All the known examples of these belong to the great Mesozoic group of the Belemnitidœ; and as this family is much more largely developed in the succeeding Jurassic period, the consideration of its characters will be deferred till that formation is treated of. Amongst the chambered Cephalopods we find quite a number of the Palæozoic Orthoceratites, some of them of considerable size, along with the ancient Cyrtoceras and Goniatites; and these old types, singularly enough, occur in the higher portion of the Trias (St Cassian beds), but have, for some unexplained reason, not yet been recognised in the lower and equally fossiliferous formation of the Muschelkalk. Along with these we meet for the first time with true Ammonites, which fill such an extensive place Fig. 145
Fig. 145.—Ceratites nodosus, viewed from the side and from behind. Muschelkalk. in the Jurassic seas, and which will be spoken of hereafter. The form, however, which is most characteristic of the Trias is Ceratites (fig. 145). In this genus the shell is curved into a flat spiral, the volutions of which are in contact; and it further agrees with both Goniatites and Ammonites in the fact that the septa or partitions between the air-chambers are not simple and plain (as in the Nautilus and its allies), but are folded and bent as they approach the outer wall of the shell. In the Goniatite these foldings of the septa are of a simply lobed or angulated nature, and in the Ammonite they are extremely complex; whilst in the Ceratite there is an intermediate state of things, the special feature of which is, that those foldings which are turned towards the mouth of the shell are merely rounded, whereas those which are turned away from the mouth are characteristically toothed. The genus Ceratites, though principally Triassic, has recently been recognised in strata of Carboniferous age in India.

From the foregoing it will be gathered that one of the most important points in connection with the Triassic Mollusca is the remarkable intermixture of Palæozoic and Mesozoic types which they exhibit. It is to be remembered, also, that this intermixture has hitherto been recognised, not in the Middle Triassic limestones of the Muschelkalk, in which—as the oldest Triassic beds with marine fossils—we should naturally expect to find it, but in the St Cassian beds, the age of which is considerably later than that of the Muschelkalk. The intermingling of old and new types of Shell-fish in the Upper Trias is well brought out in the annexed table, given by Sir Charles Lyell in his 'Student's Elements of Geology' (some of the less important forms in the table being omitted here):—

GENERA OF FOSSIL MOLLUSCA IN THE ST CASSIAN AND HALLSTADT BEDS.

Thus, to emphasise the more important points alone, the Trias has yielded, amongst the Gasteropods, the characteristically Palæozoic Loxonema, Holopella, Murchisonia, Euomphalus, and Porcellia, along with typically Triassic forms like Platystoma and Scoliostoma, and the great modern groups Chemnitzia and Cerithium. Amongst the Bivalves we find the Palæozoic Megalodon side by side with the Triassic Halobia and Myophoria, these being associated with the Carditœ, Hinnites, Plicatulœ, and Trigoniœ of later deposits. The Brachiopods exhibit the Palæozoic Athyris, Retzia, and Cyrtina, with the Triassic Koninckia and the modern Thecidium. Finally, it is here that the ancient genera Orthoceras, Cyrtoceras, and Goniatites make their last appearance upon the scene of life, the place of the last of these being taken by the more complex and almost exclusively Triassic Ceratites, whilst the still more complex genus Ammonites first appears here in force, and is never again wanting till we reach the close of the Mesozoic period. The first representatives of the great Secondary family of the Belemnites are also recorded from this horizon.

Amongst the Vertebrate Animals of the Trias, the Fishes are represented by numerous forms belonging to the Ganoids and the Placoids. The Ganoids of the period are still all provided with unsymmetrical ("heterocercal") tails, and belong principally to such genera as Palœoniscus and Catopterus. The remains of Placoids are in the form of teeth and spines, the two principal genera being the two important Secondary groups Acrodus and Hybodus. Very nearly at the summit of the Trias in England, in the Rhætic series, is a singular stratum, which is well known as the "bone-bed," from the number of fish-remains which it contains. More interesting, however, than the above, are the curious palate-teeth of the Trias, upon which Agassiz founded the genus Ceratodus. The teeth of Ceratodus (fig. 146) are singular flattened plates, composed Fig. 146
Fig. 146.—a, Dental plate of Ceratodus serratus, Keuper; b, Dental plate of Ceratodus altus, Keuper; (After Agassiz.) of spongy bone beneath, covered superficially with a layer of enamel. Each plate is approximately triangular, one margin (which we now know to be the outer one) being prolonged into prongs or conical prominences, whilst the surface is more or less regularly undulated. Until recently, though the master-mind of Agassiz recognised that these singular bodies were undoubtedly the teeth of fishes, we were entirely ignorant as to their precise relation to the animal, or as to the exact affinities of the fish thus armed. Lately, however, there has been discovered in the rivers of Queensland (Australia) a living species of Ceratodus (C. Fosteri, fig. 147), with teeth precisely similar to those of its Triassic predecessor; and we thus have become acquainted with the use of these structures and the manner in which they Fig. 147
Fig. 147.—Ceratodus Fosteri, the Australian Mud-fish, reduced in size. were implanted in the mouth. The palate carries two of these plates, with their longer straight sides turned towards each other, their sharply-sinuated sides turned outwards, and their short straight sides or bases directed backwards. Two similar plates in the lower jaw correspond to the upper, their undulated surfaces fitting exactly to those of the opposite teeth. There are also two sharp-edged front teeth, which are placed in the front of the mouth in the upper jaw; but these have not been recognised in the fossil specimens. The living Ceratodus feeds on vegetable matters, which are taken up or tom off from plants by the sharp front teeth, and then partially crushed between the undulated surfaces of the back teeth (Günther); and there need be little doubt but that the Triassic Ceratodi followed a similar mode of existence. From the study of the living Ceratodus, it is certain that the genus belongs to the same group as the existing Mud-fishes (Dipnoi); and we therefore learn that this, the highest, group of the entire class of Fishes existed in Triassic times under forms little or not at all different from species now alive; whilst it has become probable that the order can be traced back into the Devonian period.

The Amphibians of the Trias all belong to the old order of the Labyrinthodonts, and some of them are remarkable for their gigantic dimensions. They were first known by their footprints, which were found to occur plentifully in the Triassic sandstones of Britain and the continent of Europe, and which consisted of a double series of alternately-placed pairs of hand-shaped impressions, the hinder print of each pair being much larger than the one in front (fig. 148). So like were these impressions to the shape of the human hand, that the at that time unknown animal which produced them was at once christened Cheirotherium, or "Hand-beast." Further discoveries, however, soon showed that the footprints of Cheirotherium were really produced by species of Amphibians which, like the existing Frogs, possessed hind-feet of a much larger size than the fore-feet, Fig. 148a Fig. 148b
Fig. 148.—Footprints of a Labyrinthodont (Cheirotherium), from the Triassic Sandstones of Hessberg, near Hildburghausen, Germany, reduced one-eighth. The lower figure shows a slab, with several prints, and traversed by reticulated sun-cracks: the upper figure shows the impression of one of the hind-feet, one-half of the natural size. (After Sickler.) and to which the name of Labyrinthodonts was applied in consequence of the complex microscopic structure of the teeth (fig. 149). In the essential details of their structure, the Triassic Labyrinthodonts did not differ materially from their predecessors in the Coal-measures and Permian rocks. They possessed the same frog-like skulls (fig. 150), with a lizard-like body, a long tail, and comparatively feeble limbs. The hind-limbs were stronger and longer than the fore-limbs, and the lower surface of the body was protected by an armour of bony plates. Some of the Triassic Labyrinthodonts must have attained dimensions utterly unapproached amongst existing Amphibians, the skull of Labyrinthodon Jœgeri (fig. 150) being upwards Fig. 149
Fig. 149.—Section of the tooth of Labryinthodon (Mastodonsaurus) Jœgeri, showing the microscopic structure. Greatly enlarged. Trias. Fig. 150
Fig. 150.—a, Skull of Labyrinthodon Jœgeri, much reduced in size; b, Tooth of the same. Trias Württemberg. of three feet in length and two feet in breadth. Restorations of some of these extraordinary creatures have been attempted in the guise of colossal Frogs; but they must in reality have more closely resembled huge Newts.

Remains of Reptiles are very abundant in Triassic deposits, and belong to very varied types. The most marked feature, in fact, connected with the Vertebrate fauna of the Trias, and of the Secondary rocks in general, is the great abundance of Reptilian life. Hence the Secondary period is often spoken of as the "Age of Reptiles." Many of the Triassic reptiles depart widely in their structure from any with which we are acquainted as existing on the earth at the present day, and it is only possible here to briefly note some of the more important of these ancient forms. Amongst the group of the Lizards (Lacertilia), represented by Protorosaurus in the older Permian strata, three types more or less certainly referable to this order may be mentioned. One of these is a small reptile which was found many years ago in sandstones near Elgin, in Scotland, and which excited special interest at the time in consequence of the fact that the strata in question were believed to belong to the Old Red Sandstone formation. It is, however, now certain that the Elgin sandstones which contain Telerpeton Elginense, as this reptile is termed, are really to be regarded as of Triassic age. By Professor Huxley, Telerpeton is regarded as a Lizard, which cannot be considered as "in any sense a less perfectly-organised creature than the Gecko, whose swift and noiseless run over walls and ceilings surprises the traveller in climates warmer than our own." The "Elgin Sandstones" have also yielded another Lizard, which was originally described by Professor Huxley under the name of Hyperodapedon, the remains of the same genus having been subsequently discovered in Triassic strata in India and South Africa. The Lizards of this group must therefore have at one time enjoyed a very wide distribution over the globe; and the living Sphenodon of New Zealand is believed by Professor Huxley to be the nearest living ally of this family. The Hyperodapedon of the Elgin Sandstones was about six feet in length, with limbs adapted for terrestrial progression, but with the bodies of the vertebræ slightly biconcave, and having two rows of palatal teeth, which become worn down to the bone in old age. Lastly, the curious Rhynchosaurus of the Trias is also referred, by the eminent comparative anatomist above mentioned, to the order of the Lizards. In this singular reptile Fig. 151
Fig. 151.—Skull of Rhynchosaurus articeps. Trias. (After Owen.) (fig. 151) the skull is somewhat bird-like, and the jaws appear to have been destitute of teeth, and to have been encased in a horny sheath like the beak of a Turtle or a Bird. It is possible, however, that the palate was furnished with teeth.

The group of the Crocodiles and Alligators (Crocadilia), distinguished by the fact that the teeth are implanted in distinct sockets and the skin more or less extensively provided with bony plates, is represented in the Triassic rocks by the Stagonolepis of the Elgin Sandstones. The so-called "Thecodont" reptiles (such as Belodon, Thecodontosaurus, and Palœosaurus, fig. 152, c, d, e) are also nearly related to the Crocodiles, though it is doubtful if they should be absolutely referred to this group. In these reptiles, the teeth are implanted in distinct sockets in the jaws, their crowns being more or less compressed and pointed, "with trenchant and finely serrate margins" (Owen). The bodies of the vertebræ are hollowed out at both ends, but the limbs appear to be adapted for progression on the land. The genus Belodon (fig. 152, c) is known to occur in the Keuper of Germany and in America; and Palœosaurus (fig. 153. e) has also been found in the Trias of the same region. Teeth of the latter, however, are found, along with remains of Thecodontosaurus (fig. 153, d), in a singular magnesian conglomerate near Bristol, which was originally believed to be of Permian age, but which appears to be undoubtedly Triassic.

The Trias has also yielded the remains of the great marine reptiles which are often spoken of collectively as the "Enaliosaurians" or "Sea-lizards," and which will be more particularly spoken of in treating of the Jurassic period, of which they are more especially characteristic. In all these reptiles the limbs are flattened out, the digits being enclosed in a continuous skin, thus forming powerful swimming-paddles, resembling the "flippers" of the Whales and Dolphins both in their general structure and in function. The tail is also long, and adapted to act as a swimming-organ; and there can be no doubt but that these extraordinary and often colossal reptiles frequented the sea, and only occasionally came to the land. The Triassic Enaliosaurs belong to a group of which the later genus Plesiosaurus is the type (the Sauropterygia). One of the best known of the Triassic genera is Nothosaurus (fig. 152, a), in which the neck was long and bird-like, the jaws being immensely elongated, and carrying numerous powerful conical teeth implanted in distinct sockets. The teeth in Simosaurus (152, b) are of a similar nature; but the orbits are of enormous size, indicating eyes of corresponding dimensions, and perhaps pointing to the nocturnal habits of the animal. In the singular Placodus, again, the teeth are in distinct sockets, but resemble those of many fishes in being rounded and obtuse (fig. 153), forming Fig. 153
Fig. 153.—Under surface of the upper jaw and palate of Placodus gigas. Muschelkalk, Germany. broad crushing plates adapted for the comminution of shell-fish. There is a row of these teeth all round the upper jaw proper, and a double series on the palate, but the lower jaw has only a single row of teeth. Placodus is found in the Muschelkalk, and the characters of its dental apparatus indicate that it was much more peaceful in its habits than its associates the Nothosaur and Simosaur.

The Triassic rocks of South Africa and India have yielded the remains of some extraordinary Reptiles, which have been placed by Professor Owen in a separate order under the name of Anomodontia. The two principal genera of this group are Dicynodon and Oudenodon, both of which appear to have been large Reptiles, with well-developed limbs, organised for progression upon the dry land. In Oudenodon (fig. 154, B) the jaws seem to have been wholly destitute of teeth, and must have been encased in a horny sheath, similar to that with which we are familiar in the beak of a Turtle. In Dicynodon (fig. 154, A), on the other hand, the front of the upper jaw and the whole of the lower jaw were destitute of teeth, and the front of the mouth must have constituted a kind of beak; but the upper jaw possessed on each side a single huge conical tusk, which is directed downwards, and must have continued to grow during the life of the animal.

It may be mentioned that the above-mentioned Triassic sandstones of South Africa have recently yielded to the researches of Professor Owen a new and unexpected type of Reptile, which exhibits some of the structural peculiarities which we have been accustomed to regard as characteristic of the Carnivorous quadrupeds. The Reptile in question has been named Cyanodraco, and it is looked upon by its distinguished discoverer as the type of a new order, to which he has given the name of Theriodontia. The teeth of this singular form agree with those of the Carnivorous quadrupeds in consisting of three distinct groups—namely, front teeth or incisors, eye teeth or canines, and back teeth or molars. The canines also are long and pointed, very much compressed, and having their lateral margins finely serrated, thus presenting a singular resemblance to the teeth Fig. 154
Fig. 154.—Triassic Anomodont Reptiles. A, Skull of Dicynodon lacerticeps, showing one of the great maxillary tusks; B, Skull of Oudenodon Bainii, showing the toothless, beak-like jaws. From the Trias of South Africa. (After Owen.) of the extinct "Sabre-toothed Tiger" (Machairodus). The bone of the upper arm (humerus) further shows some remarkable resemblances to the same bone in the Carnivorous Mammals. As has been previously noticed, Professor Owen is of opinion that some of the Reptilian remains of the Permian deposits will also be found to belong to this group of the "Theriodonts."

Lastly, we find in the Triassic rocks the remains of Reptiles belonging to the great Mesozoic order of the Deinosauria. This order attains its maximum at a later period, and will be spoken of when the Jurassic and Cretaceous deposits come to be considered. The chief interest of the Triassic Reptiles of this group arises from the fact that they are known by their footprints as well as by their bones; and a question has arisen whether the supposed footprints of birds which occur in the Trias have not really been produced by Deinosaurs. This leads us, therefore, to speak at the same time as to the evidence which we have of the existence of the class of Birds during the Triassic period. No actual bones of any bird have as yet been detected in any Triassic deposit; but we have tolerably clear evidence of their existence at this time in the form of footprints. The impressions in question are found in considerable numbers in certain red sandstones of the age of the Trias in the valley of the Connecticut River, in the United States. They vary much in size, and have evidently been produced by many different animals walking over long stretches of estuarine mud and sand exposed at low water. The footprints now under consideration form a double series of single prints, and therefore, beyond all question, are the tracks of a biped—that is, of an animal which walked upon two legs. No living animals, save Man and the Birds, walk habitually on two legs; and there is, therefore, a primâ facie presumption that the authors of these prints were Birds. Moreover, each impression consists of the marks of three toes turned forwards (fig. 155), and therefore are precisely such as might be Fig. 155
Fig. 155.—Supposed footprint of a Bird, from the Triassic Sandstones of the Connecticut River. The slab shows also numerous "rain-prints." produced by Wading or Cursorial Birds. Further, the impressions of the toes show exactly the same numerical progression in the number of the joints as is observable in living Birds—that is to say, the innermost of the three toes consists of three joints, the middle one of four, and the outer one of five joints. Taking this evidence collectively, it would have seemed, until lately, quite certain that these tracks could only have been formed by Birds. It has, however, been shown that the Deinosaurian Reptiles possess, in some cases at any rate, some singularly bird-like characters, amongst which is the fact that the animal possessed the power of walking, temporarily at least, on its hind-legs, which were much longer and stronger than the fore-limbs, and which were sometimes furnished with no more than three toes. As the bones and teeth of Deinosaurs have been found in the Triassic deposits of North America, it may be regarded as certain that some of the bipedal tracks originally ascribed to Birds must have really been produced by these Reptiles. It seems at the same time almost a certainty that others of the three-toed impressions of the Connecticut sandstones were in truth produced by Birds, since it is doubtful if the bipedal mode of progression was more than an occasional thing amongst the Deinosaurs, and the greater number of the many known tracks exhibit no impressions of fore-feet. Upon the whole, therefore, we may, with much probability, conclude that the great class of Birds (Aves) was in existence in the Triassic period. If this be so, not only must there have been quite a number of different forms, but some of them must have been of very large size. Thus the largest footprints hitherto discovered in the Connecticut sandstones are 22 inches long and 12 inches wide, with a proportionate length of stride. These measurements indicate a foot four times as large as that of the African Ostrich; and the animal which produced them—whether a Bird or a Deinosaur—must have been of colossal dimensions.

Finally, the Trias completes the tale of the great classes of the Vertebrate sub-kingdom by presenting us with remains of the first known of the true Quadrupeds or Mammalia. These are at present only known by their teeth, or, in one instance, by one of the halves of the lower jaw; and these indicate minute Quadrupeds, which present greater affinities with the little Banded Anteater (Myrmecobius fasciatus, fig. 158) of Australia than with any other living form. If this conjecture be correct, Fig. 156
Fig. 156.—Lower jaw of Dromatherium sylvestre. Trias, North Carolina. (After Emmons.) Fig. 157
Fig. 157.—a, Molar tooth of Micro estes antiquus, magnified; b, Crown of the same, magnified still further. Trias, Germany. these ancient Mammals belonged to the order of the Marsupials or Pouched Quadrupeds (Marsupialia), which are now exclusively confined to the Australian province, South America, and the southern Fig. 158
Fig. 158.—The Banded Ant-eater (Myrmecobius fasciatus) of Australia. portion of North America. In the Old World, the only known Triassic Mammals belong to the genus Microlestes, and to the probably identical Hypsiprymnopsis of Professor Boyd Dawkins. The teeth of Microlestes (fig. 157) were originally discovered by Plieninger in 1847 in the "bone-bed" which is characteristic of the summit of the Rhætic series both in Britain and on the continent of Europe; and the known remains indicate two species. In Britain, teeth of Microlestes have been discovered by Mr Charles Moore in deposits of Upper Triassic age, filling a fissure in the Carboniferous limestone near Frome, in Somersetshire; and a molar tooth of Hypsiprymnopsis was found by Professor Boyd Dawkins in Rhætic marls below the "bone-bed" at Watchet, also in Somersetshire. In North America, lastly, there has been found in strata of Triassic age one of the branches of the lower jaw of a small Mammal, which has been described under the name of Dromatherium sylvestre (fig. 156). The fossil exhibits ten small molars placed side by side, one canine, and three incisors, separated by small intervals, and it indicates a small insectivorous animal, probably most nearly related to the existing Myrmecobius.

LITERATURE.

The following list comprises a few of the more important sources of information as to the Triassic strata and their fossil contents:—

CHAPTER XVI.

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.