It remains now to consider briefly the leading characters of six wholly extinct orders of Reptiles, the peculiarities of which are very extraordinary, and are such as are exhibited by no living forms.
The body was fish-like, without any distinct neck, and probably covered with a smooth or wrinkled skin, no horny or bony exoskeleton having been ever discovered. The vertebra were numerous, deeply biconcave or amphicoelous, and having the neural arches united to the centra by a distinct suture. The anterior trunk-ribs possess bifurcate heads. There is no sacrum, and no sternal ribs or sternum, but clavicles were present as well as an interclavicle (epistemum); and false ribs were developed in the walls of the abdomen. The skull had enormous orbits separated by a septum, and an elongated snout. The eyeball was protected by a ring of bony plates in the sclerotic (fig. 313). The teeth were not lodged in distinct sockets, but in a common alveolar groove. The fore and hind limbs were converted into swimming-paddles, the ordinary number of digits (five) remaining recognisable, but the phalanges being greatly increased in number, and marginal ossicles being added as well. A vertical caudal fin was in all probability present.
The order Ichthyopterygia includes only, or principally, the gigantic and fish-like Ichthyosauri (fig. 311), all exclusively
Fig. 311. - Ichthyosaurus communis.
Mesozoic, and abounding in the Lias, Oolites, and Chalk, but especially characteristic of the Lias. If, however, the Eosaurus Acadiensis (Marsh) of the Coal-measures of Nova Scotia be rightly referred to this order, then the Ichthyopterygia date from the Carboniferous period. Moreover, Prof. Marsh has recently described, from the Jurassic deposits of North America, a Reptile, which he has named Sauranodon, and which agrees in all important respects with Ichthyosaurus, except that it has no teeth. Prof. Marsh regards it as the type of a new order, but we may in the meanwhile include it here. There is no doubt whatever but that the Ichthyosauri were essentially marine animals, and they have been often included with the next order (Sauropterygia) in a common group, under the name of Enaliosauria or Sea-lizards.
Fig. 312. - A, Pectoral arch and fore-limbs of Ichthyosaurus: a Interclavicle ; b b Clavicles; c c Scapulae; d d Coracoids; e Humerus; f Radius; g Ulna. (Somewhat altered from Huxley.) B, Pelvis of Ichthyosaurus: p Pubis; il Ilium; is Ischium.
In the biconcave vertebrae and probable presence of a vertical tail - fin, the Ichthyosaurus approaches the true fishes. There is, however, no doubt as to the fact that the animal was strictly an air-breather, and its reptilian characters cannot be questioned, at the same time that the conformation of the limbs is decidedly Cetacean in many respects. Much has been gathered from various sources as to the habits of the Ichthyosaurus, and its history is one of great interest. From the researches of Buckland, Conybeare, and Owen, the following facts appear to be pretty well established: That the Ichthyosauri kept chiefly to open waters may be inferred from their strong and well-developed swimming apparatus. That they occasionally had recourse to the shore, and crawled upon the beach, may be safely inferred from the presence of a strong and well-developed bony arch, supporting the fore-limbs, and somewhat resembling in structure the scapular arch of the Or-nithorhynchus or Duck-mole of Australia. That they lived in stormy seas, or were in the habit of diving to considerable depths, is shown by the presence of a ring of bony plates in the sclerotic, protecting the eye from injury or pressure (fig. 313). That they possessed extraordinary powers of vision, especially in the dusk, is certain from the size of the pupil, and from the enormous width of the orbits. That they were carnivorous and predatory in the highest degree is shown by the wide mouth, the long jaws, and the numerous, powerful, and pointed teeth. This is proved, also, by an examination of their petrified droppings, which are known to geologists as "coprolites," and which contain numerous fragments of the scales and bones of the Ganoid fishes which inhabited the same seas.
Fig. 313. - Skull of Ichtkyosaitrus, showing the sclerotic plates.
Order VI. Sauropterygia, Owen ( = Plesiosauria, Huxley). This order of extinct reptiles, of which the well-known Plesiosaurus may be taken as the type, is characterised by the following peculiarities:
The body, as far as is known, was naked, and not furnished with any horny or bony exoskeleton. The bodies of the vertebrae were either flat or only slightly cupped at each end, and the neural arches were anchylosed with the centra, and did not remain distinct during life. The transverse processes of the vertebrae were long, and the anterior trunk-ribs had simple, not bifurcate heads. No sternum or sternal ribs are known to have existed, but there were false abdominal ribs. The neck (fig. 314) in most was greatly elongated, and composed of numerous vertebrae. The sacrum was composed of two vertebrae. The orbits were of large size, and there was a long snout, as in the Ichthyosauri, but there was no circle of bony plates in the sclerotic. The limbs agree with those of the Ichthyosauri in being in the form of swimming-paddles (fig. 315), but differ in not possessing any supernumerary marginal ossicles. A pectoral arch, formed of two clavicles and an interclavicle (epi-sternum), appears to have been sometimes, if not always, present. The teeth were simple, and were inserted into distinct sockets, and not lodged in a common groove.
Fig. 314. - Plesiosaurus dolichodeirus.
The most familiar and typical member of the Sauropterygia is the Plesiosaurus (fig. 314), a gigantic marine reptile, chiefly-characteristic of the Lias and Oolites. As regards the habits of the Plesiosaurus, 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 striking 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."
The geological range of the Plesiosaurus is from the Lias to the Chalk inclusive, and specimens have been found indicating a length of from eighteen to twenty feet.
Of the other genera of the Sauropterygia, Simosaurus and Nothosaurus are from the Trias, and are chiefly characteristic of its middle division, the Muschelkalk. Placodus is another genus, also from the Muschelkalk, and is characterised by the extraordinary form of the teeth, which resembled those of many fishes in forming broad crushing plates, constituting a kind of pavement.
Fig. 315. - Left fore-paddle of Plesiosaurus. a Humerus; b Radius; c Ulna.
Order VII. Anomodontia, Owen (= Dicynodontia, Huxley). The leading characters of this order are to be found in the structure of the jaws, which appear to have been sheathed in horn so as to constitute a kind of beak, very like that of the Chelonians. In the genus Oudenodon (fig. 316), both jaws seem to have been altogether destitute of teeth; but in Dicy-nodon (fig. 316) there were two long tusks, growing from persistent pulps, placed one on each side in the upper jaw. The pectoral and pelvic arches were very strong, and the limbs were well developed and fitted for walking, and not for swimming.
Fig. 316. - A, Skull of Dicynodon lacerticeps, showing the maxillary tusk. B, Skull of Oudenodon Bainii. From the Trias of South Africa. (After Owen.)
Dicynodon and Oudenodon are known only from strata of supposed Triassic age in South Africa and India, but Rhyncho-saurus occurs in the Trias of Europe. This last genus, however, is placed by Huxley amongst the Lacertilia.