Oudenodon, of which several species have been described, is so much like Dicynodon, except for the complete absence of teeth, that it has been suggested that these skulls belong to females of this genus. This view is strengthened by the fact that tusk-like canines exist, or are absent in some of the species which have been described as Cistecephalus, a genus closely allied to Dicynodon. The latter, which, like Oudenodon and Cistecephalus, occurred in Africa, extended also into India, D. orientalis having been found in the Panchet formation of Bengal, of transitional age between the Permian and Triassic epochs. Oudenodon rugosus, on the other hand, has been described from the Ural.
Gordonia and Geikia, of the New Red Sandstone of Elgin, are known from their skulls only, but these are so well preserved that there is no doubt about their close relationship to the typical South African Dicynodontia. The skull of Gordonia is about 7 inches long and 4 inches high. The canines (Fig. 54, D, p. [280]) are reduced to short, but thick, conical tusks. The most remarkable feature is the very elongated squamoso-jugal arch, which arises moreover from the dorsal end of the long squamoso-quadrate pedicle. The two wide and long temporal fossae are dorsally divided by narrow parietal crests. There is a distinct interparietal bone, and the usual interparietal foramen. The choanae are united and lie within the palatines, which themselves are united; the large lateral palatal foramina are otherwise enclosed by the pterygoids, quadrates, and laterally by the squamoso-jugal arch.
Order IV. PLACODONTIA.
These are the latest and last members of the Theromorpha, unfortunately known from skulls only, from the Muschelkalk or Middle Trias of Germany and Russia. The skull of Placodus gigas is about one foot long, rather high and triangular owing to the lateral expansion of the temporal arches, which diverge posteriorly. The squamoso-jugal arch is very broad, and most of the posterior border of the orbit is formed by the large postorbital bone. The maxillary bone seems to extend back to beyond the level of the orbits. The choanae lie behind the premaxillaries. The palatines and pterygoids are fused in the middle line, forming a broad bony palate, which, owing to the broad, posteriorly extended wings of the pterygoids, much resembles that of the crocodiles. The teeth are very remarkable. There are two or three stout, conical, or chisel-like teeth in each premaxillary bone, and three to five broad and flat maxillary teeth; three pairs of huge, broad, and quite flat teeth are crowded together and fill up the whole vomerine and palatine portion of the palate. These crushing teeth indicate that Placodus probably lived upon hard-shelled molluscs, and this would be in conformity with its occurrence in the Muschelkalk, which is a strictly marine deposit and full of shells. Another closely allied genus is Cyamodus, one species of which is known from Russia. The teeth are fewer in number and not so large as those of Placodus.
CHAPTER IX
CHELONIA–ATHECAE–THECOPHORA
Sub-Class IV.–CHELONIA.
There is no mistaking a tortoise. The shell and the horn-covered toothless jaws separate them from all other four-footed creatures.
They may be described as terrestrial or aquatic, pentadactyle reptiles, with walking limbs or with paddles; ribs with capitular portions only, two sacral vertebrae, humerus with entepicondylar foramen, pubes and ischia forming symphyses, quadrate bones fixed, jaws without teeth, but with cutting horny sheaths. Trunk encased in a bony shell, composed of numerous dorsal and ventral dermal bones, forming a carapace and a plastron, which may or may not be covered with horny shields. Copulatory organ unpaired, cloacal opening more longitudinal than round, never transverse. Oviparous.
It is customary to distinguish the marine, paddle-limbed kinds as Turtles, the others as Land- and Water-tortoises.