Water Reptiles of the Past and Present

by Samuel Wendell Williston

The skull also is very peculiar in that it has some very primitive characters and others very aberrant. The temporal roof, as has been said, has no holes through it, though it is often reduced by the emargination of the borders, whether from below or behind, until in some the whole temporal region is exposed, and not at all covered over. There is no parietal foramen, so constantly present in all the early reptiles and in the lizards and the tuatera of modern times. There are no teeth or vestiges of teeth, but the jaws have usually a horny cutting edge, which seems to be quite as serviceable; in the river turtle the lips are fleshy. There is no transverse or transpalatine bone. There is a single vomer only, not paired as in other reptiles, whence comes the doubtful theory that the vomers of other reptiles are not the real vomers originally so named in mammals, and hence often called prevomers. The vomer of the turtles under this theory is believed to be the real homologue of the mammalian bone. The palate is always slightly, sometimes nearly wholly, underfloored, as in mammals, carrying the internal nostrils far back in the mouth. In the occipital region of the skull there is a separate bone on each side called the paroccipital or opisthotic, which has been indistinguishably fused with the exoccipital in all other reptiles except the ichthyosaurs since Triassic times.

Fig. 115

Fig. 116

Figs. 115 and 116.—Trachemys. (From Hay.)

Fig. 115.—Skull from above: fr, frontal; ju, jugal; pa, parietal; paoc, paroccipital; pfr, prefrontal; pof, postfrontal; pro, proötic; qu, quadrate; sq, squamosal; soc, supraoccipital.

Fig. 116.—Skull from below: alv, alveolar surface of maxilla; boc, basioccipital; bap, basisphenoid; exoc, exoccipital; mx, maxilla; pal, palatine; paoc, paroccipital; pmx, premaxilla; pro, proötic; pt, pterygoid; qu, quadrate; qj, quadratojugal; sq, squamosal; vom, vomer.

In the feet the numbers of phalanges—that is, the bones of the free digits—are like those of mammals, that is, two in the first and three in each of the other four digits. The land tortoises have lost some of these, while the river turtles have either gained one or two in the fourth finger and fourth toe, or else have enjoyed an uninterrupted descent from the primitive reptiles which normally possessed that number. All other reptiles, save those phylogenetically allied to the primitive mammals, that is, the Theriodontia and their allies, have normally the phalangeal formula 2, 3, 4, 5, 4. It was partly because of this similarity of the numbers of toe bones that the turtles have been classed in the great group of reptiles that includes the ancestors of the mammals; that is, under this theory, the turtles would enjoy a nearer relationship to the mammals and to man himself than any other living reptiles! But this classification has been shown to be quite artificial.