Water Reptiles of the Past and Present

by Samuel Wendell Williston

Because of the reliance which must be placed upon the skeleton it will be necessary to speak somewhat in detail of its structure in the reptiles, and to use not a few terms in its description that are unfamiliar to the general reader. So far as possible technical terms will be avoided, though some must be used, as there are no equivalents in the English language for them. The reader may use this chapter as a sort of explanatory index or glossary for the better elucidation of the necessary details of the following chapters.

It is needless to say that the skeleton of a reptile is arranged on essentially the same plan as that of our own; the bones have the same names that they have in our own skeleton, but there are more of them, and the individual bones, as a general rule, are less highly specialized, that is, are not so well adapted for special functions. In a word, the skeleton of a reptile for the most part is generalized, though particular parts may be highly specialized for particular uses. As a rule, if not as a law, the course of evolution has been to reduce the number of parts and to adapt those which remain more closely to their special uses, either by increase in size, or by modifications of their shape and structure.

SKULL AND TEETH

The skull of reptiles is much more primitive or generalized in structure than is that of mammals, to such an extent, indeed, that there is yet much doubt as to the precise homologies of some of the bones composing it; and, inasmuch as the names were originally given, for the most part, to the bones of the human skull, there is still some confusion among students as to the proper names in all cases, a confusion that doubtless will not be wholly dissipated until we know much more about the early or more primitive reptiles than we do at present.

Fig. 4

Fig. 5

Fig. 4.—Seymouria, a primitive cotylosaurian. Skull, from above: pm, premaxilla; n, nasal; l, lacrimal; p, prefrontal; f, frontal; pf, postfrontal; it, intertemporal; st, supratemporal; sq, squamosal; ds, dermosupraoccipital; t, tabulare; j, jugal; po, postorbital; m, maxilla; s, surangular; ang, angular; pa, parietal.

Fig. 5.—Seymouria, skull from the side. Explanations as in fig. 4.