Page:Quarterly Journal of the Geological Society of London, vol. 34.djvu/509

Rh Or A HIGHER FORM OP LIFE ON A LOWER FORM. 427

Crocodilia. But in the vertebral, the dermoskeletal, and the palato- narial characters there is no known exception to the modifications which severally characterize the amphicoelian and proccelian Crocodiles.

We find, as the Secondary approach the Tertiary periods, that the Goniopholis, for example, of the Purbeck and Wealden series shows a nearer approach, in the shortness and breadth of the facial part of the skull, to the majority of the Tertiary and existing Cro- codiles than the Oolitic and Triassic amphiccelians exhibit ; while the long and narrow jaws, with the series of small, sharp, uniformly- sized teeth, in a straight row, common to Teleo- and Steneo- saurians are now exceptionally manifested by the single genus Gavialis, with a very limited geographical range. Such forms and proportions of the jaws and teeth, with some minor dermal modifica- tions affecting scutes and foot-webs, justify the generic status of the Indian Gharrials.

The claims of the American Alligators to generic honour rest .on feebler foundations ; and in regard to the procoelian Crocodiles of the present day, the work of what our German friends have termed the " Gattungsmacherei," has been most productive. Tomistoma, Oopholis, Halcrosia, Palceosuchus, Rliynchosuchus, Rhamphostoma, Mecistops, Bombifrons, Palinia, Molinia, Caiman, and Jacare hardly exhaust the multitude of names generic with which herpetology has been encumbered. What is of main importance to know is, that all the Crocodiles bearing the above names have the following characters in common : — the upper temporal apertures smaller than the orbits ; the palato-nares small, formed by the pterygoids exclu- sively, placed far back and on a slope towards the occiput ; the ver- tebrae procoelian.

These Crocodiles vary in respect to the proportions of length to breadth of skull, in the relative length of the nasal bones, in the proportions and course of sutures of the premaxillaries, in the degree of difference of the size of the teeth in the same species, and in the relation of some of the larger lower teeth to the upper jaw when the mouth is shut, also in the number and ar- rangement of the dermal scutes and in the extent of the toe-webs. And such characters are available, by their observed constancy, in the discrimination of the existing species of Crocodilia.

The connoting of the more essential characters common to the procoelian series, and of those which in like manner were common to the amphicoelian series, has led to the consideration of the con- comitant and seemingly influential conditions of such respective organic characters, more especially of the relation of the procoelian modifications to the coming in of the mammalian class, and perhaps to the going out of certain members of the reptilian class : and such considerations are now submitted to the judgment of the Geological Society.