66 AMERICAN PERMIAN VERTEBRATES 



has urged that the reptiles arose from the Microsaurs — that is, the 

 Microsauria as usually accepted — but while I do not agree with 

 him I do believe that he comes nearer to the real truth than is the 

 belief that reptiles have arisen from the known types called tem- 

 nospondyli. The divergence of the reptilian stem must have 

 occurred early, in Pennsylvanian if not Mississippian times, and 

 what we have in the Permian, or even Permo-Carboniferous beds 

 of Texas, Illinois, France, and elsewhere are the modified, perhaps 

 very much modified, descendants of these early divergent phyla. 

 The view of Osborn that the Cotylosauria were the primitive 

 reptiles from which all modern forms have arisen has been 

 doubtfully accepted; and his conclusion that the reptilia are 

 divisible into two subclasses, the Diapsida and Synapsida, is no 

 longer received by students of the reptilia. At the beginning of 

 the Permian or even earher we have, as is well known, at least four 

 divergent phyla of reptiles, the Cotylosauria, the Theromorpha or 

 therocrotaphic forms, the Proganosauria, with aquatic adaptions, 

 and the Proterosauria or double-arched forms. It is a very inter- 

 esting fact that the chief divergences in all these phyla are found 

 in the temporal region of the skull. The pectoral and pelvic 

 girdles, the vertebrae, and the limbs even, differ less among them all 

 than do they in some later orders of reptiles ; we surely will find 

 greater modifications among the modern Squamata. Did we not 

 know the skulls of any of these forms we would have little cause 

 to separate them ordinally. The chief differences in the skull 

 consists of the absence or presence of one or more vacuities in the 

 temporal region. I still believe with Cope, Woodward, and Osborn 

 that these temporal characters are the most crucial ones in reptilian 

 taxonomy. And yet were such characters as those of Cacops or 

 Dissorophus and Trematops to be found among reptiles, we would 

 probably accept them as of ordinal value. Among the cotylosaurs 

 we see a considerable divergence in the known forms. The presence 

 of a vestigial cleithrum in some cotylosaurs and some theromorphs 

 attests a not very remote common ancestry, and this ancestry 

 could not, of course, have been any of the known Microsauria, since 

 a bone or organ once lost never reappears, even as a vestige. 

 Furthermore, Araeoscelis, which is now known definitely to possess 



