Williston — New Genera of Permian Reptiles. 579 



fauna of amphibians and reptiles — and the end is not yet ; that 

 the local faunas of regions so little remote as those of Texas 

 and New Mexico, though closely allied, should show such dis- 

 tinct generic differences, suggest that the world's fauna of rep- 

 tiles at least, in early Permian times was richer in genera and 

 species than it has been at any succeeding epoch in geological 

 history. Divergence was limited by primitive characters, for 

 evolution had not gone far enough to affect the more funda- 

 mental ones, but no other known epoch of approximately equal 

 duration has yielded so extensive a fauna of cold-blooded air- 

 breathing animals as has the Lower Permian of America. 

 Of the genotypes, or specimens upon which the accepted 

 described genera have been based, thirty are preserved in the 

 collections of the American Museum, eighteen in the collec- 

 tions of the University of Chicago, six in those of Yale, four 

 at Munich, and the remainder at the universities of Oklahoma, 

 Kansas, Michigan and the Carnegie Institute. 



