PHYLOGENY AND CLASSIFICATION OF REPTILES 421 



homoplastic; nor am I aware of any constant character in the 

 girdles, limbs, or ventral ribs. Goodrich, in a recent paper on 

 the phylogeny of reptiles, relies greatly upon the structure of the 

 feet, especially of the fifth metatarsal. I cannot accept his con- 

 tentions, some reasons for which I have published elsewhere. 



Just when the animals we call reptiles arose in geological history 

 we do not know; certainly it was in early Pennsylvanian times, 

 probably in Mississippian. That they arose from what we call 

 the Amphibia, forms with temnospondylous vertebrae, is certain, 

 though there is probably not much more reason for calling the 

 ancestral stock Amphibia than Reptilia. I prefer to call it, 

 provisionally, Protopoda. It was ancestral to both, and both 

 classes have advanced since their divergence, the Amphibia some, 

 the Reptilia much. Could we find, as some time we hope that we 

 may, in mid-Mississippian or late Devonian times, a skeleton of 

 one of those ancestral creatures, we should perhaps not call it by 

 the name of any known order; it would be the old question over 

 again of the differences between animals and plants. At present 

 we know the Protopoda only by their footprints. As it is, we are 

 dealing chiefly with archaic forms, even by the close of Penn- 

 sylvanian times, forms which have retained in various degrees 

 their primitive characters while adding or losing others in different 

 ways. Just as the most primitive mammals now living have 

 become highly specialized in the loss of teeth, so too the amphibians, 

 as we know them in Paleozoic times, were more or less specialized 

 or degenerated. The only known distinctive characters between 

 the two classes, as represented by their known skeletons in Permo- 

 carboniferous times, are found in the atlas and feet, and doubtless 

 some time we shall find these differences bridged over. For a 

 long time we relied upon the open palate of the Amphibia, but 

 Watson has deprived us of that support, and now we are compelled 

 to group the characters at length in any differential diagnosis of 

 the two classes. However, with nearly every character in common 

 to the Amphibians and reptiles, we are never in doubt as to which 

 class a given form belongs if we know it well enough, for all the 

 common characters have never been found in the same specimen, 

 and doubtless never will be in any specimen from rocks later than 

 the Mississippian. 



