NORTH AMERICA AND THEIR VERTEBRATE FAUNA. 1 23 



tarsus and carpus. The possibility that all the specimens described by Cred- 

 ner are immature, as sviggested by Osborn, must not be lost sight of. 



We know nothing of the feet of the Poliosaiu-idae, other than in the genus 

 Varanosaurus, but these are the feet of a typical land reptile, with well- 

 ossified interlocking elements in the wrists and ankles. I can not but believe 

 that the ideas proposed in my first monograph upon the Pelycosaurs were 

 correct ; that the series represented by Poliosaurus, Clepsydrops, and Dime- 

 trodon is a morphological series, representing stages in the development of 

 the phylum, and if we add to this series, as a beginning member, a Protero- 

 saurid individual, we would have representatives of the group from its incep- 

 tion to its extinction. 



Not improbably the Caseasauria, Kadaliosauria, and Edaphosauria were 

 derived from some lower member of this line. This would in no wise exclude 

 Osbom's hypothesis that the Dinosauria were derived from the Proterosauria. 



The origin of the Cotylosauria directly from the Stegocephalia has not 

 been questioned by those familiar with the group. That no one of the known 

 forms can be considered as a connecting link is well accepted. Seymouria 

 and Pantylus are both very close to the Amphibia in the characters of the 

 skull, but neither of these is conceivably in the direct line. It is practically 

 certain that we must turn to one of the smaller, less well-known forms, 

 Pariotichus, Isodectes, Sauravus, Eosauravus, Ectocynodon, or some similar 

 form, for the connecting link. As in every case of a proposed phyletic series, 

 we can approach more or less closely to a point of union from both sides, but 

 the known forms are all too highly specialized to occupy the exact position. 



The Amphibia are all closely connected with the true Carboniferous 

 amphibians^so closely that we can only see in them a direct continuation of 

 the phylum. 



