INTRODUCTION 



Nevertheless, there were still so many inherited characters among 

 both the amphibians and reptiles of early Permian times that nothing 

 distinctive of either class can be found in the skeleton, except in the 

 atlas and feet, with a considerable gap in the structure of their ver- 

 tebrae. In the vertebral column there was a general change among 

 the Temnospondyli from the embolomerous to the rhachitomous 

 t3^e, that is, from the more simply divided centrum of two disks to 

 the tripartite centrum composed of wedges; while all reptiles had ac- 

 quired a reduced embolomerous form 

 with one disk, the centrum, and one 

 wedge, the intercentrum. Doubtless 

 all amphibians of Lower Carbonif- 

 erous times had embolomerous verte- 

 brae, but only a very few of their 

 stock persisted as late as the Per- 

 mian. In general literature the Am- 

 phibia are distinguished from the 

 Reptilia by the possession of two 

 occipital condyles. The earliest am- 

 phibians doubtless all had a single 

 occipital condyle, an inheritance from 

 their ancestral fishes — all that we 

 know from the Lower Carboniferous 

 had — of which only one known de- 

 scendant with that character survived 

 to the Permian. The reptiles, how- 

 ever, retained the single condyle until 



the beginning of their evolution into mammals, when they too de- 

 veloped a double condyle. We relied, until recently, upon the 

 widely open palate of the Amphibia as a final distinguishing char- 

 acter of their class, but we now know that some, if not all, of the 

 earliest amphibians had a [closed] palate like that of the [earliest] rep- 

 tiles, but of these none is known at the beginning of Permian times. 

 In other words, a single condyle and a closed palate are more prim- 

 itive characters of the tetrapods than those we had assumed as 

 characteristic of the Amphibia. We know no amphibians with as 

 many bones in the digits as the early reptiles possessed, and no rep- 

 tiles with as many bones in the tarsus as the early amphibians had, 



Fig. a. Seymouria (Cotylosauria). A, 

 from above; B, from side. One third 

 natural size. 



