The most primitive known reptile. 



287 



The large ventral exposure of the basioccipital is a reptilian 

 character not achieved in Amphibia. 



Basisphenoid. — The presence of large powerful t libera basi- 

 sphenoidales in the basis cranii of Seymouria is a reptilian 

 character. Definite tubera do occur in certain Temnospondyls 

 (Eryops), but these are always small. In Embolomeri tubera are 

 absent. 



i The short basipterygoid processes of Seymouria supporting the 

 pterygoid through the intervention of a special bone are unique, 

 nothing similar occurring in any other known adult reptile or 

 amphibian. The shape and position of the pterygoid render it 

 certain that these special bones cannot be the epipterygoids, 

 which in Dimetrodon are known to articulate with the basi- 

 sphenoid. Swinnerton and Howes showed that in the develop- 

 ment of the skull of Sphenodon special articular cartilages are 

 developed between the basipterygoid processes and the pterygoid, 

 and it is not impossible that these are the representatives of the 

 articular bones in Seymouria. 



On the other hand, Gaupp has shown that in Lacerta the 

 basipterygoid processes contain independent centres of ossification, 

 and it is feasible and attractive to regard the Seymouria bones as 

 permanently separate autogenous basipterygoid processes. 



The way in which the sides of the lower surface of the basi- 

 sphenoid of Seymouria pass on directly into the outer surface of 

 the prootic is reptilian, agreeing exactly with the conditions in 

 most Ootylosaurs. The structure of this region is, however, 

 equally similar to that of Pteroplax and another Middle Coal- 

 Measure Embolomerous amphibian ; but it is quite different 

 from the conditions in Rachitomi. 



The parasphenoid of Seymouria is of normal reptilian type, 

 and also agrees with that of Embolomeri. It differs in its small 

 size from that of Rach-itomi. 



Brain-case and Otic region. — The brain-case and otic region of 

 Seymouria present a wonderful mixture of features occurring in 

 other Ootylosaurs and in Labyrinthodontia. 



Seymouria is unique amongst reptiles in not possessing an 

 ossified supraoccipital. It also differs in this respect from the 

 Embolomeri, but agrees exactly with the conditions in most 

 Rachitomi. 



The whole structure of the exoccipital, its relations to the 

 basioccipital and paroccipital, and to the skull-roof especially in 

 its connection with a special descending lappet from the dermo- 

 supraoccipital, is that of such an amphibian as T rimer or achis, 

 which resembles Seymouria even in the unusual position of the 

 hypoglossal foramen. 



The Rachitomous amphibian in which the brain-case is most 

 completely known is Eryops, where the structure has been 

 described by v. Huene, Broom, the present writer, and Williston. 



Although Prof. Williston criticises certain points of my 



