98 



THE OSTEOLOGY OF THE REPTILES 



its exogenous chevron is typically rhachitomous, in that it is wedge- 

 shaped. And this very probably represents the real intermediate 

 condition between the embolomerous and holospondylous vertebrae. 

 Evidence that reptilian vertebrae arose in this way is also seen in the 

 dorsal vertebrae of a young Seymouria, the most amphibian-like, 



Fig. 76. Vertebrae: A, B, C, Cr;Vo/Mj (Temnospondyli), dorsal, basal 

 caudal, and median caudal, from the side and front. D, Eryops 

 (Temnospondyli), caudal, from the side. E, Seymouria (Cotylo- 

 sauria), median dorsal, from the side. F, Dimetrodon (Pelycosauria), 

 dorsal intercentrum from behind and below. G, Trimerorhachis 

 (Temnospondyli), intercentrum from side and below. 



otherwise, of all known reptiles (Fig. 76 e). The intercentrum is here 

 remarkably large for a reptile, nearly half as long as the notochordal 

 centrum or pleurocentrum. And it is also almost the condition found 

 in the first vertebra of primitive reptiles, the atlas (Fig. 79), as will 

 be shown in the discussion of that bone. Additional evidence is fur- 

 nished by the fact that while truly embolomerous vertebrae occur in 

 fishes, in the modern Amia, for instance, real rhachitomous vertebrae 



i 



