65 



this point of divergence may be placed the extinct Ichthyo- 

 sauria and Plesiosauria as independent branches from the 

 Proto-x\mniota, between the ancestral Amphibia and Eeptilia. 

 Their position is however a little doubtful. They may 

 possibly have arisen further back than they are placed in 

 the table, and may really be offshoots from the Proto- 

 Pentadactyloidei, close to, but independent of the ancestral 

 Amphibia. 



In the primitive Sauropsida, the epidermis must have 

 come to take a prominent part in the formation of the exo- 

 skeleton. The internal skeleton also, and especially the 

 skull, became more completely formed and more thoroughly 

 ossified. This group then probably split into two ancestral 

 series, of which the one eventually produced the Crocodilia, 

 the Ornithoscelida, and the Aves ; and the other the 

 Dicynodontia, the Chelonia, the Pterosauria, the Lacertilia, 

 and the Ophidia. 



The latter branch must be regarded as the more direct 

 continuation of the primitive Sauropsida, and is probably 

 most nearly represented by the Lacertilia of the present 

 day. They form the central group in the Reptiles just 

 as the Urodela do amongst Amphibians. The Ophidia are 

 a degenerate group of Reptiles allied to the Lizards ; they 

 may be regarded as having diverged from the ancestral 

 Lacertilia. The extinct Pterosauria branched off from the 

 Lacertilian stem further back, and became considerably 

 modified ; while the Chelonia and the extinct Dicynodontia 

 diverged still earlier, and probably not far above the point 

 where the primitive Sauropsida branched. The Chelonia 

 and the Dicynodontia probably left the ancestral Lizards 

 at much the same point, or they may possibly have had a 

 short common ancestry. The Chelonia have become greatly 

 modified, chiefly in respect to their exoskeleton and body- 

 form. 



