PEDIGREES AND RELATIONSHIPS 



21 



branch from the belodonts is that of the crocodiles which retain 

 the dermal armour and socketed teeth, but have lost their 

 clavicles and the pubis has become excluded from the aceta- 

 bular cavity of the pelvis. Specialisation early displays itself 



A 



Fig. 2. — Upper (A) and lower (B) views of the skull of a Belodont (Phyto- 

 saurus) a primitive reptile from the Upper Trias : nar. nostrils; p. na. posterior 

 nostrils ; pmx. premaxilla. C. Upper view of skull of an extinct Crocodile (Metrio- 

 rhynchus) to show the open temporal region and the small premaxilke as con- 

 trasted with the corresponding parts in Phytosaurus. 



in the backward transference of the posterior openings of the 

 nostrils into the mouth, by means of the development of a false 

 floor to the palate of the skull, but it was not until the Tertiary 

 period that this feature obtained its full development. The 

 culmination of this adaptive modification was attended by the 



