234 MAMMALIAN DENTITION 



tion in the form of the teeth which is itself evidence of the 

 comparatively recent differentiation of the dentition. But this 

 does not mean that the Odontoceti and the Pinnipedia are 

 related. It simply indicates that similar habits call forth a sim- 

 ilar adaptation though in varying degree. In other words the 

 similar secondary simplification of the teeth in these two sub- 

 orders is one of convergent evolution. We have seen that the 

 Pinnipedia are probably related to the Miocene "Bear dogs" 

 but although the Odontoceti are also carnivorous they are cer- 

 tainly not related to any known carnivorous Mammals. 



Fig. 83. — Dentition of common Porpoise (Phocaena communis, 9.53-1). Note the great 

 number of similar cylindrical teeth and the extreme specialization of skull and jaws. 



There may have been typical Odontocetes in the Oligocene 

 but it is quite possible that the Whales are of more recent 

 origin.' There is nothing in their anatomy to suggest great 

 antiquity as for example in the cases of the Edentata and the 

 Rodentia. The derivation of all the Whales, toothed and tooth- 

 less, seems to be from Insectivore-Creodont stock but the 

 earliest examples at present known are certain fossils from 

 the middle Eocene of Egypt, Protocetus and Mesocetus by 

 name, primitive creatures which, while perhaps not represent- 

 ing the direct ancestral stock, are at least collateral to it. 



