56 University of California Publications in Geology [Vol. 13 



of mollusks, which they secure by diving to the bottom of the sea and 

 digging the mollusks up with their tusks. According to Elliott their 

 swimming in the open sea was remarkable in comparison with their 

 clumsy movements in shallow water, while their size and weight made 

 them relatively helpless on land. In landing and in climbing over 

 low, rocky shores they used their fore-flippers, and progressed very 

 slowly. 



The enlargement and lengthening of the upper canines was par- 

 alleled by a pinching in or a constriction of the symphysial portion 

 of the mandible. This constriction of the mandible became more 

 pronounced as the symphysial portion was increasingly abraded by 

 the enlarging tusks. The earliest known ancestor of the Odobenidae, 

 Prorosmarus, already possessed a mandible of typical odobenid type, 

 but it also possessed certain peculiarities which leave little doubt as 

 to how this modification was brought about. The ancestral odobenid 

 had the same number of teeth as the Otariidae, but, incident to pro- 

 gressive change and extreme specialization, the true molars became 

 reduced and finally disappeared. 



We may presume, as stated before, that the earliest offshoots of 

 the otarid stock were merely local differentiations in adaptation to 

 different feeding zones. If our preliminary conclusions are correct, 

 we may assume that the more immediate ancestors of Prorosmarus 

 gradually left the otarids in possession of the feeding grounds where 

 sea fish, squids, and cuttlefish were plentiful and sought new types 

 of food, first, partially, and finally, entirely bivalve mollusks. The 

 dentition required to feed upon mollusks would necessarily be of a 

 different type from that required to crush squids and cuttlefish, for 

 in the latter case the dental battery is not required to crush anything 

 stouter than the flesh or tough quills from the backs of squids, while 

 in the former the dentition must of necessity be able to withstand the 

 crushing of heavy shells. 



The present shape of the rostral region of the walrus skull appears 

 to be correlated with, or the result of, changes in the form of the dental 

 battery. The upturning of the facial portion of the skull is undoubt- 

 edly the result of the enormous increase in size of the upper canines 

 while the massiveness of the skull as a whole is correlated with the 

 type of dental battery and the uses to which it is put. This may be 

 either an orthogenetic trend or simply the response to some unknown 

 factor. 



