Mammalia: ( Classification 



767 



Fig. 583. Skeleton of baleen whale. (Courtesy, Owen: '•Comparative Anatomy an 

 Physiology of Vertebrates," London, Longmans, Green & Co., Ltd.) 



I he statement that the orbit is usually completely separated from the 

 temporal fossa by a bony wall. In fact, it may almost be said of pri- 

 mates, as of insectivores, that their most characteristic feature is the 

 absence of distinctively specialized structures. The search for them 

 brings to light only such apparently trivial things as the flat nails on 

 the digits, instead of claws or hoofs — and, in fact, some animals classed 

 as primates have claws on some of the digits. Another small anatomic 

 peculiarity makes possible the rotation of the first digit so that it 

 becomes opposed to the other four, a position favorable for prehension 

 and especially for grasping branches of trees, for most primates are 

 arboreal. Either the pollex or the hallux or both may be opposable, 

 but in some of the more primitive primates there is little or no power of 

 opposition. The human primate, although not arboreal, finds the 

 opposable thumb exceedingly useful. The feature of most outstanding 

 significance is one that involves no new type of specialization but 

 merely carries to a higher level something which is already well ad- 

 vanced in other mammals, especially ungulates and carnivores — 

 namely, the development of the cerebral cortex. But, here again, many 



Fio. 584. Jaws of baleen whale. (Courtesy, American Museum of Natural History. 

 New York.) 



