Mammalia: Classification 765 



digits completely enclosed within a common covering of skin (Fig. 579). 

 Pelvic limbs represented only by small vestiges of the pelvic girdle 

 (possibly also of the femurs) embedded in the body-wall. 



Skin very thick and nearly or quite hairless in adult; thick sub- 

 cutaneous layer of fat ("blubber"), important as insulation against 

 loss of heat. One pair of mammary organs in extreme posterior (in- 

 guinal) region. External nares on dorsal surface of head. Complex 

 stomach — divided into several compartments. Brain relatively large 

 and cerebral hemispheres highly convoluted. 



There are two Suborders: 



Suborder ODONTOCETI. Toothed whales; carnivorous. Homo- 

 dont teeth usually numerous and of simple conical form (Fig. 580); 

 exceptionally, teeth reduced to a few pairs or even only one pair. 

 Skull asymmetric — bones of right anterior region larger than corre- 

 sponding bones of left side; reason unknown. External nares join in a 

 single external opening. Sperm whale (Physeter: Fig. 581); beaked 

 whale; bottlenose; narwhal (Monodon); porpoises and dolphins 

 (Delphinidae). 



Suborder MYSTACOCETI. Whalebone whales (but "whale- 

 bone" is not bone). Adults toothless; horny plates (baleen), fringed 

 along their lower edges, depend, like curtains, from the upper jaw, 

 constituting a sieve through which water expelled from the oral cavity 

 is strained, the animal's food being whatever small organisms (mostly 

 fishes) are thus obtained (Figs. 582-584). Skull symmetric. External 

 nares open separately. Greenland or Arctic right whale (or ''bow- 

 head"; BalaenaJ, important as a source of commercial whalebone: 

 humpback whale (Megaptera); rorqual or finback (Balaen- 

 optera), including some of the largest of whales. 



Cetaceans range all the oceans from the equator to the arctic and 

 antarctic regions. Some of the smaller members of the Order inhabit 

 fresh water — large rivers of Asia and South America. 



Order 19: PRIMATES. For purposes of a precise definition, the 

 primates are perplexingly lacking in distinctive anatomic features. 

 The limbs are devoid of any extreme specialization such as marks those 

 of cetaceans, bats, and the several orders of ungulates. All of the primi- 

 tive live digits are retained. Erect posture and bipedal locomotion 

 characterize only a few primates; most of them "go on all fours," and 

 all are plantigrade. The dentition is complete and heterodont, but no 

 one kind of tooth is highly specialized. The dentition is perhaps even 

 less specialized than that of Insectivora. The stomach is usually simple. 

 The uterus may be either bicornis or simplex. Most definitions include 



