324 MAMMALIA. 



employs the two front feet as supports to fix the body, and then by 

 bending its back drags the hinder part forward. In swimming the 

 anterior extremities are applied to the body, and are used as rudders, 

 while the hind feet serve as swimming fins. 



The dentition, which is usually complete, indicates a predatory 

 mode of life, resembling that of the true Carnivora, to which order 

 the Pinnipedia are also allied in other anatomical characters, as in 

 the possession of a two-horned uterus and a zonary placenta. With 

 regard to the dentition, however, there are essential differences between 



o 



the families of the Walruses and Seals. The Seals have _, more 



2 g 5 



rarely - chisel-shaped incisors, small canines in each jaw, and 



1 5 



jagged grinders, of which one or two are true molars. The Walruses 

 have a complete dentition only in the young stage ; the incisors, 



3 1 



which at first are -, are soon reduced to _ in the prsemaxilla. The 

 3 1 



canines in the upper jaw are transformed into huge tusks, which are 

 used when the animal crawls on land to fix the anterior part of the 

 body. There are five grinders in the upper jaw and four in the 

 lower, with masticatory surfaces which wear away, in course of time, 

 obliquely from within outwards. The change of teeth usually takes 

 place during embryonic life. The Seals live principally on fish ; the 

 Walruses on sea-weeds, Crustacea, and Molluscs, the shells of which 

 they crush with their grinders. 



Fam. Phocidae (Seals). Pinnipedes with complete dentition, short canines, 

 and jagged molars. Haliclioerus grypus Nilss., Utsel. PJwca vitulina L., 



OIK 



Common Seal, -. ; Pli. grcerilandica Nilss., Northern Seas ; Cystophora 



cristata Fabr., Greenland ; Otaria jubata Forst., Sea- lion of South America ; 

 0. ( Callorhiwus) ursina Per., Sea-bear, Greenland. 



Fam. Trichechidae (Walruses). The upper canines are large, rootless, down- 

 wardly-directed tusks. The grinders are at first bluntly pointed, but are 

 gradually worn down, and eventually reduced to three in each ramus, while in 

 the upper jaw there is an internally placed incisor. TricJiechus rosmarus L., 



Walrus of the Polar Seas. Dentition : ..W - 1 -^ . 



Order 11. 



Carnivorous Mammalia with predatory dentition, without or with 

 a rudimentary clavicle, and with strongly-clawed digits. 



Vhe Carnivora are distinguished from the Insectivora by their larger 



