AQUATIC MAMMALS 



as the first and both are longer than the three middle toes. The pre- 

 acetabular length of the pelvic bones is always less than the post-acetabu- 

 lar. The humerus is relatively massive and the bones of the forearm 

 very broad at one end, while the femur is much reduced in length and 

 is flattened. There is no clavicle. In adults there is probably always 

 a large hepatic sinus of the vena cava. Nictitating membranes are 

 said to be present, and retia mirabilia occur to an unknown extent. 



Otariidae 



These are the sea-lions and fur seals or sea bears. The pelage is 

 short, either entirely hairy or with a dense and fine underfur, and the 

 young do not have a coat that is exclusively woolly. The external ear 

 is small and narrow. Both fore and hind feet are used in limited ter- 

 restrial locomotion but the former exclusively in swimming. The area 

 of the fore foot is great and the axilla is situated at about the middle 

 of the forearm. The hind foot assumes a plantigrade posture during 

 terrestrial locomotion and the astragalus is without a posterior exten- 

 sion. Each digit of both fore and hind foot has a cartilaginous ex- 

 tension and the palms and soles are naked. The nails of the fore foot 

 and those of the first and fifth digits of the hind foot are vestigeal. 

 The testes are scrotal. The canines are not unusually developed. The 

 vertebral spines of the anterior thorax are well developed. The greater 

 tuberosity of the humerus is higher than the lesser. The ilium is 

 slightly curved — not markedly and abruptly bent laterad — and the femur 

 has a lesser trochanter. 



Odobenidae 



This is the walrus, of the north Atlantic and north Pacific. 

 This family, containing a single genus, is in most respects nothing 

 but a specialized otariid, although in some points it is fairly inter- 

 mediate between that group and the phocids. The body is almost 

 hairless. There is no external ear. Both fore and hind feet are used 

 in terrestrial locomotion, which is more limited than in otariids, but 

 the hind feet are used in swimming by being moved from side to side, 

 while the fore feet are also used, but alternately. In major details the 

 feet are most like those of an otariid but the cartilaginous extensions 

 are not as long, the fore foot is relatively smaller and the hind foot 

 broader and more comparable in shape to that of a phocid. The hind 

 foot assumes a plantigrade position during terrestrial locomotion and 

 there is a slight posterior extension to the astragalus. The testes are 



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