ART. 15 ANATOMY OF THE EAKED AND EABLESS SEALS HOWELL 123 



Tiie pelvis of the Otariidae is somewhat weak, while that of the 

 Phocidae is very strong and angular. For somewhat different rea- 

 sons the innominate bones of these two families have become con- 

 siderably differentiated in the same general direction from the nor- 

 jnal fissiped type of pelvis, chiefly by the trailing position persist- 

 <^ntly assumed by the limbs. Most of the muscles originating from 

 the anterior ilium are normally rotators of the femur, and as the 

 ilium is long in fissipeds (measured from the acetabulum, 59 per 

 cent of the innominate length in the cat) it must be important that 

 tliese rotators have a long leverage. Strong rotation of the femur 

 in pinnijjeds can be of less use, however, because of the extreme 

 shortness of this bone and the position assumed by the leg. It is 

 true that these muscles do act as rotators of the femur in Zalophus, 

 but more as abductors of the femur in Phoca. And in neither can 

 tlie flexors of the femur nor the femoral extensors of the shank have 

 much function as such because of the shortness of the thigh and fixed 

 posture of the leg. All of these details conspire to obviate the 

 need for a long ilium and the result has been an extreme shorten- 

 ing of this part of the innominate (32 per cent of the total length 

 of the bone in Zalojjhus and but 16 per cent in the Phoca) . In the 

 fissiped the more caudal part of the innominate gives rise chiefly to 

 muscles which extend the femur and flex the shank, and caudad 

 of the acetabulum this portion of the pelvis is 33 per cent of the 

 innominate length in the cat, 55 in Zalophus, and 74 in the Phoca. 

 In the pinnipeds the whole leg is so bound down that but little exten- 

 sion of the femur or flexion of the shank is possible. In this order, 

 therefore, both these groups of muscles are, by virtue of the trailing 

 position maintained by the legs, as much if not more concerned 

 with the actions of abduction and adduction, which is of especial 

 use to the Phocidae. The farther the innominate extends caudad 

 the longer will be the lever arm of these muscles and hence we find 

 this rearward extension more pronounced in the phocids. In the 

 otariids the pelvic muscles are used for a great variety of move- 

 ments, none of which is likely of great importance to the animal, but 

 the hind feet are flapped and rotated this way and that in their 

 function of rudders, and the pelvic musculature is correspondingly 

 specialized and split up into numerous divisions — the adductors to 

 as many as six, the sartorius into two, etc. On the other hand the 

 hind feet of Phoca are used for but one thing— flapping from side 

 to side — and the musculature is correspondingly specialized, but in 

 the direction of fusion and simplification. The gluteal mass has 

 little to do but act upon the greater trochanter in a way to cause 

 chiefly abduction of the femur, and the pubo-ischial muscles as 

 adductors of the shank, but with some function of elevation and 

 depression according as origin is from the dorsal ischium or the 



