AQUATIC MAMMALS 



considered. In the Phocidae the spinal muscles, both apaxial and hypax- 

 ial, are enormously developed and play a more important part in swing- 

 ing the feet from side to side than do the intrinsic muscles of the limbs. 

 The chief effect which this development has had upon the pelvis of this 

 group is the fact that the ilium has been projected sharply to the side 

 (figure 49) so that it may accommodate a powerful attachment of the 

 iliocostalis lumborum. In other respects the innominates of these two 

 groups exhibit many resemblances, chiefly brought about by the fact 

 that in both the legs are always maintained in a trailing position. In 

 both, also, a significant result of the latter posture has been a binding 

 down of the shank so that the "crotch" consisting of the tissue joining 

 the limb to the vertebral column, falls opposite the heel. As a con- 

 sequence there is little independent movement of the limb bones, which 

 means that flexion and extension of the thigh is very definitely circum- 

 scribed, and that although rotation and abduction may be strong, they 

 can be only through a short distance. This has resulted in reduction in 

 the length of the femur, and of the ilium, or pre-acetabular part of the in- 

 nominate, which accommodates the lesser gluteal muscles. The latter 

 is but 16 per cent of the entire innominate length in a species of Phoca, 

 and 32 per cent in Zalophus (an otariid), while in such a terrestrial 

 carnivore as the cat this percentage is 59- In compensation the post- 

 acetabular measurement, or pubo-ischial portion, is 74 per cent of the 

 innominate length in Phoca, 55 in Zalophus, and 33 in a cat. Appar- 

 ently the stimulus for the increase of this posterior part of the pelvis has 

 been the increase in importance of the function of the muscles attached 

 thereto as adductors of the shank. Especially is this true in the case of 

 the seal, and parts of the ischium and of the pubis have been projected 

 in a dorsal and a ventral direction respectively, which furnishes greater 

 leverage chiefly to the superficial division of the biceps femoris and the 

 gracilis (fig. 49). Ryder (1885) has claimed that the pinniped pel- 

 vis is degenerate but this statement indicates an incorrect viewpoint. It 

 is permissible to view the ilial part as having suffered degeneracy in 

 length, and the sea-lion innominate is definitely less robust in some par- 

 ticulars, but the posterior part has experienced relative hypertrophy and 

 the whole is little or no shorter in relation to body length than in the cat 

 (21 per cent in Zalophus, 26 in Phoca, and 25 in a cat). Neither is 

 the sacropelvic connection weaker than in many terrestrial mammals. 



Accompanying and intimately involved with the shortening of the 

 pinniped ilium is shortening of the femur. In Zalophus this segment is 

 now 11 per cent of the body length, 12 in Phoca and 35 per cent in a 



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