AQUATIC MAMMALS 



The partial binding down of the hind limbs effects a static posture 

 with these members more trailing than would otherwise be the case, 

 and for the same reason the terrestrial posture must be with the back 

 more arched, so that the sacrum is more elevated toward the perpendicu- 

 lar in a manner suggestive of the sea-lion's posture. The evidence clearly 

 shows that the feet are placed in plantigrate position with ease, although 

 I have seen it stated in a letter that this is impossible. 



In examining a skeleton of Enhydra in comparison with one of Lutra 

 it is seen that although the proportions of the ilium in regard to post- 

 acetabular dimension is the same, the entire innominate is somewhat 

 heavier, to accommodate stronger musculature, and the ilium flares con- 

 siderably in a lateral direction, for a stronger iliocostal attachment. Tay- 

 lor (1914) has stated that the pelvis is more nearly parallel to the ver- 

 tebral column also and mentions that there is no evidence of a liga- 

 mentum teres, as there is in Lutra. He also showed by drawings reduced 

 to the same size that the thigh and shank bones of the sea otter are the 

 heavier and that their relative proportions are not appreciably altered. 

 The greater trochanter is slightly broader, because of the specialization 

 of the lesser gluteal complex, and more seal-like. The metatarsals and 

 phalanges are slightly flattened, and manipulation of the hardened spirit 

 specimen seems to show that there is unusual freedom of movement in 

 the articulation of the astragalus and calcaneum with the centrale and 

 cuboid a functional tarsal joint as in the seal, making the foot more 

 handlike to facilitate oscillating movements when the pes is held on a 

 line with the shank. 



Why the sea otter should have developed a swimming method essen- 

 tially different from that seemingly in course of elaboration by the river 

 otters is not entirely clear, but the fundamental reason which appears 

 mose likely is that in the former animal the tail has always been too 

 short and light to be used for propulsion alone. Presumably the hind 

 feet will continue their specialization and become still more efficient, 

 while the tail may either continue to function as an accessory, central 

 stiffener for the swimming organ formed by the hind feet, as it now 

 appears to do, or become shorter as the feet have less need for a support 

 of this kind. Conceivably, however, the tail may ultimately increase in 

 power and take over the function of primary propeller. 



2. Aquatic mammals which use the hind feet chiefly for equilibra- 

 tion include Ormthorhynchus, the Otariidae or sea-lions and fur seals, 

 and the Odobenidae or walruses partly. The latter need no further 

 consideration here save to mention once more that their feet partake of 



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