AQUATIC MAMMALS 



In the sea-lion manus the terminal border of the foot is practically 

 straight, but in the pes it is deeply serrated, because the predigital car- 

 tilages extend considerably beyond the interdigital membrane borders. 

 The length of foot including cartilages, in the fur seal (Callorhinus) 

 appears to be relatively greater than in the sea-lion (Zalophus). Also, 

 judging by but two specimens measured, this length is proportionately 

 greater in young of the latter genus than in an adult, which may indi- 

 cate that a longer rudder was needed in the past, with somewhat less 

 perfected swimming apparatus, than is now requisite. 



The development of the pes in the Otariidae is somewhat puzzling in 

 certain respects, chiefly because of its length in respect to breadth, the 

 former augmented by the peculiar predigital cartilages. It may really 

 be said to be more highly specialized than either the manus or the pes 

 of the Phocidae. At one time I was inclined to entertain the possibility 

 that at a not very distant geologic date it might even have been of some 

 definite, active aid in locomotion, but this I have abandoned. Evidently 

 the stimulus for a long neck that is highly mobile is a strong one, and 

 this mobility necessitates a highly specialized and highly efficient rudder 

 fully capable of compensating for abrupt movements of the head, say 

 in snatching at prey, that would otherwise tend to deflect the animal 

 from its desired course. Such a rudder should project well to the rear, 

 and does, but its "hinge" should be at the body junction, and is. Hence 

 the leg has been shortened and is contained within the body covering 

 while the foot has been lengthened by predigital cartilages, either be- 

 cause these fulfill some need which longer digital bones could not ac- 

 complish or because they were more easily developed than longer 

 phalanges. 



3. Under the third heading mentioned on page 000 will be dis- 

 cussed those aquatic mammals in which the hind feet are absent or de- 

 finitely subordinate in swimming or steering. Under this will be in- 

 cluded the mink for the sole reason that there is nowhere else to place 

 it without making a separate heading for its reception. It swims chiefly 

 by all four feet, as far as I can learn, but it should not be placed with 

 most other mammals employing this method of propulsion, for its ten- 

 dencies are different. It is clearly what may be called an incipient otter 

 which may be expected to develop its swimming abilities along pre- 

 cisely the same lines as has the latter, and accordingly it will be given no 

 further attention. 



For discussion under this heading there are the aquatic Tenrecidae, 

 Limnogale, and Potomogale, the Lutrinae or river otters, the three re- 

 cent families of the Sirenia, and the Cetacea. 



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