AQUATIC MAMMALS 



we may not derive much help in this from a scrutiny of brachial condi- 

 tions in aquatic reptiles, but to do so will at least be interesting. 



In discussing the modification which the pectoral limb has undergone 

 in both mammals and reptiles of aquatic habits Williston (1914) stated 

 that the humerus has become greatly shortened in aquatic types having 

 a tail fitted for primary propulsion and even in some having short tails, 

 as seals, and to a lesser degree, sea otters. In those which use the legs 

 for direct propulsion, as plesiosaurs and marine turtles, the humerus 

 is elongated. In all save seals and otters whose limbs are used rather 

 as sculls than oars the lower limb bones are always shortened. These 

 statements are not entirely accurate. In all highly specialized aquatic 

 mammals now living, regardless of the method of propulsion em- 

 ployed, the humerus has become shortened. On the other hand, so 

 far as I am acquainted with the facts, in all aquatic reptile the sequence 

 seems to be for the antibrachial segment first to experience shortening, 

 followed later by a shortening of the humerus. At times, as in the 

 leatherback turtle (Dermochelys, fig. 34) , the disparity in size of 

 these segments is still very pronounced, the humerus being huge and 

 the antibrachium really of insignificant size. Apparently this is funda- 

 mentally characteristic of the order and different from what seems to be 

 the usual sequence in aquatic mammals. The likelihood is that this 

 difference is chiefly attributable to the dissimilarity in the osteological 

 plan and basic muscular equipment of the two orders. For the present 

 it is hardly profitable to pursue the subject further. 



In spite of the elongation of the sea-lion manus the total bony arm 

 length from the shoulder joint is, relative to length of body vertebrae, 

 very much shorter than in such a terrestrial carnivore as a cat, while 

 in the seal it is 20 per cent less. This may be said to be due entirely 

 to a diminution in the length of the brachial and antibrachial segments. 

 Relative to length of body the humerus in the sea-lion is but 65, and 

 in the seal but 45 per cent of the length of this bone in the cat. The 

 radius, as representing the antibrachial length, is found to be 105 in 

 the sea-lion and 96 per cent in the seal of their respective humeral 

 lengths, while in the cat this percentage is about 104. It is somewhat 

 surprising to find the proportions of these two segments so uniform 

 in these three mammals, and it is an indication that the stimulus for re- 

 duction in the arm length of these pinnipeds has apparently been quite 

 uniform, or at least has had a uniform result, in both segments between 

 the scapula and manus. In the walrus, however, the radius is only about 

 80 per cent of the humeral length. 



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