AQUATIC MAMMALS 



diverse that but little more can be deciphered. There seems to be but 

 Httle doubt, however, that the more falcate shape of the scapula in Phoca 

 signifies a more caudal extension of the glenovertebral angle for the chief 

 purpose of furnishing increased leverage for the enormous triceps, rather 

 than to any extrinsic limb muscle. 



In all aquatic mammals the ball-and-socket character of the shoulder 

 joint is retained. In the Pinnipedia, Sirenia and the Zeuglodonts (at 

 least in some of them) the synovial character of the elbow joint is also 

 retained, and this is true also of the carpal bones of pinnipeds, save that 

 in the Otariidae the mobility of the latter region is reduced. In modern 

 whales, however, the joints distad of the shoulder have all lost their 

 synovial character and instead are entirely fibrous. That part of the limb 

 that projects beyond the body contour of whales is thus incapable of 

 bending movement save that the fibrous interosseous tissue gives con- 

 siderable elasticity to the flipper. This character of resilience is appar- 

 ently of much value and it is to be expected that it will increase to a 

 certain ideal optimum. One way in which this might be accomplished 

 is in the reduction in length of the interfibrous (i.e osseous) elements. 



The advantage of an elastic but non-jointed paddle to a whale is too 

 obvious to need discussion. Apparently it might be of equal desirability 

 to a swimming sea-lion, but this animal is obliged to have adequately 

 bendable joints in the arm if it is to continue movement on land. Al- 

 though the latter function is of no use to a sirenian, it does need to bend 

 the elbow during feeding, for the flipper is then employed for drawing 

 herbage toward the mouth, and Murie mentioned that in the manati the 

 joints are very lax and their ligaments simple. 



But a fore limb in the shape of a true paddle is needed by whale, 

 sirenian and sea-lion — not a spatulate enlargement of the manus upon 

 the end of a long arm. I think this is obvious and that it may be accepted 

 without argument that the normal tendency in such aquatic mammals as 

 whales and sea-lions is for increase in relative size of the paddle part 

 (the manus) and decrease in relative size of the non-paddle part (the 

 antibrachium and humerus) . There undoubtedly are several different 

 factors that may help this development, and likely some unknown ones 

 that tend to hinder it. 



In such a non-jointed paddle aswe may presume to be ideal for pro- 

 pulsion or equilibration in the water the only essential limb muscles are 

 those which operate the arm as a whole, bending it upon the shoulder 

 joint. Not only must there be adequate provision for flexion and ex- 

 tension, abduction and adduction, but also for tilting or rotation, in or- 



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