AQUATIC MAMMALS 



I would deem that in life the latter figure might be cut to one third by 

 the confining effect of the integument. Supination and pronation, even 

 in the partially dissected specimen, was through only some 45 degrees 

 with respect to the humerus (this therefore including rotation of the anti- 

 brachium) . Flexion of the manus is through an arc of 90 degrees, which 

 is more than one would expect because one cannot see that such an action 

 would be useful. 



It has been seen that the effectiveness of the otariid paddle is in- 

 creased by the extension of its border beyond the digital tips. Unlike 

 the case of less modified aquatic mammals in which the dorsal and pal- 

 mar integument of the digital webbing is in close contact with practi- 

 cally no subcutaneous tissue intervening, there is a liberal amount of 



\ 



^ 





FiGURE 41. Outlines of cetacean and pinniped flippers: (a) Globiocephah 

 (after Murie) \ {b) Orcinus (from a photograph by E. P. Walker) ; {c) se3 

 hon {Zalophiis) ; and {d) seal {Phoca) . 



such tissue in the manus of the sea-lion and whale, as well as the 

 sirenians. This has the effect of padding out the interdigital depres- 

 sions so that the flipper presents a plane surface both above and below. 



In the Otariidae the effective length of each digit is increased by 

 a cartilaginous rod, the extent of which can be determined in the entire 

 animal by the distance from the respective nails to the flipper border. 

 Presumably there has been a strong stimulus either for a flipper longer 

 than the bony elements of the manus have been capable of supplying, 

 or else for an elastic border. 



It has been mentioned that the predigital part of the membrane of 

 platypus is strengthened by thickenings of the integument, and it is 

 not unlikely that these may eventually be replaced by cartilage. But 

 [252} 



