THE PECTORAL LIMB 



That there has been a pronounced reduction in brachial and anti- 

 brachial length in sirenians is apparent and yet it is difficult to compare 

 its degree with this detail of the Pinnipedia. It is clear, however, that 

 in relation to arm length there has not been as great reduction in this 

 dimension of the sirenian humerus, while there has been a greater 

 amount in the radius and ulna. But it is unexpected to discover that 

 in both genera considered the radius is just 68 per cent of the humeral 

 length. 



In living cetaceans there is invariably a phenomenal shortening of 

 the humerus to the extent where this bone is at times two thirds as 

 broad as long, but its proportion to the radial length is extremely vari- 

 able. In almost all sorts it is definitely shorter than the radius, but in 

 Physeter, Kogia, Stenodelphis and Platanista it is longer in the latter 

 over 200 per cent of the radial length. In other odontocetes it is slightly 

 shorter, while in mysticetes this is considerably more pronounced. Thus 

 in Rhachianectes the humerus is about 68 per cent of the radial length, 

 and 60 per cent in Sibbaldus. Speculation of the above cetacean facts 

 is, however, sadly complicated by the circumstance that the arm of 

 Basiolosaurus, more primitive than any recent whale, shows a much 

 reduced antibrachium and a rather long humerus. 



As with the carpus and digits of the Cetacea the elbow joint in this 

 order is entirely fibrous in character. Apparently this is due to the non- 

 development of the articular structures characteristic of a synovial joint, 

 rather than to the alteration of these. It seems that the fibers of the 

 existing joint develop directly from the perichondrium or periosteum 

 to the degree where they supply the amount of stiffening that the animal 

 finds essential. 



Whereas the external axilla of the sea-lion falls midway of the anti- 

 brachium and of the seal opposite the wrist, it is apparently situated 

 just proximad of the elbow in the Sirenia, at about the proximal third 

 of the antibrachium in at least some odontocetes (porpoises), and at 

 about the same point in Mysticeti. The optimum position of the elbow 

 in respect to the body contour in aquatic mammals thus seems to be 

 somewhat variable, depending upon individual requirements, and of 

 course upon the amount of specialization. 



In no other mammals than those discussed above does it seem safe to 

 say that there has been alteration in the osteology of the fore limb in- 

 duced by life in the water. Taylor (1914) has made the observation 

 that in the sea otter this member is relatively smaller than in the river 

 otter, and the short legs of the hippopotamus may be considered either 



[231] 



