AQUATIC MAMMALS 



mm. broad at its narrowest point, thence widening to cover the whole 

 wrist before splitting in two sheets, the more robust going to the anterior 

 and the lesser to the posterior border of the flipper. This effects a 

 definite cupping action of the palmar surface as discussed later. 



In pinnipeds as well as Cetaceans there is no appreciable tendency 

 for either the fusion of the radius and ulna, nor for the reduction of 

 one of these bones at the expense of the other, which is as one would 

 expect. The stimulus is for each bone to become independently broad, 

 which is apparently facilitated by non-fusion, and ultimately after the 

 entire abandonment of the land, for the formation of a toughly fibrous 

 connection of all the bony elements, which gives a desirable amount of 

 resilience. The antibrachial bones of the Sirenia, however, are anoma- 

 lous among aquatic mammals of a high degree of specialization. In the 

 dugong the radius and ulna are distinct and quite simple, without any 

 real indication of broadening. In the manati there is some slight ten- 

 dency toward flattening of the ulna and this bone is firmly fused at both 

 extremities with the radius, although the shafts of the two bones are 

 curved and quite wide apart. In Hydrodamalis there is little or no pro- 

 pensity for flattening but the extremities not only are firmly fused but 

 the shafts as well (fig. 40), save for one small interval that has doubt- 

 less remained as a foramen for the passage of nerves and blood vessels. 

 Hence one finds the unexpected condition that in this detail the Steller 

 sea cow does not resemble its (presumably) nearest relative, Halicore, 

 but the more distant Trichechus, and it must be inferred that in the first 

 and last mentioned genera there is no need either for pronation or 

 supination of the manus save what is possible through the carpus, while 

 presumably in the dugong some little rotation of this segment is possible. 

 Stronger extension of the antibrachium in the dugong is indicated by the 

 better definition of the olecranon. A synovial elbow joint presumably 

 of the normal sort is possessed by this order. 



In all known cetaceans, both living and fossil, the radius and ulna 

 are distinct and neither is appreciably reduced in comparison with the 

 other. Almost always the radius is slightly more robust than the ulna, 

 which might be expected because of its exposed position upon the an- 

 terior border of the arm, where it would encounter full water resistance ; 

 but at times (as in Platanista) , the ulna may be the larger. Apparently 

 zeuglodonts retained the synovial character of the elbow joint, and this 

 may also have been the case with some Miocene whales, for Eurhino- 

 delphis, and likely others, have articular surfaces upon the bones, al- 

 though these are of lessened area and definition, so that it is more logical 



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