AQUATIC MAMMALS 



to be available for examination, the alteration of the foot posture in 

 the direction of pronation, or tendency for the elevation of the outer 

 border of the pes. The aquatic shrews Neosorex and Atophyrax, the 

 genus Galemys, and the muskrat (Ondatra) all show this pronation to an 

 extent of about 30 to 45 degrees. This is accomplished not through 

 the ankle joint, or even, apparently, in the tarsus, but through the meta- 

 tarsus and digits, thus affecting mainly the part of the foot which fur- 

 nishes the chief propulsive power, and is very noticeable in the alcoholi- 

 cally hardened specimens. Just what degree of further pronation, or 

 of supination, from this posture is possible in life I do not know. Nor 

 is it possible to determine the precise reason for this modification. All 

 that can be said at present is that it is an adaptation by means of which 

 these animals can employ the feet in a more effective manner for the 

 purpose of swimming, for the action of the feet against the water de- 

 pends entirely upon the position in which the legs are held. If the axis 

 of the femur be maintained in the vertical plane then the feet would also 

 be kicked in this plane, and the pronated posture of these members 

 would tend to force the hinder part of the body mediad at each stroke, 

 thus operating to neutralize the propensity which the anterior end 

 would otherwise have of swinging toward the opposite side after each 

 vigorous kick. Or if the femur and pes be held largely in the horizontal 

 plane during swimming, then the pronated foot posture would tend 

 to force the rear of the animal in a downward direction, which would 

 have to be overcome by some antagonistic body posture or action. 



The pronated foot posture of these mammals, and of Ornithorhynchus 

 as well, results in a slight rotation of some of the toes so that when 

 the digits are slightly flexed the nails of the more lateral toes have a 

 tendency to point in a lateral direction. 



In feet of small aquatic mammals a comparatively early modification 

 is the acquisition of webbing between the toes or a fringe of stiff, bristly 

 hairs upon the sides of the foot and toes. The latter is more often en- 

 countered, but such fringes would be of little or no help in the case of 

 a body of large size and in such instances they are never encountered. 

 It seems that it is easier for a small foot to develop fringes than webbing 

 but it is utterly impossible to state the reason for this difference. Ap- 

 parently it is just a case of two mammals being able to respond differently 

 to the same stimulus, as seems to be the case so frequently in other details. 



There is often a great difference in the precise character of the bristle 

 fringes. They are present in varying definition and form in Desmana, 

 Galemys, Neosorex, Atophyrax, Neomys, Chimarrogale, Crossogale, Nec- 



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