THE PECTORAL LIMB 



other three digits has a pair of such thickenings, however, which diverge 

 very shghtly as they extend distad. Apparently they all have a sort of 

 hinge at the base, for the predigital part of the membrane is folded 

 back against the palm well out of the way when the animal is not only 

 scratching and digging, which is accomplished chiefly by means of the 

 well developed claws, but also during terrestrial progression. The 

 claws have undoubtedly been retained for these purposes. Wood Jones 

 has given the length of the digits as in the order of 4, 3, 2, 5 and 1, 

 but it is seen that their development is to all intents bilaterally sym- 

 metrical. 



It is not at all easy to determine from the literature the precise ac- 

 tions of the platypus manus that are involved in swimming. From an 

 examination of the skeleton it seems that the humerus must operate 



Figure 40. Bones of the right fore arm of the Steller sea cow {Hydrodamalis). 

 from a specimen in the U. S. National Museum. 



chiefly in the transverse vertical plane, and the fore arm mostly parallel 

 to the body axis and also in the vertical plane. Hence the manus prob- 

 ably operates on the thrust-and-recovery method, rather than on some 

 system which involves oblique action against the water. 



In some important respects the aquatic stimuli encountered by the 

 hmbs of the capybara and the hippopotamus are similar. It is true that 

 the former animal lacks the markedly broad beam of the latter, which 

 makes it convenient for the hippopotamus to counteract the tendency 

 for the alternate kicks of the hind legs to deflect its course first to one 

 side and then to the other. But both lack a functional tail and in 

 neither has the development of one pair of limbs gained decided ascend- 

 ency over the other pair, which may be considered as somewhat unusual 

 in mammals of their degree of aquatic predilection. Apparently, as 



[245] 



