THE PELVIC LIMB 



fogale, Ondatra, Ichthyomys, Rheomys and Anotomys. They may occur 

 as a single fringe upon the outer border of the foot (Desmana, and 

 Galemys fig. 43, b), upon both borders, upon the toes only, or the 

 entire foot, including toes, may be heavily fringed (Ondatra, fig. 43, i). 

 Webbing may be either absent or present. Needless to say this fringing 

 either supplements webbing or else is a make-shift to take its place. 

 I say make-shift for I can hardly believe that it can be as efficient as 

 webbing, although it undoubtedly helps in propelling a small body 

 through the water. 



In many sorts of terrestrial mammals the bases of the toes are joined 

 by rudimentary webs, and it is only natural that as these are of such 

 importance in the economy of an aquatic mammal they should increase 

 in area. Evidently they are easily developed by all classes of vertebrates, 

 but why they should be lacking in such an essentially aquatic animal as 

 the muskrat remains a puzzle. 



The form and relative proportions of the toes of the above sorts of 

 aquatic mammals differ. In some the plan of these members is essen- 

 tially normal. In some there is lengthening of the hallux so that the 

 foot is practically symmetrical (Chironectes and Castor), and this is 

 undoubtedly the most suitable shape for such mammals as swim by mo- 

 tions that involve actions resulting in equal water force being applied 

 to both borders of the feet. But while it is true that most rodents and 

 insectivores are not as yet sufficiently aquatic for there to have been very 

 profound changes in the pedal equipment, it must not be assumed 

 that an asymmetrical foot falls short of the animal's requirements, for it 

 may employ some method of swimming that involves asymmetrical foot 

 action. 



Nails of the normal sort are present in the aquatic mammals men- 

 tioned and where differences occur these are undoubtedly attributable to 

 some habit that has nothing to do with the aquatic life. In Ornithor- 

 hynchus the nail of the first digit is straighter and more spike-like than 

 the others. In Chironectes the hallux nail is not claw-like as are those 

 of the other digits but is of the same general shape as the nails in man 

 and is little more than a callosity (fig. 43, c ) . 



The pes of the beaver (Castor) appears to be a more efficient paddle 

 than that of any existing rodent. It is large, broad and fully webbed, 

 and is operated by powerful muscles and heavy limb bones. Parsons 

 (1894) found that its calcaneal tendon is very readily separated into its 

 component parts. In this animal the nails of the first and second pedal 

 digits are slightly more slender than those of the others (fig. 43, h) . 



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