AQUATIC MAMMALS 



grate toward the rear and there will very often be a broadening of the 

 superficial division of the biceps femoris, usually to cover most of the 

 shank. It may, or more often will not, be correspondingly broad at 

 origin. There will frequently occur a great increase in the power of 

 the semitendinosus, possibly accompanied by a similar development of 

 the gracilis, and semimembranosus, the insertions of all three extend- 

 ing farther distad toward the heel. 



If the animal be large and the feet need not be used for extensive ter- 

 restrial progression the pes will usually be webbed, and symmetrical 

 providing it be used in a symmetrical fashion, in which case it may be 



CALCANEU/A 

 ASTRAGALUS- 



CUBOID 

 SE5AWOIO 

 TARS /\\_ES- 



FIGURE 53. Dorsal view of left tarsus and metatarsus of (Z) sea-lion (Zalophus] 

 and (P) seal (Pboca}. 



expected that the first and fifth toes be of equal length and longer than 

 the middle three. If the animal be of small size the pes may be either 

 webbed or fringed, is less apt to be symmetrical, and the tendency will 

 most frequently be for the gradual enlargement of the member. No 

 matter in which active manner the Cetacea once employed the hind 

 limbs for swimming the present result could have been attained pro- 

 viding they were equipped with a relatively long and rather heavy tail 

 and that swimming methods entailed curvature of the backbone (and 



[308] 



