THE PELVIC LIMB 



the character of those of the sea-hon, with cartilaginous extensions of the 

 digits, and in form somewhat resemble those of the seal. Apparently 

 they are rather intermediate in their uses as well. 



The hind feet of the platypus are much less modified than the anterior 

 ones, and in a somewhat different way. In all probability all four feet 

 were formerly used for propulsion, on the mud-turtle principle, until 

 the tail had become flattened and by more frequent use the fore feet 

 were enabled to gain developmental ascendancy. Now the hind feet ap- 

 parently are but rarely used to assist propulsion, but rather is their ac- 

 tion comparable to that in the marine turtles — mainly for vertical and 

 to a lesser extent for horizontal steering, in the former direction aiding 

 the tail in depressing, or in elevating the body. They are fully webbed 

 and articulation of the four lateral toes is such that these digits flex in a 

 direction more toward the outer border of the foot and away from the 

 hallux than in a strictly palmar direction. A variation in this flexional 

 peculiarity has been noted in the case of Galemys, Sorex, Phoca and 

 Ondatra, the advantage evidently being either for attaining a slightly 

 different angle of the membranes with respect to the articulation of the 

 ankle, or more probably so as more easily to secure the maximum ex- 

 pansion of the membranes. 



For the purpose of more readily evaluating the differences between the 

 pelvic limbs of seals and sea-lions these two groups of pinnipeds will be 

 discussed together under the present heading. But first it should again 

 be mentioned that seals swim by oscillations from side to side of the 

 hind feet, placed palm to palm, and of the entire hinder end of the body, 

 and that the hind feet are useless in terrestrial progression for the reason 

 that the sole cannot be placed flat on the ground. The sea-lion, however,, 

 can place the sole upon the ground, but only by curving the sacral ver- 

 tebrae in a position perpendicular to the surface. In the water the feet 

 are trailed, usually sole to sole much as in the seal, but they seem 

 never to be used in the slightest degree as an aid to purposeful propul- 

 sion. When playing or lolling about in the water the feet may be flexed 

 in all manner of positions, assisting to alter the posture of the body, and 

 I have watched a young fur seal progress very slowly by repeatedly roll- 

 ing the entire body over and over, the hind feet thus acting to some ex- 

 tent in a cork-screw manner as a propeller; but such inconsequential 

 actions need not be considered. 



In scrutinizing the influences to which the pelvic limbs of the Pinni- 

 pedia have been subject one must begin with the spinal musculature. 

 In the Otariidae the latter is not appreciably modified and need not be 



C 291 1 



