ABT. 15 ANATOMY OF THE EARED AND EARLESS SEALS HOWELL 131 



and the opposite in Phoca; but adpression of the palms is assumed 

 with ease in both animals, and much pronation and supination is pos- 

 sible from this position. There is but slight pronation or supination 

 through the actual ankle joint of either animal, but the articulations 

 distad are loose and movement much facilitated, especially in the 

 phocid. 



In the Zalophiis the flexion-extension action of the ankle proper is 

 through an arc of about 75° and of the tarsus through 10° or 15°, 

 allowing the foot to assume a plantigrade position or to trail in the 

 same plane with the axis of the shank. In Phoca^ however, condi- 

 tions are very different. Movement at the ankle joint is from the 

 extreme trailing position through an arc of only 30° toward planti- 

 gradism, the inhibitional factor being the tendon of the flexor 

 hallucis longus, as discussed later. In the oscillating swimming 

 movements of the feet of this animal it is requisite, however, that 

 there be more play and flexibility than such an ankle joint will allow, 

 and this is attained through the articulation between the astragalus- 

 calcaneum and centrale-cuboid. And this joint, called the tarsal 

 joint, is actually more flexible than that of the true ankle, this being 

 through an arc of 65° or more. The result is that the mechanism 

 of the phocid pes is very handlike, and the tarsus may be flexed 

 at a right angle to the axis of the heel, thus assisting by a follow- 

 through movement the adductor motion of the distal leg in swim- 

 ming. In the otariid the tibial facet of the astragalus is prolonged 

 onto the neck but stops short of the junction with the heel. In the 

 phocid this condition is just reversed, showing that the joint is con- 

 siderably better fitted for maintaining the foot perpetually in a 

 trailing posture. In the entire specimens, however, there is no appre- 

 ciable difference in this respect, which is a further instance of the 

 fact that from observation one can not always tell what a structure 

 is best fitted to do. 



Comparative osteology is not yet at the point where the signifi- 

 cance of slight changes in the interrelationship of the tarsal bones are 

 perfectly understood, and not a great deal can be said with confidence 

 regarding the structure of the otariid tarsus. That of Phoca, how- 

 ever, is somewhat more illuminating. The calcaneum, is rather 

 weak but is deeply grooved for the passage of the peroneal tendons. 

 Partly accountable for the lack of strength in the posterior process 

 of this bone is the fact that the phocid has no soleus and that in this 

 animal as well as the Zalophus the plantaris is entirely distinct 

 from the "tendo calcaneus" and passes mediad to it. In fact, in 

 view of the permanently extended position of the foot, together with 

 the development of the flexor hallucis longus, one would rather 

 expect to find that this process was still more reduced in the earless 

 seal. 



