132 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 73 



It is, however, in the phocid astragalus that interest chiefly centers. 

 Its articular facets differ from those of Zalophue as already dis- 

 cussed, but in addition there extends caudad a large, long process — an 

 accessory heel, as long and almost as deep as that of the calcaneum. 

 It is broadly grooved caudo-ventrad for the passage of the stout 

 tendon of the flexor hallucis longus, and it is the strong tension of 

 this muscle that primarily, if not solely, inhibits the assumption of 

 a plantigrade posture by the foot in this animal. This is, perhaps, 

 the most significant single detail of the specialization of the Phocidae. 

 "Why is it that the hallucis longus was so highly and peculiarly 

 developed to flex the foot with a considerable leverage to a somewhat 

 excessive degree rather than the muscles going to the calcaneum, 

 with equal leverage ready to hand is a puzzling circumstance. It is 

 likely, however, that one important factor was that the hallucis, as it 

 occurs in this animal, is also well fitted for facile flexion of the digits. 

 In its distal portion also the astragalus differs much from the con- 

 dition in Zalophus. Whereas in the latter the facet for articulation 

 with the centrale is prominent and highly convex, and with a pro- 

 tuberant process mediad, in the Phoca this facet — very much 

 smaller — is flattish and somewhat irregular. In addition, the distal 

 part of the bone is rendered much more narrow by the absence of the 

 medial process, probably caused partly by the more dorsal position 

 in this animal of the tendons of the extensor hallucis and tibialis 

 anticus, and partly by the reduction in width of this part of the 

 tarsus. By the shape of this facet and its relation in regard to the 

 cuboid the excessive amount of movement of which this tarsal joint is 

 found to be capable is evidently facilitated, and the narrowness of the 

 tarsus at this point permits a certain amount of motion in the trans- 

 verse plane. Another point worth mention is that as the calcaneum 

 and astragalus are of the same length, they can be, and are, closely 

 bound together by ligaments and therefoie together constitute an 

 unusually solid base for the remainder of the pes. 



The lateral side of tlie cuboid has a groove for the passage of the 

 jDeroneus longus tendon that is completely roofed over by a process 

 of the bone in contact with metatarsus 5. These tarsal grooves, ab- 

 sent as such in ZaJophus and formed exclusively by the peroneal 

 muscles in Phoca^ are an indication of unusual and persistently re- 

 curring contraction of these muscles. In the trailing position in 

 which the feet of this animal are ahvays carried, action of the 

 peroneal group results exclusively, if we except the usual binding 

 action upon the tarsal bones of the peroneus longus, in elevation of 

 the feet, and therefore to some slight degree as an aid to the spread- 

 ing of the digits. These facts suggest that there is somewhat more 

 of a twisting, sculling movement of the rear flippers in swimming 

 than one is able to distinguish during observation of a live individual. 



