128 PROCEEDINGS OF THE ISTATIOjSTAL MUSEUM vol.73 



these bones end even and the articular surface of the fibula is almost 

 as extensive as of the tibia. Zalophus has few malleolar grooves 

 which are broad and shallow, but in Phoca these are exceedingly 

 deep, narrow, and pulley-like, which form has undoubtedly been 

 developed by the constant see-saw movements of the tendons per- 

 forming precisely repeated motions during swimming. 



The shank bones, especiall}^ the fibula, of Phoca are more ridged 

 and show more indications of every kind of strength of muscle. 

 Perhaps the most remarkable muscular modification of this region 

 is without osteological indications, however, and includes the ham- 

 string muscles — biceps femoris, gracilis, semimembranosus, and semi- 

 tendinosus. In the Zalophus the biceps is of a very remarkable 

 rhomboid shape. Insertion of the superficial sheet is fascial over 

 the proximal four-fifths of the shank, and origin is practically as 

 extensive over the anterior caudal spines. The result is that the 

 shank is bwimd down nearly in contact with the innominate and its 

 mobility is almost inhibited. Equally effective in this function but 

 less strikingly modified are the gracilis, semimembranosus and semi- 

 tendinosus whose insertions are distributed over the distal two-thirds 

 of the shank, and with origins confined to the posterior part of the 

 innominate. In Phoca, whose hind limb is apparently useless for 

 all purposes save lat^eral oscillations, one would expect the biceps to 

 be even more modified for binding down the shank, but such is not 

 the case. The muscle is of prime importance in adduction of the 

 shank, so although insertion is extensive over the shank (the proxi- 

 mal seven-eighths) for long leverage, the muscle is robust and tapers 

 to a tendinous origin from the superior spine of the ischium, where 

 it can act to good advantage as an adductor, as well as an elevator 

 to some extent. The other hamstring muscles are also strong, and 

 disposed in insertion much as in Zalophus; but origin, especially of 

 the anterior semitendinous, is not so entirely confined to the posterior 

 border of the innominate, thus giving a slightly greater length to 

 these muscles. The hamstring group are the only muscles which can 

 adduct the shank with any real power, and hence, are of fundamental 

 importance to the swimming of Phoca. They act in the most inti- 

 mate cooperation with the muscles of the lower back in performing 

 the lateral oscillations of the posterior end, and it must not be for- 

 gotten that because of the adpressed palmer position of the feet, 

 adduction of the ankle and foot of one side maj^ furnish much of 

 the impulse to abduct those of the opposite side. 



As mentioned elsewhere, there has been a slight movement proximad 

 of the origins of the flexor digitorum longus, and peronei longus and 

 brevis of these pinnipeds, and a movement distad of the origins of 

 the flexor hallucis longus and extensor digitorum longus. The stimuli 

 for these changes are obscure, however. Because the muscles of the 



