120 PROCEEDINGS OF THE NATIONAL MUSEUM tol. 73 



been shifted distad and of the fifth, both proximad and laterad, 

 making the latter more opossable than is the pollex. The same 

 agency that has operated to constrict the proximal part of the carpus 

 has crowded the carpales so that these bones have become somewhat 

 pyramidal and fit together so as to occupy the minimum of space. 



In the Zalophus the manus externally is a rather broad and long 

 paddle, tapering almost to a point, thicker upon the cranial or radial 

 border and thin caudad. It is tough and elastic but the articulations 

 of the metacarpals and phalanges are not as freely mobile as in many 

 mammals. The palmaris longus is developed to a phenomenal degree 

 and probably has an involuntary action in resisting an undesirable 

 amount of extension of the manus during swimming, and by means 

 of its insertion chiefly upon digits 1 and 5, in maintaining a slightly 

 concave palmar surface. Extension of the pollex beyond the manual 

 axis is largely inhibited also by all the other flexors which go to this 

 digit and the interaction of the tendons concerned, as well as those 

 of the remainder of the manus, is probably complicated and well 

 developed for best efficiency. Specialization of the palmar tendons 

 is not so strikingly marked in Phoca, but there is a broadening of the 

 flexor carpi radialis to form a second deeper palmar fascia, and the 

 middle part of the flexor digitorum communis has broadened 

 greatly. 



Such highly specialized aquatic animals as the whale and the 

 turtle retain a short pollex, but this is not the case in the Pinnipedia. 

 In the Phoca this digit is the longest and the most robust, the others 

 being evenly and slightly subequal in sequence toward the fifth. In 

 Zalophus the pollex is much the longest and much the most robust, 

 while the fifth digit is relatively short. The flipper therefore tapers 

 rather gradually to a point, and the entire axillary border is very 

 thin — a condition which may be presumed of importance to the 

 animal. The metacarpals and phalangeal bones of the Otariidae 

 only are slightly flattened, which is a character which probably 

 inevitably, sooner or later, follows the assumption by the manus of 

 a paddle shape. This flattening is especially pronounced in the 

 terminal phalanges of Zalophus. As mentioned elsewhere there are 

 cartilagenous extensions of the digits in the Otariidae. The stimulus 

 for this has evidently been at least to some degree attributable to a 

 need for a longer manus having outstripped the lengthening ability 

 of the phalanges The subject of the formation of these cartilages 

 can not of course be exhausted with any such casual statement, but 

 a number of theories which may be advanced to account for them are 

 too speculative to merit acceptance at the present time. 



There are interdigital membranes in Phoca while in Zalophus these 

 have developed to the point where the whole manus is virtually a 

 homogeneous paddle. This webbing need not be discussed, for it, 



