THE PECTORAL LIMB 



is rather basic. In the former case the cartilages are added as fast as 

 there is room for them between the original digit tip and the slowly 

 extending flipper border. In the sea-lion it appears probable that the 

 flipper border extended considerably beyond the phalanges before there 

 was any indication of cartilaginous stiff^ening. This suggests that com- 

 paratively recently, geologically speaking, the latter animals were in the 

 habit of folding back the predigital part of the flipper in precisely the 

 same manner as they now fold back the predigital part of the pes. 



Leboucq (1889) claimed to have found a thickening of the cetacean 

 epidermis above the digits well back from the flipper border. This he 

 interpreted as the remnant of a nail, and Kiikenthal held the same view. 

 I believe this is no longer considered as probable, if for no other reason 

 than that after the disappearance of a true nail any thickening of the 

 epidermis in the same situation would speedily be obliterated. Assum- 

 ing that the vestige of nails still persists, however, there has been con- 

 siderable argument over the question of where such rudiments would 

 be situated — whether upon the flipper border of farther back. The an- 

 swer to this hypothetical question depends entirely upon what one be- 

 lieves to have been the origin of hyperphalangy. If one subscribe to the 

 belief that this had intercalary origin of some sort then he must also 

 believe that the true ungual phalanx, and therefore any rudiment of 

 nail, is situated upon the flipper border. If he entertain the conviction 

 that accessory phalanges were added distad of the original digit, then 

 must he argue that any possible nail rudiment must be situated above 

 the fourth phalangeal element distad of the carpus. 



[ 267 .1 



