ADAPTIVE MODIFICATIONS OF LIMB SKELETON 467 



In the Cetacea it is of rare occurrence, and never more than 

 one extra digit is present. These extra digits always appear on 

 the posterior side of the paddle. Baur offered the suggestion 

 for the Ichthyosaurs that the sixth digit was produced by 

 elongation of the pisiform or its homolog, but there is no direct 

 evidence in favor of this view, while the breaking up of the 

 digits toward the end in such a form as IcJitliyosaurus communis 

 indicates rather that they have been produced by a splitting 

 of the digits. At any rate Baur's suggestion would not be suffi- 

 cient to cover cases where more than one extra digit appeared. 

 Kiikenthal has shown in the case of the whales that the sixth 

 digit is produced directly by the longitudinal splitting of the 

 fifth. 



As to the reason for the appearance of extra digits, Kiiken- 

 thal has given what appears to me to be a very satisfactory ex- 

 planation, and one for which he, together with Leboucq, has ad- 

 vanced considerable evidence in the whales. The extension of 

 the swimming membrane on the posterior margin calls for extra 

 support which is at first met by a widening of the last digit. 

 Finally, retarded ossification causes a separation of the digit 

 into two parts in somewhat the same way as the double epiphyses 

 are separated from the diaphysis in hyperphalangy. Thus the 

 need of widening the supporting area of the posterior border, 

 together with the tendency toward the formation of smaller 

 skeletal parts in the swimming limb, would account for the ap- 

 pearance of the additional digits. 



Formation of Smaller Skeletal Parts. 



This principle, which, I believe, Kiikenthal was the first to 

 mention, is exhibited by nearly all forms which have progressed 

 so far in aquatic adaptation as to lose their land relation. It is 

 best seen in the Ichthyosaurs, but the Cetacea, Plesiosaurs and 

 later Mosasaurs also show it well. It is about coextensive with 

 hyperphalangy by which it is in part produced, but it also in- 

 volves the direct reduction in size of certain parts and a con- 

 centration towards the proximal end of the limb. The purpose 

 is evidently to produce a limb that shall be uniformly flexible — 



