244 OUTLINES OF EVOLUTIONARY BIOLOGY 



flying membrane is formed chiefly of feathers, and the distal 

 part of the wing is supported mainly by the second digit ; 

 the first and third digits are greatly reduced, and the 

 other digits are absent. In all these different types of wing 

 the skeleton is again clearly of the pentadactyl type, modified 

 by enlargement, diminution or total suppression of certain of its 

 constituent bones. 



In the paddles of the modified aquatic mammals, such as the 

 sirenians, seals (Fig. 100) and whales (Fig. 101), the whole limb 



FIG. 100. Skeleton of right fore Foot (A) and right hind Foot (B) of a 

 Seal (Otaria hookeri), X &. (From photographs.) 



c. carpals ; m.c. metacarpals ; m.t. metatarsals ; ph. phalanges ; t. tarsals ; I V, digits. 



is very much shortened and flattened and the digits are enclosed in 

 a common integument so as to offer a greater resistance to the 

 water, but the skeleton still reveals the essential pentadactyl 

 features, and the same is true of those aquatic reptiles, the 

 turtles, plesiosaurians (Fig. 141) and ichthyosaurians (Fig. 142). 

 In the whales, however, a curious increase in the number of 

 phalanges (hyperphalangy) may often be observed, and in the 

 ichthyosaurians supplementary small bones are developed in 

 such positions that the entire skeleton of the paddle assumes 

 the form of a mosaic pavement. 



