20 A BOOK OF WHALES 



digits which have but three phalanges, the thumb 

 indeed possessing but two. In whales the number 

 of phalanges may reach so great a number as seven- 

 teen. In the fish fin, on the contrary, the required 

 area is obtained : firstly, by the multiplication of rays, 

 and, secondly, by the continuation of the fin as an 

 expansion supported by the horny or calcified fin 

 rays, which have nothing to do with the cartilages 

 of the fin, but are exoskeletal structures. Rarely, as 

 in the Batoid fishes (Skates), the cartilages of the fins 

 increase and the horny fin rays disappear. 



The closest analogy with the whales is offered 

 by those extinct aquatic reptiles, the Ichthyosauria. 

 Like the whales, they are clearly to be derived from 

 terrestrial reptiles ; there is no suggestion that is at 

 all tenable that they have sprung separately, on their 

 own account, from fishes. Their hand is still further 

 advanced than is that of the whale, but along the 

 same lines. There are, it is true, only five fingers, 

 of which the last is split into two, so far resembling 

 the whales ; but the number of phalanges is great 

 in all these fingers ; not only is the hyperphalangy 

 of the ichthyosaurian manus more pronounced than 

 is that of the Cetacea, but the individual elements 

 are less separable by their distinctive characters. 

 A recognisable humerus is followed by a series of 

 bones which can hardly be classified into radius and 

 ulna, carpus and metacarpus, by their position and 

 relations, so much alike are they in general appear- 

 ance. 



But it must be noted that the number of phalanges 



