290 THE VERTEBRATE SKELETON. 



The limbs are as a rule adapted for walking, but in Ichthyo- 

 sauria, Sauropterygia, Pythonomorpha and some Chelonia, they 

 have the form of swimming paddles, the relative size of the manus 

 and pes being increased, while that of the proximal and middle 

 portions of the limbs is reduced. This reduction is carried to its 

 furthest extent in the Ichthyosauria in which radius and ulna, 

 tibia and fibula, have the form of short polygonal bones similar 

 to those constituting the manus and pes. In the Pythono- 

 morpha the reduction of the limb bones is not quite so marked, 

 in the Sauropterygia it is less, and still less in the Chelonia. 

 In the earlier Ichthyosauria too, the limb bones are not so 

 short as they are in the later forms. The Ichthyosaurian limb 

 is also remarkable, firstly for the fact that both humerus and 

 femur are terminated by concave articulating surfaces instead 

 of by convex condyles, and secondly for the great multiplication 

 of the phalangeal bones, each digit being sometimes composed of 

 a series of over twenty. Sometimes too the number of series is 

 increased, either by the bifurcation of some of the digits or by 

 the development of marginal bones. In the Sauropterygia the 

 phalanges are likewise increased above the normal but not so 

 much as in Ichthyosauria. The humerus and femur of Sauro- 

 pterygia are noticeable for the enormous size of the terminal 

 epiphyses which form in each case by far the greater part of 

 the bone. 



THE ANTERIOR LIMB. 



The anterior limb is usually approximately equal in 

 length to the posterior, but in many Dinosauria it is consider- 

 ably the shorter of the two. The humerus is generally with- 

 out distinct condyles, but they are well developed in the Thero- 

 morpha, the Lacertilia and Sphenodon. 



In the Therornorpha, some Rhynchocephalia, and some 

 Sauropterygia, such as Mesosaurus, the humerus has an ent-epi- 

 condylar foramen ; in Lacertilia, Chelonia and some Dinosauria 

 there is an ect-epicondylar foramen or groove ; Sphenodon 



