SKELETON 



481 



between them. They articulate with three carpal bones, the ulnare, 

 intermedium, and radiale, while a small pisiform bone lies on the 

 outer side, between the ulnare and the outer distal carpal bone. 

 In Ichthyosaurus, from the Liassic period onwards, the ulna and 

 radius are much shortened, broader than long, and touch each other 

 without any intervening space ; the pisiform element is enlarged. 

 Lastly, in Ophthalmosaurus of the Middle Oolite (but not in con- 

 temporary species of Ichthyosaurus) the ulna and radius are still 

 more reduced, and the pisiform has moved up to the humerus, so 

 that the latter articulates with three bones. 



B 



FIG. 114. A, Ventral view of the shoulder-girdle and right fore-limb of an Ichthyosaurus, 

 from the Lias ; B, part of the fore-limb of a Mixosaurus, from the Trias ; C, part 

 of the fore-limb of an Ophthalmosaurus, from the Chalk. c 1; c 2 , first and second 

 centrale carpi ; Cl, clavicle ; Co, coracoid ; H, humerus ; /, interclavicle ; i, inter- 

 medium carpi ; .p, pisiform ; R, radius ; r, radial carpal ; Sc, scapula ; U, ulna ; 

 u, ulnar carpal. 



Other important features of these paddles are not only the 

 much -increased number of phalanges (sometimes up to twenty or 

 more), but also the increase of digits to six or more, produced 

 apparently by a splitting of the third finger into two series, and 

 by the development of additional rows of phalanx-like bones 

 on the outer and inner margins of the paddle. This increase of 

 fingers exists, for instance, in Ichthyosaurus communis, but not in 

 /. tenuirostris. Owing to this peculiar development of paddles the 

 constituent bones are extremely numerous, and from the radius 

 and ulna downwards they are all closely packed, and have 

 assumed a polygonal, often hexagonal, shape, dwindling to more or 



VOL. VIII 2 I 



