SKELETON 



475 



ventral surface of the clavicle ; the latter and the irregularly 

 T-shaped interclavicle being, however, still visible from below. 

 InCryptocIidus the two acromial processes meet each other and form 

 a long ventral symphysis, which meets that of the much-enlarged 

 coracoids, the latter enclosing with the scapulae a pair of roundish 

 foramina. The clavicles are not visible from below ; they rest upon 

 the dorsal surface of the scapular symphysis, and the inter- 

 clavicle seems to be suppressed. Young Cryptoclidus (Fig. 113, B) 

 and various species of Plesiosaurus show intermediate con- 

 ditions. 



This unique arrangement is correlated with the enormous 







Fig. 11.3. —A, Restored outliues of a Plesiosaurus, x -^V ; B, dorsal view of the pectoral 

 arch of an immature Cryptoclidus, from the middle Oolite ; C, fore-limb of a Plesio- 

 saurus, from the Lias. A, Acromial process of scapula ; 01, clavicle ; Co, cora- 

 coid ; H, humerus ; i, carpale intermedium ; Mi to m^, first to fifth metacarpals ; 

 p, pisiform bone ; R, radius ; r, radial carpal ; S, scapula ; U, ulna ; u, ulnar carpal. 



development of the fore-limbs, although nothing of the kind has 

 taken place in the Ichthyosauri, which have similar large 

 paddles. The limbs exhibit considerable differences in the 

 various groups of Plesiosauria, but they are all pentadactyle. In 

 the oldest, the Mesosauri and :NTothosauri, the limbs are still of 

 the terrestrial type, although fitted for swimming; the chief 

 bones are still slender and elongated, and none of the five 

 fingers and toes have more than five phalanges, the usual 

 number of which seems to be 2, 3, 4, 5, 3 for the first to fifth 

 digits respectively. In the Plesiosauri the limbs are trans- 

 formed into long hyperphalangeal paddles, unfit for progression 

 on land, rather like those of the Ichthyosauria, with much 



