THE ICHTHYOSAURIA, 213 



end of the humerus presents two facets, which articulate with 

 a couple of short flattened polygonal bones, which represent 

 tlie radius and the ulna (7?, U). To these succeed two rows 

 of smaller polygonal ossicles in the place of a carpus : three, 

 representing the radiale, inter medimn^ and ulnar e (r. i. ii.)^ 

 lie in the proximal row, and three or four carpalia ( Q:>.) in 

 the distal row. With the distal carpal bones are connected, 

 by means of the metacarpal ossicles [Wc), longitudinal series 

 of very numerous polygonal bones, adapted together by their 

 edges, and becoming gradually smaller toward the distal ex- 

 tremity of each series. The number of complete series does 

 not exceed five, and may be reduced to three — so that the 

 paddle may be pentadactyle, tetradactyle, or tridactyle. An 

 apparent multiplication of the number of digits arises from 

 two causes : First, the occasional bifurcation of some of the 

 digits ; secondly, the superaddition of marginal hones * to the 

 radial and to the ulnar edges of the manus [in.i/., m.r.). There 

 is thus formed a paddle, which is unlike either that of a Ce- 

 tacean, or that of a Plesiosaiirus^ or that of a Turtle — depart- 

 ing more than any of these structures from the ordinary form 

 of vertebrate limb. 



There is no trace of any sternum behind the pectoral arch, 

 but the abdominal walls were strengthened by a number of 

 transverse arcuated bones, similar to those observed in the 

 Plesiosauria^ though not so strong. Each is composed of a 

 median piece with pointed ends, and of some three, or more, 

 lateral pieces, overlapping each other's ends, on each side, 



(Fig. 76, C, Y'^') . 



The pelvis (Fig. 76, F) is not connected by bone with the 

 vertebral column. It consists of an ilium (jT/.), an ischium 

 (J^.), and a pubis {JPh.)^ uniting together to form an acetabu- 

 lum, while the pubis and ischium of each side meet in the mid- 

 dle line. The ischium is a narrow and almost rod-like bone, the 

 pubis is somewhat broader, especially at its symphysial end. 



The hind-limb (Fig. 76, D) has substantially the same 

 structure as the fore-limb, but is always smaller, and generally 

 of much less size. 



The only other bony structure apiDertaining to IchtJiyosau- 

 nis that need be noticed, is a circle of plates developed in the 

 sclerotic of the enormous eye, which is frequently met with in 

 a very perfect state of preservation. 



* 1 leave open the question whether these series of marginal ossicles are 

 remains of the digits of a pel} dactyle manus, such as exists in the Elasmo* 

 branch fishes. 



