BRITISH FOSSIL REPTILES. 67 



the present species the costal articular surface begins to descend 

 from the neurapophysis upon the body at the fiftieth vertebra, 

 and this and the succeeding vertebra may consequently be 

 reckoned as sacral ; their relative position to the dislocated 

 ileum verifies the supposition that the same character, with re- 

 ference to the costal articular surface, points out the sacral ver- 

 tebrae in the PL macrocephalus, as it does those of the PL 

 HawJiinsii. The surface which supported the spinal marrow 

 in the sacral vertebra is small and flattened, slightly impressed, 

 bounded by two gently curved lines, whose convexities are turned 

 towards each other. A comparison of the medwUary canal at 

 this part and at the cervical region shows that the usual law of 

 the increase of the spinal cord at the parts where larger nerves 

 were required to be given off to supply the locomotive extremi- 

 ties, obtained in the extinct Enaliosaurs, but only in the same 

 degree as in their existing cold-blooded congeners. 



In the caudal vertebrae the length of the centrum is to its 

 transverse diameter as 2 to 5. 



Extremities. — In the bones of the paddle, or radiated ap- 

 pendage of the pectoral arch, the following diiferences exist be- 

 tween the PL macrocephalus and PL Hawkinsii. The hu- 

 merus, as was before observed in the description of the PL 

 Hawkinsii, is less contracted at its proximal extremity, and 

 less curved backwards ; the anterior margin being rather con- 

 cave, instead of convex as in the PL Hawkinsii ; it is also 

 broader in proportion to its length ; its greatest breadth (at 

 the distal extremity) being more than half its length, while in 

 PL Haivkinsii it is less, being about f ths of the same length. 

 The distal end terminates in a slight but reguly,r convex curve, 

 while in PL Plawkinsii the separate facets for the radius and 

 ulna are distinctly marked, and meet so as almost to form an 

 obtuse angle. 



In the Plesiosaurs generally the radius is nearly straight, 

 while the ulna is bent with the concavity towards the radius : 

 both bones are flattened, as in other Enaliosaurs. In PL ma- 

 crocephalus the margin of the radius next the ulna is more con- 

 cave than in PL Haivkinsii, and the bone is relatively broader 

 at its distal extremity, which is terminated by a convex, instead 

 of a nearly straight, line. The ulna equals in length the radius, 

 as in the PL dnlichodeirus, while it never attains the same 

 length in the PL Haivkinsii ', it is also relatively broader than 

 in either the PL dolichodeirns or PL Haivkinsii ; and presents 

 a more regular reniform figure j the humeral articular surface 

 not being so straight, or so distinctly marked off from the outer 

 convex margin. The carpus consists, in the PL macrocephalusy 



V 2 



