BRITISH FOSSIL REPTILES. 59 



8 to 5 through the greater part of the dorsal region. The tail 

 includes 2^ lengths of the head, and from the posterior end of 

 the ischia consists of 35 vertebrae. The same general form and 

 proportions of the vertebrse are preserved throughout the verte- 

 bral column ; and it is this fact, established upon a comparison 

 of four nearly entire specimens in the collections of the British 

 Museum and of Mr. Hawkins, which enables me to speak with 

 confidence of the specific importance of well-marked characters, 

 -though they may be afforded by detached vertebrae only. 



I have few observations to offer on the specific peculiarities 

 of the head of the PL Hawk'msii, as it is principally from the 

 perfect specimens of this species that the description of this 

 part in the general account of the Plesiosauri has been taken. 

 Its specific character will be manifest when I come to compare 

 with it the head of the PL macrocephalus. I may remark here, 

 however, that the orbit occupies a position halfway betv/een the 

 occipital condyle and the end of the snout. The bones have a 

 smooth surface, except at the anterior part of the head, where 

 there are many pits and grooves, like those in the head of the 

 Crocodile. The teeth participate in the general character of the 

 Plesiosaurian type, being long, slender, slightl)^ recurved, finely 

 but distinctly grooved in the longitudinal direction on the outer 

 surface, w^ith a long pulp-cavity within. There are about 40 

 teeth on each side of the upper and 35 on each side of the lower 

 jaw : those towards the anterior extremities of the jaws are 

 longer than the rest, but the disproportion is more strongly 

 marked in other species than in the present. 



EA'tremities. — The fore and hind extremities of this species 

 are nearly equal in size, but the latter are a little longer. 



The pectoral arch accords with the general Plesiosaurian 

 type. 



The sternum has no median process ; it presents a well-marked 

 concavity anteriorly. 



The anchylosed scapula and clavicle form a triradiate bone, of 

 which the scapular portion is short and compressed, directed 

 obliquely backwards at an angle of 45° with the clavicular part, 

 and equalling two thirds of the extent of this bone. 



The coracoid equals in antero-posterior extent eight of the 

 cervical vertebrae immediately anterior to it. 



The humerus equals in length 6^ of the posterior cervical 

 vertebrae : its anterior margin is slightly convex : the breadth 

 of its distal flattened extremity equals twice its length. 



The radius is longer than the ulna, the breadth of which docs 

 not quite equal its length. 



Six appears to be the normal number of carpal bones. 



