Structure of the Plesiosaurus Macrocephalus. 517 



the root of the neck : and the seventh, eighth, and ninth cervical vertebrge are 

 displaced, and their component parts dislocated, so as to bring- into view the 

 upper surface of the bodies. 



The dorsal vertebras are also inclined to the leftside, presenting principally 

 their sides to the observer. Dislocation of the component parts of all these 

 vertebrae has taken place to a greater or less degree, but principally in the 

 lumbar and sacral regions. 



A great proportion of the caudal vertebra is lost ; and of those that re- 

 main, all the apophyses are separated from the bodies. 



The fore and hind paddles of the left side are in a beautiful state of pre- 

 servation. The greater part of the right fore-paddle is also present; but 

 there is no trace of the hind paddle of the corresponding side, excepting the 

 dislocated portions of the pelvic arch to which it was attached. Owing to the 

 position of the skeleton, the structure of both the pectoral and pelvic arches is 

 more or less obscured. 



Of the Trunk. 



The number of vertebrae, from the atlas to the first caudal inclusive, is 

 fifty-two. They are generally characterised by having a less antero-posterior 

 extent of body, and a greater antero-posterior extent of the spinous process, 

 than in the Plesiosaurus Hawkinsii. Those vertebrae, as the seventh, eighth, 

 and ninth cervical, in whfch I have been able to examine the under-surface 

 of the body, have presented a broad and slightly-produced longitudinal ridge 

 along the middle line ; a character, however, which may be observed in a 

 slighter degree in the cervical vertebrae of the PL Hawkinsii. 



In the generic characters afforded by these parts of the skeleton, the ver- 

 tebrae of the Plesiosaurus Macrocephalus correspond with those which Mr. 

 Conybeare has described in the original and admirable memoir * devoted to 

 the determination of this most interesting genus of Enaliosaurs. 



Before, however, proceeding to compare the vertebrae of the several regions 

 of the spine in the Plesiosaurus Macrocephalus and Plesiosaurus Hawkinsii, 

 I would offer a few observations on the composition of a vertebra in the 

 abstract. 



At the commencement of my examination of the fossil remains of the Enalio- 

 saurians, I endeavoured to apply to the parts of the vertebrse, (which in these 

 animals are frequently complicated, and with the elements more or less dislo- 

 cated) the views and nomenclature of M. Geoffroy St. Hilaire, whose analysis 



* Geol. Trans., Lond., vol. v., part 2. 

 3x2 



