58 REPORT — 1839. 



In consequence of the unequivocal presence of ribs through- 

 out so great a proportion of the vertebral column, the ordinary 

 characteristics of cervical, dorsal, lumbar, and caudal vertebrae 

 are wanting, and a definition founded on the relative size or form 

 of the costal elements becomes, from the gradual manner in 

 vi^hich they alter in these respects, very ambiguous and difficult 

 in its application. I have therefore proposed, in order to gain 

 a surer point of comparison of the different species of Plesio- 

 sauri, to reckon those vertebrae as cervical in which the centrum 

 exhibits the whole or a part of the costal articular surface. Tlie 

 body of a cervical may always be distinguished from that of a 

 caudal vertebra in being without any trace of haemapophysial 

 pits. The dorsal vertebrae are those in which the costal surface 

 is situated wholly on the neurapophysis. The caudal vertebrae 

 are characterized by having both costal and haemapophysial 

 impressions on the body, except the terminal ones, which are 

 readily distinguished by their small size, the absence of both the 

 above-named impressions, and by the concave character of the 

 articular surfaces of the bodies. 



The cervical vertebrae present the following characters in the 

 species under consideration : taking the transverse diameter of 

 the body of the vertebra at 10, the vertical diameter of the same 

 is 9, and the antero-posterior 8. The articular surfaces present 

 the normal Plesiosaurian character, being slightly concave, with 

 a gentle convex rising in the centre of the concavity. 



The exposed or nonarticular surfaceof these vertebras is smooth. 

 The costal pit is longitudinally elliptical, situated near the lower 

 part of the centrum in the anterior two thirds of the cervical 

 region, and having a space equal to its vertical diameter inter- 

 vening between it and the lower extremity of the neurapophysis : 

 and here I may observe that the character afforded by the rela- 

 tive extent of this space is a very useful one, as it is variable in 

 the species but constant in each ; and as it is indicated by the 

 centrum alone, it serves as a term of comparison when the other 

 elements of the vertebra may be lost. The articular base of the 

 neurapophysis is bounded below by two lines meeting at an open 

 angle. From tlie apex of this angle to the articular process the 

 distance is less than the extent of the centrum below the apex. 

 The spines are compressed throughout, slightly curved back- 

 wards, with the anterior angle and apex rounded off. This cha- 

 racter is gradually changed at the base of the neck for a quadrate 

 form of the spine with a straight truncate apex ; and towards 

 the posterior part of the dorsal region this apex is slightly 

 thickened. 



The height of the spine is to its antero-posterior diameter as 



