284 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 6, 



number given is thirty-one* ; and thirty-one is stated to be the 

 number in this species in the same author's memoir on P. macro- 

 cephalus (p. 523). No less contradictory are the statements as to 

 the number of dorsal vertebrae. At pages 57 and 58 of the * Report ' 

 they are by implication estimated at twenty-five ; but at page 66 

 they are said to be twenty-three. I can nowhere find the slightest 

 indication that Prof. Owen imagines the number of cervical or dorsal 

 vertebrae to be variable in the same species of Plesiosaurus. The 

 opposed statements which I have quoted are wholly devoid of the 

 comment which would have been naturally evoked by the discovery 

 of so remarkable a fact. 



The specimens of Plesiosaurus Hawkinsii, on which the description 

 of the species, contained in the * Report on British Fossil Reptilia/ 

 is chiefly based, are, I believe, those now contained in the Collection 

 of the British Museum. Of these specimens three, viz. that num- 

 bered ^ and figured by Mr. Hawkins in his plate 24, that num- 

 bered 14,549 and figured in plate 28 of the same work, that num- 

 bered H,54i and figured in Hawkins's plate 27, are but little dis- 

 turbed, and retain the head and neck in situ. 



In a fourth specimen, 14,550, which is in many respects ex- 

 tremely valuable and instructive, the head is unfortunately displaced 

 and the anterior cervical vertebras are absent. 



Besides these four specimens, there is a fifth Plesiosaurus, num- 

 bered 2000 and named dolichodeirus ; it is however certainly either 

 Hawkinsii or Etheridgii, and I believe the latter, although the ab- 

 sence of the head and anterior cervical vertebrae renders it hazardous 

 to give a confident opinion. 



I will speak of these specimens in the order here named, under the 

 heads of Nos. 1, 2, 3, 4, and 5. But I must first remark, that no 

 one of them affords the means of determining the number of the 

 dorsal vertebrae with so much certainty as in P. Etheridgii. To 

 ascertain the number of the dorsal, or dorso-lumbar, vertebrae in 

 any vertebral column, it is obviously necessary that we should be 

 able to assure ourselves of these facts: — 1st, that we know which 

 is the last cervical ; 2nd, that we know the first sacral ; and 3rd, 

 that we know how many vertebrae intervene between these. 



In No. 1 the vertebral column is so obscured by the ribs and pec- 

 toral and pelvic girdles, that no one of these points can be ascertained 

 with accuracy. In No. 2 the anterior part of the sixth vertebra from 

 the skull is gone, and it is impossible to be certain that a whole ver- 

 tebra may not have disappeared ; at the same time an uncertain 

 number of vertebrae have been displaced from the middle region of 

 the back. 



* At least, this is the only conclusion consistent with the definition of a cer- 

 vical vertebra at page 58. Prof. Owen there proposes to consider as cervical those 

 vertebras whose centrum exhibits the whole or a part of the costal articular sur- 

 face. At page 57 he states with respect to P. Hawkinsii, "In the first or ante- 

 rior 31 vertebra; the centrum supports the whole or part of the costal pit." 

 Therefore, according to the definition, these 31 vertebrae are cervical. 



