526 ON A NEW SPECIES OF PLESIOSAURUS 



in this species in the same author's memoir on P. macrocephaliis 

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

 of dorsal vertebras. 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 oi Plesiosaurus Hawkinsii, on which the description 

 of the species, contained in the ' Report on Britist Fossil Reptilia,' is 

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

 the British Museum. Of these specimens three, viz. that numbered 

 ~ and figured by Mr. Hawkins in his plate 24, that numbered '■•■549 

 and figured in plate 28 of the same work, that numbered '4.54i and 

 figured in Hawkins's pla,te 27, are but little disturbed, and retain the 

 head and neck in situ. 



In a fourth specimen, m.ss", which is in many respects extremely 

 valuable and instructive, the head is unfortunately displaced and the 

 anterior cervical vertebrae are absent. 



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

 bered =«" and named dolichodeiriis ; it is however certainly either 

 Hawkinsii or Etheridgii, and I believe the latter, although the absence 

 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. i, 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 

 vertebrse with so much certainty as in P. Etheridgii. To ascertain the 

 number of the dorsal, or dorso-lumbar, vertebrje in any vertebral 

 column, it is obviously necessary that we should be able to assure 

 ourselves of these facts : — ist, 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. I the vertebral column is so obscured by the ribs and 

 pectoral and pelvic girdles, that no one of these points can be ascer- 

 tained 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 

 vertebra may not have disappeared ; at the same tirhe an uncertain 

 number of vertebrae have been displaced from the middle region of 

 the back. 



