1858.] HUXLEY PLESIOSAURUS. 283 



and their ends truncated. The three or four last caudals have ap- 

 parently neither spines nor neurapophyses. 



There are more than thirty named species of Plesiosaurus. Of 

 these, however, far more than half are founded upon detached bones, 

 and I am not aware that entire, or nearly entire, specimens of more 

 than four species, viz. dolichodeirus, Hawkinsii, macrocephalus, and 

 brachycephalus, have as yet been described. This point is worthy 

 of notice, when we consider that the proportion of the head to the 

 body constitutes an important datum in the determination of the 

 species of this genus. I will compare P. Etheridgii first with those 

 of which complete or nearly complete skeletons have been ob- 

 served. 



In P. hrachycephalus, according to Prof. Owen, the head equals 

 one-eighth of the body in length ; in P. macrocephalus the length of 

 the head equals one-half that of the neck ; they are therefore at once 

 excluded. 



The classical authority on Plesiosauri, Mr. Conybeare, states that 

 the head of P. dolichodeirus equals one-thirteenth of the entire body, 

 or one-fifth of the neck, while the head and neck together are to the 

 body as six to seven*. These proportions approach those of P. 

 Etheridgii. But they are not the same ; and besides, the neural 

 spines of the cervical vertebrse of P. dolichodeirus are quite differ- 

 ently shaped from those of P. Etheridgii. And though the total 

 number of vertebrae in P. dolichodeirus is the same, viz. 90, 35 are 

 said to be cervical, 27 dorsal, 2 sacral, and 26 caudal. Clearly then 

 the specimen described has nothing to do with dolichodeirus. 



Plesiosaurus Hawkinsii approaches it much more closely in size, 

 form, and general proportions. 



Several magnificent specimens of this species are to be seen at the 

 British Museum, and afford excellent materials for the determination 

 of its distinctive characters. Nevertheless the account of its charac- 

 ters in the i Report/ already cited, presents some difficulties to the 

 reader. At page 57, for instance, it is stated that in this species 

 " the neck equals three lengths of the head, and the neck and head 

 together equal the trunk and tail." If this be true, of course the 

 length of the head must equal one-eighth of that of the whole body. 

 Nevertheless, at page 61 of the same * Report,' it is said that the 

 head equals less than one-tenth part of the body. 



Again, at page 6 1 (and by implication at page 63 ?), P. Hawkinsii 

 is said to possess twenty-nine cervical vertebrae ; but at page 57 the 



* In his well-known memoir (Geol. Trans, ii. 1, 1824) Mr. Conybeare states at 

 page 382, " the neck is fully equal in length to the body and tail united ; " but 

 at page 385 he says, " taking the head as 1, the neck will be 5, the body as 4, 

 and the tail as 3 : the total length being, as before remarked, 13 times that of the 

 head." Prof. Owen, in his ' Report on the British Fossil Reptilia,' quotes Mr. 

 Conybeare's first statement, but omits to refer to the last. Prof. Owen further 

 states (Report, p. 61) that in PL dolichodeirus the head is four times the length 

 of the neck. I suppose this to be a misprint, and that what is meant is, that the 

 neck is four times the length of the head ; but even this is at direct variance with 

 Mr. Conybeare's assertions and figures. 



YOL. XIV. — PART I. U 



