ON A NEW SPECIES OF PLESIOSAURUS 525 



In p. bracliycephalus, according to Prof. Owen, the head equals 

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

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

 excluded. 



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

 the head of P. dolickodeirus 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 vertebrae of P. dolicliodeirus are quite differently shaped from: 

 those of P. Etiieridgii. And though the total number of vertebrs in 

 /-*. dolicliodeirus 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 dolichodeirits. 



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 characters- 

 in the ' 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 61 (and by implication at page 63?), P. Hawkinsii 

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

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



' In his well-known memoir (Geol. Trans, ii. 1, 1824) IMr. 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 i, 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 PI. dolickodeirus 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. 



^ At least, this is the only conclusion consistent with the definition of a cervical vertebra 

 at page 58. Prof Owen there proposes to consider as «rz;z'ca/those vertebree whose centrum 

 exhibits the whole or a part of the costal articular surface. At page 57 he states with respect 

 to P. Hawkinsii, "In the first or anterior 31 vertebra the centrum supports the whole or 

 part of the costal pit." Therefore, according to the definition, these 31 vertebrce ar& 

 cervical. 



