CEYPTOCLEIDTJS OXONIEKSIS. 165 



ascribes to this species a shoulder-girdle (figured and described as a pelvis — op. cit. 

 p. 310, fig. cxvi.) and a paddle (op. cit. p. 312, fig. cxvii.), both of which probably 

 belong to a species of Murcenosaurus. 



Phillips, in his original account of the cervical vertebrse which must be regarded as 

 the type specimens, simply states that they are biconcave with narrow tumid inter- 

 foraminal space, while his figures and measurements show that the centra are short ; 

 the vertebrae figured, however, are not anterior examples as Phillips supposed, but from 

 some little distance back in the neck. The dorsals and caudals described are not 

 definitely stated to have been associated with the cervicals, and, as above noted, the 

 shoulder-girdle and paddle are those of a Muraenosaur, consequently the determi- 

 nation of the species must rest entirely on the cervical vertebrae. Comparison of these 

 with the corresponding vertebrae of the commonest type of Plesiosaur from the Oxford 

 Clay of Peterborough, shows such similarity of form that both are clearly of the same 

 species, and therefore the name Cryptocleidus oxoniensis is applied to them. Lydekker 

 employed the name Cwioliosaurus osoniensis for the smaller individuals of this type, 

 while he called the larger C. eiirymerus, a name which had been given by Phillips to a 

 large broad paddle from the Oxford Clay of Bedford associated with vertebrae similar to 

 those of C. oxoniensis. The series of specimens in the Leeds Collection tends to show 

 that, as Lydekker himself suggested, these two forms are probably only a single 

 species, and that the difference in size and form are merely the result of increased age. 

 Since, however, the ossification of the shoulder-girdle is completed while the individual 

 is considerably smaller in some cases than in others, it is possible that the difference 

 may be a sexual one, as I have already suggested in a paper on the development of 

 the shoulder-girdle (Ann. Mag. Nat. Hist. [6] vol. xv. (1895) p. 333). The differences 

 in the form of the limb-bones which led Professor Seeley to establish his species 

 Cryptocleidus platymerus for a specimen (Leeds Coll., R. 2412) in this collection, are 

 probably due entirely to the advanced state of ossification that had been reached in 

 this case. 



One of the most important points about this species is, that remains of individuals 

 of all ages are common in the Peterborough deposits, and are easily distinguished 

 from the other forms. From these specimens it has been possible to make out the 

 history of the development of several parts of the skeleton, notably that of the 

 shoulder-girdle (see Ann. Mag. Nat. Hist. [6J vol. xv. (1895) p. 333). 



The description of the skeleton given below is founded mainly on the almost perfect 

 adult skeleton (R. 2860) which has been mounted in the Gallery of Fossil Reptiles ; 

 reference will also be made to other specimens, especially to the mounted skeleton of 

 a young individual (R. 2417), a brief account of which has been published in the 

 Geological Magazine (1895), p. 241. 



Skull (PI. IX.). — The skull, so far as known, is very similar in its structure to 



