on the Fossil Genera Ichthyosaurus and Plesiosaurus. 119 



PLESIOSAURUS. 



The researches of Mr. De la Beche, during the last year, have not been 

 crowned with the success of meeting an entire skeleton of this new genus ; 

 but many important parts have been brought to light. 



The first of these is a very perfect dental bone of the lower jaw, which is 

 represented under three points of view in the figures of Plate XVIII , being 

 there one half the natural size. We may with great certainty ascribe this to 

 the plesiosaurus : for it certainly belonged to some animal which had a com- 

 posite jaw like the Saurians ; since the posterior end of the bone thins off too 

 much to have formed part of a single maxilla. Again, in the lias at Lyme, the 

 only vertebrae of size enough to have belonged to such a jaw are those of the 

 ichthyosaurus and plesiosaurus. It is probable, therefore, that it belonged to 

 one of these ; but the dental bone of the former is well known, while that of 

 the latter (till the occurrence of the present bone) had never been found. It 

 cannot with probability be objected that it might have belonged to some third 

 animal so rare that its vertebrae have not yet been met with ; for fragments of 

 a similar dental have in other instances been found : so that the suspicion of 

 extreme rarity, which might have attached to a single specimen^ is precluded. 

 On the whole then the evidence stands thus : — We find in the same place ske- 

 letons of a Saurian animal wanting the jaw, and the jaw of a Saurian animal 

 wanting the other bones ; and no other claimants exist for either. 



This dental bone resembles that of the crocodile more than that of the other 

 lacertae. The teeth which, with the exception of one young tooth, have un- 

 fortunately been displaced, were lodged in separate alveoli; on the inner side 

 of those alveoli is a series of small holes, designed apparently for the passage 

 of nerves and vessels (as in the crocodile) ; and along the outer side of the jaw, 

 dispersed irregularly in small points, are holes for the passage of the branches 

 of the lower maxillary nerve, which is the case in the crocodile also, as con- 

 tradistinguished from the other lacertte. 



Head. — Of the head of the plesiosaurus, only a single specimen approaching 

 to completeness has yet occurred ; which was discovered by Mr. Thos. Clarke 

 in the Lias of Street near Glastonbury. It is represented in Plate XIX.* This 

 specimen is unfortunately much crushed; but is nevertheless sufficiently per- 

 fect to exhibit its most essential osteological characters. 



* la this and ia the former Plates the same letters have been employed to designate the aua. 

 logous bones. 



