BRITISH FOSSIL REPTILKS. 113 



figured in the Philosophical Transactions for 18] 4, Plates XVII. 

 — XX. , belong to the present species. 



The name jjlatyodon, proposed by Mr. Conybeare for the 

 present species, is expressive of the form of the crown of the 

 tooth, which is conical, subcompressed, the convex surfaces 

 meeting on each side at a sharp edge : it further differs from the 

 tooth of the Ich. communis in not having the basal grooves con- 

 tinued so deeply upon the crown, which, on the contrary, often 

 present a smooth and polished surface*. The most prominent 

 and distinctive character of this species is the equality of the fore 

 and hind paddles as to size, and the comparative simplicity of 

 their structure as to the number of digital phalanges and ossi- 

 cles composing them. The discovery of this structure is due to 

 Mr. Hawkins. 



The head is relatively longer in proportion to the trunk than 

 in the Ichthyosaurus communis or Ich. intermedins. The length 

 of the trunk, e. g., includes only one length and a half of the 

 head, while in the Ich. commy^iis it includes rather less than two 

 lengths of the head, and in the Ich. intermedins rather more. 



The head of the Ich. platyodon is also longer in proportion 

 to its breadth than in the Ich. intermedins, but the jaws are 

 relatively stronger on account of their greater relative breadth, 

 and their less gradual attenuation to the rostral extremity. 

 The lower jaw is remarkably massive and powerful, and pro- 

 jects further backwards beyond the joint than in the preceding 

 species. 



The orbit is relatively longer than in the preceding species : 

 the upper maxillary boue is excluded from the formation of any 

 part of its circumference by the union of the malar with the 

 lachrymal bone. In a magnificent fragment of the cranium of 

 this species in the collection of Mr. Johnson of Bristol the orbit 

 measures one foot in diameter. 



The nostril has a more elongated elliptical form than in the 

 Ichthyosaurus intermedins or Ich. communis. 



In the composition of the lower jaw I find that the angular 

 bone is continued further forwards before it is covered by the 

 overlapping dental piece than in the Ich. iyitermedius or Ich. 

 communis, a structure which contributes to the strength of the 

 lower jaw in this gigantic species. 



The teeth I have not found to exceed 45-45 in the upper and 

 40-40 in the lower jaw. Their crowns are more frequently 

 found to be snapped off than in the smaller species, a circum- 

 stance which is indicative of the greater violence with which 

 they had been used. 



* In the collection of Mr. Johnson of Bristol there is a tooth oH\\q Ich.j)laty~ 

 odon which measures two inches and a half in length : its crown, which, as usual, 

 is pretty smooth and compressed, measures one inch. 

 1 8.39. I 



