508 E. TT7LLEY NEWTON ON THE REMAINS OF 



Hypsodon lewesiensis, Agassiz. 



Hypsodon lewesiensis, Ag. Poiss. Foss. 1843, vol. v. p. 99, pi. 25 a. 

 figs. 1, 2, and 4, pi. 25 b. figs. 4 and 5. 



This species will still be typified by the specimens from the Lewes 

 Chalk indicated above ; but the limitations which are proposed 

 render it necessary to modify Agassiz's description. The lower jaw, 

 of which only the anterior part is preserved, is very massive, its 

 thickness being as great horizontally as vertically. Anteriorly the 

 jaw rapidly decreases in size, aud, after making a very definite curve 

 inwards to meet its fellow, terminates in a small, slightly swollen 

 symphysis. Twelve teeth * are preserved ; and these are placed at 

 intervals in one row. The spaces between them are obscured by 

 matrix : and it is possible that some of these were occupied by 

 teeth ; but, judging from the spaces existing in the upper jaw, it- 

 would seem unlikely that the teeth were ever in a continuous series. 

 The teeth are hollow, conical, curved, all very nearly of the same 

 size, and apparently ankylosed to the jaw ; probably they have 

 fangs implanted in definite sockets. There are no definite indications 

 of a vertical succession as observable in the genus Portheus. 



The maxilla is imperfect ; where it is broken across it shows a 

 triangular section and a very considerable horizontal thickness. In 

 its present condition it is provided with seven or eight teeth similar 

 to those of the mandible, and arranged in a single row. A vvell- 

 marked facet, evidently for articulation with the proemaxilla, ex- 

 tends from the anterior part of the outer surface, upwards and 

 backwards, indicating that the latter bone overlapped the maxilla 

 for a considerable distance, and in a manner quite unlike what 

 obtains in Portheus. 



The form of the praemaxilla cannot be clearly made out ; but it 

 evidently has a greater length and horizontal thickness than in 

 Portheus, while its vertical dimensions are apparently much less. 

 The outer edge of the broad dentary margin still retains four or 

 five teeth similar in form to those of the maxilla ; within these,, 

 and separated from them by a distinct space, there are two some- 

 what larger teeth. Prof. Cope, judging from the figure of the 

 present specimen, seemed to think it might be allied to his genus 

 Ichthyodeetes ; but from what has been said above it will be obvious 

 that such is not the case. The form of the lower jaw is certainly 

 more like that of Saurodon : but I am far from being convinced of 

 the propriety of placing it in the group of the Saurodontidae. 



The skull figured by Agassiz from the Chalk of Lewes may pos- 

 sibly belong to the same species ; but there is no direct evidence to 

 prove this. If this is truly the skull of Hypsodon lewesiensis, then 

 its characters and those of the vertebrae found with it and other 

 bones, upon the same block of chalk, are additional evidence of the 

 distinctness of Hypsodon and Portheus ; for while this skull is broad 



* Some of these teeth appear to have been lost since Agassiz's figure was- 

 (iravui 



