86 HADROSAUKUS. 



of the corresponding border of the crown, and forms with it a convex ridge extend- 

 ing the whole length of the tooth. Its inner border forms an obtuse angle from 

 the bottom of the enamelled face of the crown, and is excavated into a shallow 

 groove marked by vascular furrows. The grooved border appears to have been 

 adapted to fit the outer border of the crown of a successional tooth beneath. 

 The measurements of the tooth arc as follows : — 



Lines. 



Length of the tooth externally 22 



Length of enamelled crown internally . . . . . . . .14 



Breadth of enamelled crown internally at middle 5J 



Breadth of enamelled crown internally at bottom 2 



Width of crown at base from without inwardly t^ 



Length of fang internally 12^ 



The tooth just described bears a near resemblance to one of those, before alluded 

 to, upon which the genus Tradtodon was founded, as may be seen by comparing the 

 figures of the former (1-4, Plate XIII) with those of the latter (12-14, Plate II). 

 The specimen of the tooth of Trachodon has lost its fang, but its crown has the 

 same form as that of Hadrosaurihs, and nearly the same size, except that towards 

 the base it is narrower from without inwardly, and wider in the opposite direction. 

 In the tooth referred to Hadrosauriis the diameter at base, from Avithout inwardly, 

 is twice as great as that from side to side, but in that of Trachodon it is only a 

 fourth greater, while in both the crown is of nearly equal length. 



The outer portion of the crown of the tooth of Trachodon is irregularly rough- 

 ened with a multitude of granulations or minute tubercles, and, independently of 

 the triangular bevelled planes at its base, is subdivided by ridges into three sur- 

 faces, of which those lateral are flat, or even slightly depressed at the upper part, 

 and the intermediate one is moderately convex. In transverse section the outer 

 portion of the crown forms three sides of a hexagon. In Hadrosaurus the outer 

 portion of the crown is smooth, and forms the two sides of the vertical section of 

 a cone with a rounded apex. 



The upper enamel borders of the tooth of Trachodon are devoid of the character- 

 istic groups of tubercles observed in the tooth of Hadrosaui-ns, though they exhibit 

 a feeble tendency to development in a slight irregvdarity of the limit of the border 

 where it is defined from the dentinal structure of the outer portion of the crown, 

 and by a slight unevenness of the borders of the apex. 



Since writing the present memoir I have seen a " Supplement to the Fossil Rep- 

 tilia of the Cretaceous Formations," by Prof. Owen, in the publications of the 

 Pateontographical Society for 1860. At the end of the Supplement, Prof Owen 

 indicates a tooth, from the upper Green-sand, near Cambridge, England, as that of 

 a young Iguanodon, which bears a near resemblance to those above described of 

 Hadrosaurus and Trachodon. It is represented in Figs. 15, 16, Plate VII, of the 

 Supplement, and though closely related in character with the teeth of the Icjuanodon 

 Mantelli, certainly differs from all those which had been previously referred to this 

 species. The specimen has nearly the form and size as those of Hadrosaurus and 

 Trachodon, but judging from Fig. 15 it differs in the upper borders of the crown, 

 being broken into a series of mmute imbricating laminae. Perhaps the three teeth 



