86 HADROSAURUS. 
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 are as follows :— 
Lines. 
Length of the tooth externally . : : - : 5 - : 4 . 22 
Length of enamelled crown internally : : : 3 : ; : . 14 
Breadth of enamelled crown internally at middle - ; 5 : - . 2 
Breadth of enamelled crown internally at bottom 2 
Width of crown at base from without inwardly . . : - : : tes 
Length of fang internally . : c : : é c : : e . 124 
The tooth just described bears a near resemblance to one of those, before alluded 
to, upon which the genus Trachodon 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 I). 
The specimen of the tooth of Trachodon has lost its fang, but its crown has the 
same form as that of Zadrosaurus, 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 Hadrosaurus the diameter at base, from without 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 Hadrosaurus, though they exhibit 
a feeble tendency to development in a slight irregularity 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 
Paleontographical 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 Jguanodon, 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 Jyuwanodon 
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 Ha/rosaurus and 
Trachodon, but judging from Fig. 15 it differs in the upper borders of the crown, 
being broken into a series of minute imbricating laminze. Perhaps the three teeth 
