DR. MANTELL ON THE IGUANODON. 
189 
constitutes a strong line of demarcation between the inner and outer boundary of this 
area ; the latter is thick and convex, and terminates anteriorly, as already mentioned, 
in a mammillary protuberance or tubercle (Plate XVI. e, and Plate XVII. fig. 4, e). 
A series of foramina, eight in number, extends along the outer and inferior surface of 
the symphysis ; the terminal one, which is three-fourths of an inch in its transverse 
diameter, is situated immediately under the mental tubercle (e) above described. 
The mesial or inner edge of the symphysis, which in connection with the ramus of 
the left side formed the median suture of the lower jaw, is thin and expanded ; the 
articulating surfaces of the two dentary bones appearing to have overlapped each 
other (Plate XVI. fig. 1,^, and Plate XVII. fig. 1,^, ; but as the edge of the bone is 
somewhat broken, the line of junction is not quite determinable ; but the two rami do 
not seem to have been united by anchylosis. 
On the under surface of the symphysis there is a depressed oval area, bounded 
laterally and posteriorly by a slightly elevated ridge (Plate XVII. fig. 2, /, T) ; probably 
for the insertion of the protractor muscles of the tongue. 
The coronoid bone (Plate XVI. fig. l,m, and Plate XVII. fig. 4, m), of which only a 
portion remains, is much more expanded outwardly than in any recent Saurian. 
With respect to the length of the jaw to which this specimen belonged, an approxi- 
mative estimate only can be offered, since we have no means at present of deter- 
mining the relative size of all the different pieces that entered into the construction 
of the maxillary organs of the Iguanodon. From the appearance of the fractured end, 
it seems probable that the dentary bone was prolonged backwards five or six inches 
before it united with the surangular and angular : upon this supposition its entire 
length must have been two feet, and the number of teeth about twenty. In the 
Iguana and most Lizards the dentary element is half the length of the jaw ; and if 
this proportion be taken as the standard of comparison — and it appears to be the 
most probable one — the length of the jaw of this individual was four feet. An eminent 
palaeontologist* has estimated the length of the head of the largest Iguanodon at 
only thirty inches ; having taken as the basis of his calculation, the length of six dorsal 
vertebrae, which in the Iguana is equal to that of the lower jaw. But the specimen 
before us proves either that the same scale of proportions is not applicable to this 
colossal Saurian, or that much larger dorsal vertebrae than those from which the 
measurement was taken, are yet to be discovered ; for several teeth in my possession 
exceed in magnitude the largest sockets of this dentary bone. Even if we take the 
* Reports of the British Association for 1841. Article ‘ British Fossil Reptiles,’ p. 143. “ If there be any 
part of the skeleton of the Iguana which may with greater probability than the rest be supposed to have 
the proportions of the corresponding part of the Iguanodon, it is the lower jaw, by virtue of the analogy of 
the teeth and the substances they are adapted to prepare for digestion. Now the lower jaw gives the length 
of the head in the Iguana, and this equals the length of six dorsal vertebrse ; so that as five inches rather ex- 
ceeds the length of the largest Iguanodon vertebra yet obtained, with the intervertebral space superadded, on 
this calculation the length of the head of the largest Iguanodon must have been two feet six inches.” 
2 c 2 
