140 
DR. MANTELL ON A PORTION OF THE LOWER JAW 
in circumference at the shaft, and thirty-five inches at the distal or condyloid extre- 
mity ; and I have seen fragments in the rock much larger*. The femur of the Igua- 
nodon, unlike that of the Crocodile and of the fossil marine reptiles, has a medullary 
canal from one inch to an inch and a half in diameter in the largest examples (Plate 
VIII. fig. 27.) ; and the cavity is generally filled with calcareous spar. The tibia and 
fibula, though presenting deviations from the structure of the Iguana, manifest a 
close affinity to the Lacertian type. The head of the tibia (Plate VIII. figs. 3, 4. a a) 
is subtriangular, and the shaft, which has a large medullary canal (fig. 21.), is cylin- 
drical and very strong ; the distal extremity is transversely oblong, and forms a 
powerful process on the inner aspect. The fibula (Plate VIII. fig. 2.) is of a subcylin- 
drical form, flattened on its tibial aspect, and expanding at its distal termination. 
The metacarpal and digital bones have but little resemblance to the corresponding 
elements in the Iguana ; they approach more nearly to those of the large herbivorous 
Mammalia. M. Cuvier remarked on some specimens which I transmitted to the 
Mus. d’Hist. Nat., “ les fragmens d’os du metacarpe, ou du metatarse, sont si gros, 
qu’au premier coup d’oeil je les avais pris pour ceux d’un grand hippopotameT-” 
The annexed outline (Plate VIII. figs. 6, 7? 8, 9, 29 ; Plate IX. figs. 12, 14.) of some 
of the most characteristic examples, exhibit the usual type. The ungueal bones 
(Plate VIII. figs. 10, 11, 12.) of the hind-feet differ considerably from those which I 
suppose to have belonged to the anterior extremities ; instead of being curved, as 
in the Iguana, they are depressed, and closely resemble the ungueals of a gigantic 
land Tortoise. 
Although the tarsal bones are wanting, yet we have here the elements for a resto- 
ration of the pelvic extremity of the Iguanodon. I shall not, however, presume to 
intrude on the indulgence of the Society with any calculations as to the probable 
magnitude and proportions of the original, further than to state, that the largest femur, 
if clothed with muscles and integuments, would form a thigh seven feet in circum- 
ference. From the shortness of the caudal vertebrae, and the length of the spinous 
processes of the neura and hsemapophyses, indicating, as before remarked, a great 
vertical development of the tail, it seems probable that this organ was not long and 
slender, as in the Iguana, but that it approximated more nearly to the caudal process 
of the Doryphorus. 
From the structure and condition of the teeth, it appears evident that the Iguano- 
don was herbivorous ; and from the nature of the bones of the extremities, we may 
presume that, with its long slender prehensile fore-feet, it was enabled, while supported 
by its enormous hinder limbs, to pull down and feed on the foliage and trunks of the 
Clathrarice, Draciznce, Yuccce, and arborescent ferns, which constituted the flora of 
* No femur of the Megalosaurus discovered in Tilgate Forest exceeds twelve inches in the circumference of 
the shaft of the hone. 
\ Ossemens Fossiles, tom. v. part ii. p. 350. 
