DR. MANTELL ON THE IGUANODON. 
185 
found in the sandstone, is heavy, and of a rich umber colour, from impregnation 
with oxides of iron. It consists of the dentary, and part of the coronoid or comple- 
mentary bone, of the right side, and is entire at the anterior part, but the posterior or 
opposite extremity is imperfect, probably to the extent of several inches. Its original 
relative position in the jaw will be understood by referring to my former commu- 
nication*, in which the peculiar construction of the lower jaw in Saurian reptiles is 
described. In this place it may, however, be proper to remark, that of the six pieces 
on each side of which the inferior maxilla consists, that containing the teeth, and form- 
ing the anterior portion or symphysis, is termed the dentary ; and the posterior part of 
this bone is united on the outside by suture to the complementary or coronoid^ angular, 
and surangular •, while on the mesial or inner side, it is covered below and behind 
the teeth by an expansion of the opercular bone. 
The specimen is represented of the natural size in Plate XVI. ; its dimensions are 
as follow : — 
inches. 
Length from the front of the symphysis to the posterior extremity of the hone 21 
Greatest width of the outer surface measured over the convexity, from the lower margin to the 
upper alveolar edge 6f 
Greatest thickness at the posterior part 2|^ 
Length of the alveolar parapet for twenty teeth 15 
Breadth from the anterior termination of the alveolar space across to the inner margin 4^ 
Height of the alveolar parapet at the posterior part 2 
Width of the alveolar space at the posterior part ly 
Width of the alveolar space at the anterior part f 
Length from the first anterior tooth to the symphysial extremity 5 
Height of the successional tooth (Plate XVI. fig. 1, b) 1^ inch ; greatest width 
The mesial or inner aspect of the fossil (Plate XVI. fig. 1) is flattened and smooth, 
and shows the successional teeth which remain in their original places (a, b), and the 
sockets for nineteen or twenty teeth; the inner alveolar plate having been 
destroyed, and the mature molars dislodged, before the bone was imbedded in the 
rock. The deep conical groove or furrow, so constantly present on the inner side of 
the dentary bone in reptiles, and whieh from its being covered by the splenial or oper- 
cular piece, it may be convenient to designate the opercular furrow, is here entirely 
exposed (fig. \,d, d) in consequenee of the removal and destruction of that maxillary 
element. It is very large, and prolonged anteriorly {d) to within six inches of the 
symphysis ; the opercular piece must therefore have more nearly corresponded 
with that of the Varanians or Monitors than with the Iguanas, in which it is of a 
rhomboidal figure, and relatively of limited extent. The lower margin of the jaw is 
thiek and convex at the posterior part, and gradually becomes thinner towards the 
front, where it expands horizontally into a broad scoop-like process, which is termi- 
nated anteriorly by an obtuse projection or tubercle (Plate XVI. fig. 1, e) ; it thins out 
rnesially to form the symphysial suture (^) that connects it with the opposite ramus. 
The upper margin is formed by the alveolar process, which has a thick external 
* See Philosophical Transactions for 1841, Plate V. figs. 3. 7. 
