188 
DR. MANTELL ON THE IGUANODON. 
Enamelled-— -r-'"' 
face. 
— Cavity formed by 
the pressure of a 
new tooth. 
Coronal surface 
worn flat by use ; 
showing a radi- 
ated structure. 
As the surface of the crown when abraded by Xylograph, No. 2. 
mastication invariably possesses two distinct facets Lower tooth reduced by abrasion of the crown and absorp- 
. ^ ^ , , , tion of the shank. 
(Xylograph, No. 4, and Plate XVIII. figs. 1* and 2*), 
it is obvious that the arrangement of the lower teeth 
in relation to the upper was intermediate, or sub- 
alternate, as is the case in the ruminants ; the further 
consideration of the dental characters of the Igua- 
nodon will be resumed in a subsequent part of this 
memoir. 
The external aspect of the specimen (Plate XVI. 
fig. 2, and Plate XVII. fig. 4) presents in its transverse 
diameter a gentle convexity, traversed by a slightly 
elevated longitudinal ridge, which lies parallel to, 
and immediately beneath, the row of vascular foramina, commonly met with in this 
part of the lower jaw in reptiles ; and towards the posterior extremity the side of the 
bone is somewhat compressed below the longitudinal eminence ; according in this 
respect with the portion of a lower jaw of a much smaller reptile, described in a pre- 
vious memoir*. The upper edge of the bone is formed by the outer alveolar parapet, 
which is deeply scalloped or crenated by the terminations of the sockets of the teeth ; 
the angular eminences indicate the intra-alveolar ridges. A reference to the figures 
(Plates XVI. and XVII.) will impart a more correct idea of the configuration of this 
part of the fossil than any verbal description. The whole surface of the bone is 
covered with minute punctuations and striae. 
The relative thickness and proportions of the different parts of the specimen, and its 
external contour, are shown in the reduced figure of the section exposed by the frac- 
tured posterior extremity, Plate XVII. fig. 3 ; and in this view is seen the dental canal 
( 3 ) which contained the large blood-vessels and nerves that supplied the teeth and 
integuments of the lower jaw ; its longest diameter at this part is six-tenths of an 
inch. 
The numerous and large vascular foramina which afforded passage to the vessels 
and nerves from the dental canal to the external integuments, form a striking 
character in this aspect : they open obliquely forwards ; nine are distributed at re- 
gular intervals in a line with the alveolar margin, from the posterior end of the bone 
to nearly opposite the successional tooth in place. A fracture in the middle of the 
outer surface (Plate XVII. fig. 4, h) at the distance of 4^ inches from the posterior end, 
exposes the dental canal filled with sandstone : its diameter is here two-fifths of an 
ineh. 
At the anterior termination of the alveolar space, a slight protuberance (Plate XVII. 
fig. 4, i) marks the commencement of the upper margin of the symphysial region, 
which is defined by a sharp smooth ridge, that sweeps downwards and inwards to 
form the front of the jaw. A deep groove, beset with foramina (Plate XVII. fig. 4, h), 
* Philosophical Transactions, 1841, Plate V. figs. 1, 8, 9. 
