192 
DR. MANTELL ON THE IGUANODON. 
transversely with a convex outer, and a flat inner aspect, and gradually increases 
downwards in width and thickness, from the broadly-rounded excentric apex to its 
greatest longitudinal diameter. It continues to expand transversely while decreasing 
in breadth ; subconcave planes also replace the serrated edges at which the surfaces 
meet above ; it obtains its greatest thickness where the tooth is bent on itself to form 
the fang ; the latter diminishes rapidly in both diameters, and the lateral facets are 
brought in contact below, and obliterate the inner surface ; in fully-formed teeth the 
fang tapers to a point*. 
“ The inner coronal aspect has a rhomboidal outline, and is covered with a thin 
layer of enamel extending on the margins ; it is flat in the antero-posterior diameter, 
and only slightly convex vertically. The upper serrated edges ascend converging ; 
the anterior is the longest and most curved ; it sweeps rapidly backwards above to 
the excentric apex, which thus presents a broad front shoulder. The inferior mar- 
gins are striated or granulated, but destitute of serrations ; the posterior is bent 
forward below to meet the straight lower front edge. The angle to which the hinder 
margin inclines is more prominent and acute than that formed by the anterior edge ; 
and by this character the teeth belonging to the respective rami of the lower jaw 
may be distinguished. 
“ The enamelled surface is divided into two unequal channelled areas by a primary 
longitudinal ridge (Plate XVIII. fig. 4, n, and fig. 5, n) ; commencing at the apex, it 
intersects the long diagonal, and terminates behind the lower angle, from which a 
broad secondary elevated tract ascends along the floor of the wide anterior groove, 
nearly obliterating it in front. A slight convexity, rapidly subsiding, passes upwards 
in front of the superior posterior edge. 
‘‘ The relative width and depth of the longitudinal grooves, and the prominence of 
the intervening ridges, vary in different specimens ; and the edges of the ridges are 
more or less replaced by planes ; the ratio of the upper to the lower margins also 
differs. The serrations are produced by small convex marnmillated ridges separated 
by slight intervals ; the inner edges of the anterior apical ones are prolonged down- 
wards ; those on the posterior margin are abraded, apparently by absorption, during 
the upward growth of the germ. The inner convex surface of the fang is variously 
grooved and flattened, becoming ridge-like below ; it is in apposition with the outer 
alveolar parapet. The lateral planes converge inwards, and are grooved longitudi- 
nally ; the posterior plane presses forwards the inferior hinder coronal margin ; they 
extend as high as the obtuse angle of the crown, and leave between them, as they 
diverge in their ascent, an unenamelled triangular space on the inner aspect. Ex- 
pansions of the alveolar septa on each side are adapted to the lateral planes of the 
fang, and the inner parapet is deficient opposite the triangular tract above-mentioned, 
but is closed below, separating the alveolus from the cavity of reserve in the secon- 
dary dental groove. 
* Philosophical Transactions, 1841, Plate VII. figs. 1, 2. 
