DR. MANTELL ON THE IGUANODON. 
195 
and fang. The broad area between it and the primary ridge, exhibits numerous 
strise of enamel converging as they descend to a secondary ridge ; on the crest of 
which, one is prolonged before the outer edge of an anterior apical serration. The 
posterior coronal margin is nearly straight, plain above, and serrated belovv. The 
primary ridge forms a strong convex buttress subsiding towards each extremity ; it is 
slightly inclined to the hinder edge, and nearly obliterates the smaller posterior area. 
“ From the inversion of the teeth in the upper jaw, we naturally expect to find some 
alteration in the configuration of the molars ; while the position of the apex, of the 
primary ridge, and of the areas into which it subdivides the crown, will remain as in 
the lower series. On these grounds I am led to conclude that the smaller area 
always indicates the posterior part of the tooth, whether of the upper or lower jaw ; 
but the marginal angle and the reflected edge are anterior in the upper molars, and 
posterior in the lower, these characters having relation to the manner in which the 
series of teeth are arranged. In the lower jaw the posterior margin of the molar 
overlaps, or rather projects internally beyond the anterior edge of the tooth behind, 
but in the upper it is situated externally to it (Xylographs, No. 3 and 4, page 193.) ; 
a similar arrangement takes place in the Ruminants. The mechanical advantages 
resulting from the opposite curvature of the teeth in the upper and lower jaw, are 
too obvious to require comment.” 
As it is very rarely that a specimen occurs in which the absorption of the fang, 
from the upward growth and pressure of a new tooth, has not taken place in a greater 
or lesser degree, it is probable that the formation of successional teeth was in con- 
stant progress at all periods of the animal’s existence, as is the case in most of the 
Saurian reptiles. 
The internal structure of the teeth is in striking accordance with the external form 
and mechanical arrangement of the dental organs ; for the central body of dentine 
or tooth ivory is of a softer and coarser texture than in any other known Saurian, 
and closely resembles that observable in the colossal vegetable feeders of the Sloth 
tribe — the Mylodon and Megatherium*. But towards the periphery of the anterior 
part of the tooth the dentine is finer and harder ; and not only the front of the 
crown, but also the inserted base, is covered by a thick layer of enamel ; and from 
this peculiar arrangement of substances of different degrees of hardness, the tooth, in 
every stage, must have been admirably adapted for the trituration and comminution 
of vegetable food. A vertical slice of a tooth, showing the pulp-cavity, with the 
dentine traversed by vascular canals radiating from the centre, and running parallel 
with the calcigerous tubes, is represented as seen by transmitted light, and magnified 
eight diameters, in Plate XVIII. fig. 3. 
The dental pulp becomes ossified in the old teeth, so that whatever the degree of 
abrasion, the exposed masticating coronal surface is solid ; this is seen even in the 
* “ The intimate structure of the vascular dentine of the Bradypus resembles that of the inner half of the 
dentine of the Iguanodon.” — Odontography, p, 329. 
MDCCCXLVIir. 2 D 
