DR. MANTELL ON THE IGUANODON. 
187 
in the mature state; namely, with the ridged face outwards, and the smooth side 
inwards, or towards the cavity of the mouth. 
As the coronal portion of the tooth in the Ig’uanodon is not symmetrical, one late- 
ral margin presenting a gentle curvature (Plate XVIII. fig. 4, a), and the other form- 
ing a broad angle at the base of the serrated border (Plate XVIII. fig. 4, h), the teeth 
belonging to one side of the lower jaw may readily be distinguished from those of the 
other ; the lateral marginal angle {b) being always situated posteriorly. Guided by this 
character. Dr. Melville and myself examined the numerous teeth in the British 
Museum and in my own collection, and were enabled readily to determine to which 
ramus any tooth belonged. Thus, for example, the specimen represented, Plate XVIII. 
fig. 4, which is the very counterpart of that implanted in the jaw (Plate XVI. fig. 1, b), 
is a perfect successional tooth, consisting of the crown before the formation of the 
fang, and belonging to the right side {a denotes the anterior, and b the posterior 
angle). The specimen, Plate XVIII. fig. 5, appertained to the opposite or left lower 
side, as is shown by the situation of the posterior marginal angle (i). 
The position of the lower teeth in relation to the alveolar process, appears to have 
been somewhat changed during the upward growth of the coronal portion consequent 
on the progressive development of the fang ; and it seems probable that the face of 
the crown became inclined rather obliquely forwards and outwards, and that the 
mature teeth were arranged in an imbricated manner. This opinion is supported 
by the form of the alveoli in the outer parapet, and the corresponding oblique curva- 
ture in the fangs of the mature molars, as shown in Plate XVIII. fig. 5 ; but this in- 
ference does not admit of that absolute proof which the perfect adaptation of a full- 
grown tooth to one of the sockets would afford ; for the alveoli are irregular, and 
none of the detached teeth in my possession will fit either of the sockets in the 
recently discovered dentary bone. 
The situation of the germ in relation to the tooth it was destined to supplant, is 
invariably on the inside of the mouth ; in the lower molars the excavation in the 
mature tooth occasioned by the upward growth of the germ, is consequently on the 
enamelled mesial face, as is shown in my original memoir*: in the upper tooth the 
germ was lodged in an excavation on the smooth convex aspect, as will subsequently 
be demonstrated (see Plate XVIII. fig. 2“,/'). 
In the fossil represented in the annexed sketch, the cavity Xylograph, No. i. 
produced by the pressure of the germ is situated in the fang of 
the tooth in place ; in other examples, however, the successional 
dental excavation is on the base of the enamelled crown ; for in 
the Iguanodon the old teeth were retained till nearly the entire 
coronal portion was worn away, and the crown of the tooth, 
from the abrasion by use above, and the removal of the fang by 
absorption below, w^as reduced to a mere disk, before it was finally 
shed; as in the specimen (figured in Xylograph, No. 2, p. 188). Lower tooth of the right side. 
* Philosophical Transactions, 1825, Plate XIV. fig. 7 a. 
2 c 
MDCCCXLVIII. 
