186 
DR. MANTELL ON THE IGUANODON. 
parapet, deeply furrowed on the inner side, as seen in this view, by the sockets for the 
mature teeth Strongly-defined ridges occupy the interspaces, and rising above 
the sockets produce a sharp crenated upper border. The alveolar space is protected 
on the inner side by a moderately strong plate or wall, which must originally have 
nearly equalled the outer parapet in height, but is now in a great measure broken 
away : within this process the germs of the successional teeth were developed. 
The mode of implantation of the teeth appears to have been intermediate between 
the pleurodont and thecodont types ; for the teeth were not anehylosed to the alveolar 
wall as in the Iguanas, but free as in the Crocodiles ; yet as the ridges that separate 
the dental sockets are smooth and rounded, it may be inferred that these were not 
rendered complete alveoli by transverse plates extending from the outer to the inner 
parapet, as is the case in the Megalosaurus*. 
The dental sockets diminish in size, but somewhat irregularly, from the posterior 
to the anterior termination of the alveolar process ; and the latter suffers a corre- 
sponding diminution in breadth, and terminates suddenly at the distance of five inches 
from the front. At this point the upper margin becomes attenuated and contracted 
in a vertical direction, and descending with a gentle curve, expands horizontally and 
mesially to unite at the symphysial suture (.?) with the opposite ramus, the anterior 
part of the jaw being edentulous. 
From the fortunate preservation of two successional teeth in their original position, 
the mode of dental development in the Iguanodon is clearly demonstrated. The 
coronal portion of the tooth was first formed, as seen in the germ, Plate XVI, fig. 1, a ; 
and the entire crown was completed (fig, 1,&) before the secretion of the shank or 
fang commenced, as in the existing Saurians. The formative pulp was situated in a 
distinct depression or cavity, on the inner side of the root of the tooth it was destined 
to supplant : this is obvious by the positions of the teeth above described ; and also 
by the remains of a third germ towards the posterior part (fig. 1, c). 
Although the peculiar characters of the molars of the Iguanodon were described 
somewhat in detail in my former communications, and the present fossil confirms in 
every essential particular the inferences suggested by the detached teeth, as stated in 
my memoir of 1825, yet several new and important points relating to the development 
and functions of the dental organs, are elucidated by the new acquisition which Capt. 
Brickenden’s researches have brought to light. The second tooth (Plate XVI. fig. 1, h) 
which occupies its natural position in the alveolar space, consists of the entire crown, 
having the serrated margin as perfect as in the recent state ; and this is the first 
evidence I have obtained as to the mode in which the teeth were implanted. The flat 
enamelled front, characterized by its longitudinal ridges, is placed mesially, and 
})arallel to, and within the inner alveolar wall ; the smooth convex face filling up a 
depression in the outer parapet, in the interspace of two sockets of the mature molars. 
This position is the reverse of that in which the successional teeth in the Iguana are 
developed ; for in that reptile the coronal germ occupies the same relative place as 
* See Dr. Buckland’s Bridgewater Essay, Plate 23. 
