OF THE IGUANODON, AND OTHER SAURIAN REMAINS. 
133 
and the same number in the upper ; twenty-seven teeth occupy a length of l^ths 
of an inch. The teeth are arranged in a linear series, having a slight inclination 
towards the anterior part of the jaw, and they diminish in size as they approach the 
symphysis. The germs of the new or successional dental organs are situated at the 
base of the fangs of the old teeth, as shown in fig. 4. h. 
On examining the inner or mesial surface of the fossil (Plate V. fig. 1.), it will be 
found to present the essential elements of the corresponding portion of the jaw of the 
Iguana. It consists of the proximal termination of the dentary bone ( a a), and of the 
distal portion of the opercular ( b b), which, as above remarked, is of a rhomboidal 
form in the Iguana, and extends but a short distance over the dentary. The oper- 
cular has two vascular perforations (fig. 1. k k), as in the corresponding bone of the 
Iguana. A large canal for the passage of the inferior maxillary vessels and nerves 
is seen at the fractured proximal extremity (figs. 1 and 2. i i ) ; but, anteriorly, the 
bone is solid (fig. 9.). 
The dentary bone forms a strong parapet (figs. 1. 2. 5. 6. g g g ), to the mesial surface 
of which the shanks or fangs of the teeth are anchylosed, in the same manner as in 
the recent Iguana ; and the position of the germs of the successional teeth at the 
base of the old ones, is shown in four instances (see figs. 1.5. and h. h ). 
There appear to be indications of a thin, inner, alveolar process (figs. 1. and 5. o o), 
but its nature cannot be accurately determined ; and from a careful examination of 
the specimen, I am led to conclude that the fangs of the teeth had no osseous pro- 
tection on their mesial aspect, but were only covered by the integuments of the gum, 
as in the Iguana. 
This comparison appears to me to furnish conclusive proof that the fossil is a por- 
tion of the lower jaw of a reptile of the Lacertian family, allied to the Iguana. That 
it is reptilian, its complicated structure affords decisive evidence ; the absence of 
alveoli, and the anchylosis of the teeth to the mesial surface of the dental parapet, 
are characters peculiar to the Lizards ; while the relative situation, form, and size of 
the opercular bone, are alike confirmatory of its analogy to the Iguana ; with which, 
above all, its mode of dentition corresponds, as is evinced by the position of the 
successional teeth. 
But although this fragment possesses characters which supply a clue to the general 
relations of the original animal, it does not present the elements necessary for a re- 
storation of the lower jaw. The peculiar form of the teeth, as shown in my previous 
memoir*, indicates in the Iguanodon a very different masticatory apparatus from 
that possessed by any living reptile, and the jaws must necessarily have presented a 
corresponding osteological modification. None of the existing reptiles perform mas- 
tication ; their food or prey is taken by the teeth or tongue, so that a moveable cover- 
ing to the jaws, similar to the lips or cheeks of Mammalia, is not required either for 
confining substances subjected to the action of teeth as organs of mastication, or for 
* Philosophical Transactions, 1825, p. 186. 
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