132 
DR. MANTELL ON A PORTION OF THE LOWER JAW 
which J paid in 1837 to a quarry near Cuckfield, that had often proved a rich mine 
of fossil treasures. The sandstone adhered very firmly to the bone, but I succeeded 
in completely developing' the specimen, which is now deposited in the British Mu- 
seum with the other bones of the Iguanodon. The annexed drawings (Plate V.) repre- 
sent the fossil of the natural size. It is a portion of the right side of the lower jaw, 
three inches in length, and l^ths of an inch in width at the proximal extremity. It 
bears the fangs of fifteen teeth, and the impressions of three others, with indications 
of the position of four successional teeth. None of the crowns of the teeth remain, 
and this must have been the case when the jaw became imbedded in the sand, for 
there were no traces of any portions of teeth in the surrounding stone. The absence 
of these essential characters renders it necessary to enter upon some anatomical 
details to prove the correctness of the conclusion which I propose to establish. To 
assist the comparison, representations are annexed (Plate V. figs. 3. 4. 7-) of the right 
ramus of the lower jaw of an Iguana, belonging to an individual three feet and a 
half in length. 
It is well known that the lower jaw in the Mammalia, even in a foetal state, is 
composed of but one bone on each side ; but in the Lizards, Turtles, Crocodiles, and 
most reptiles, it is formed of six. These bones, as seen in the Iguana (Plate V. figs. 
3. 7-) consist of the dentary (a) which supports the teeth ; the opercular ( b ) ; the 
complementary (c) ; the surangular or coronoid (d) ; the angular (e) ; and the articular 
(/'). The modifications of form and arrangement of these bones in the Iguana and 
other true Lizards, differ very materially from those observable in the Crocodile ; 
particularly in the circumstance that the dentary bone does not carry the teeth in 
distinct alveoli or sockets, but forms a parapet or wail, composed of a strong simple 
plate of bone, to the inner surface of which the teeth adhere without any osseous 
partitions between them. In the Iguanas, the inner border of the dentary bone is 
therefore but little developed, and the teeth are not protected mesially by a process 
of bone, but are only covered by the gum, as is shown in Plate V. fig. 3. The oper- 
cular bone in the Iguana is small, and of a rhomboidal form, and does not extend 
to more than one-third the length of the dentary (see fig. 3. b). A large foramen, for 
the entrance of the blood-vessels and nerves, is situated on the mesial plane of the 
jaw, behind the coronoid process, and between the complementary, the surangular, 
and the articular bones (see fig. 3*.). On the external surface of the dentary there 
are three or four perforations for the passage of nerves (fig. 7- «) ; and on the mesial 
surface of the jaw there are two similar foramina in the opercular bone (fig. 3. b). 
Such are the maxillary elements of the Iguana ; but before applying these data to 
the examination of the fossil, it will be necessary to offer some remarks on the deve- 
lopment of the teeth in the recent Lizard. The pointed, compressed, lanceolate form 
of the crown of the tooth of the Iguana, its deeply serrated margins, and the shape 
of the fang, are so well expressed in figs. 3 and 7? and in the magnified view, fig. 4, 
as to render any description unnecessary. There are fifty-four teeth in the lower jaw. 
