DR. MANTELL ON THE IGUANODON. 
191 
surface ; and rising to a higher level, becomes faint, and terminates one inch behind 
the anterior extremity; several small vascular foramina open where it ceases. A 
rough, irregularly triangular, and excavated tract, separates it from the outer edge 
of the bone ; its apex is behind, about 1^ inch from the infra-orbital orifice ; its base 
corresponds to the rounded anterior and external angle, where it passes into the low 
external border. 
“The roof of the infra-orbital canal presents a deep concavity behind (fig. 2, c), 
with the smooth lateral surface of the groove rising into it externally ; this is evi- 
dently part of the floor of an upwardly emergent canal about an inch in diameter ; 
the thin partition continued backwards between it and the canal below is destroyed, 
and the posterior segment of the circumference removed ; a comparison of the fossil 
with the skull of an Iguana will confirm this statement. 
“The infra-orbital canal, which is eight lines wide behind and four lines high, 
bends inwards as it retrogrades from its anterior opening. The inner surface is only 
four lines from the nasal aspect of the fragment behind, so that after a course of a 
few inches, it would have emerged on the floor of the nasal cavity. The roof is 
incised obliquely outwards, and the inner portion of it extends forwards to the retro- 
grade groove. The portion of the external surface of the alveolar process that 
remains, slopes inwards, and exhibits no traces of vascular foramina.” 
From the almost entire destruction of the alveolar walls of the furrow, deep trans- 
verse excavations (Plate XIX. fig. ) are the only remains of the dental sockets. 
As the fangs of the teeth of the upper jaw, as will be shown in the sequel, were more 
curved than in the lower series, and their implantation presented a corresponding 
modification, as is the case in the dental organs of certain existing Monitors, the 
width of the alveolar space is greater than in the lower jaw. 
Dental characters. — Although the peculiar form and structure by which the teeth 
of the Iguanodon are distinguished from those of all other animals have been described 
in my former communications, yet as the mode in which the teeth were implanted 
in the jaws was then unknown from actual observation, no attempt was made to 
aseertain the dextral or sinistral position of the detached specimens ; nor to separate 
the lower from the upper series, and thus determine the dental arrangement by 
which the jaws of this colossal reptile were invested with the functions of those of 
the existing herbivorous Mammalia. To solve this highly interesting problem, it 
became necessary to institute a rigorous and minute comparison and examination of 
all the teeth of the Iguanodon to which we could obtain access; the results of the 
investigation are detailed by Dr. Melville in the following statement. 
“ Teeth of the Lower Jaw, Plate XVIII. figs. 4, 5, 6. — The lower tooth is curved, 
with the coneavity outwards, or towards the external alveolar parapet ; the upper 
and lower limbs, corresponding respectively to the wedge-shaped crown, and elon- 
gated taper fang, are not separated by a constriction or neck, but are flattened in 
opposite directions. In the upper moiety of the eoronal segment, it is compressed 
