upper jaw and thirty on the lower jaw in full-grown individuals | but in
a half-grown specimen M. Bibron found only twenty-six teeth both
above and below, and in a still younger individual only twenty above,
and eighteen below. The pterygoid teeth in this species are nine or
ten on each side ; small, slender, rounded, and they appear not to he
constant, at least in young specimens.
The spiny-tailed Cyclurus has more numerous teeth ; from fifty
to fifty-six in each jaw.
The handed Iguana, which is the type of the genus Brachylophus of
Cuvier, has between thirty-five and forty teeth in both the upper and
the lower jaw, most of which are compressed and tricuspid : there is
a slightly curved row of short and pointed teeth on each pterygoid
bone.
The dentition of the allied genus Enyalus differs only in the larger
proportion of simple pointed anterior teeth.
The Iguanoid Hyperanodon, as its name implies, has no pterygoid
teeth.M
ost of the maxillary teeth of the Proctotreti are equal, compressed,
and trilobate, a few of the anterior ones being pointed. The
pterygoid teeth are still smaller and are pointed.
The dentition of the Tropidolepes and of the toad-like Phrynoso-
mes differs from that of the Proctotretes only in the absence of pterygoid
teeth.
In the Callisauri all the maxillary teeth are simple, nearly equal
and conical | here likewise there are no pterygoid teeth.
The insectivorous Ecphymotes have pterygoid teeth.
In the Doryphorus the dentition begins to exhibit a little more
variety. the palate is edentulous; but in the upper jaw there may be
distinguished eight incisives, three laminaries, and about fourteen
molars on each side. The laniaries are a little longer than the others,
rounded and slightly curved ; both these and the incisors are separated
by intervals g the molars have compressed, tricuspid crowns with the
middle cusp longer than the rest.
The most strictly vegetable feeding reptiles are the true Iguanas
and the Amblyrhynchi; yet the size of the teeth, their mode of implantation
and the limited motions of the jaws permit only an imperfect
comminution of the food by these instruments; and their summits
are rather chipped off than ground down by use. The appearance of
abrasion is greatest in the posterior teeth, especially in the Iguana
cornuta, in which the crowns of the teeth are thicker than in the
Iguana tuberculata, and make a nearer approach to the very remarkable
form of tooth that characterizes the gigantic Iguanodon.
Before, however, proceeding to describe the teeth of this extinct
lizard, I shall offer a few observations on the microscopic structure of
the teeth of the existing Iguana. In both the common and horned
species, the teeth consist of a body of simple compact dentine, with the
crown covered externally by a thin layer of enamel, and the fang with
an investment of cement- The dentine, viewed by transmitted light in
a thin horizontal section, exhibits minute calcigerous tubes in a clear
substance, radiating from a simple conical pulp-cavity, which is-
widely open at the base of the tooth and continues in a linear form
into the crown of the tooth : the calcigerous tubes at the base of the
tooth proceed in an irregular sinuous course at right angles to the
axis of the tooth: above this part they sweep outwards in a graceful
curve, with the concavity turned towards the base of the tooth: as
they approach the summit of the tooth they gradually incline towards
it, and those from the apex of the pulp-fissure proceed directly in the,
axis of the tooth : throughout their course the calcigerous tubes are
disposed in minute undulations, and they send off from the concave
side of the primary flexures numerous short parallel branches at an
angle of 45°: these branches rise less regularly the nearer the main
tube is to its origin from the pulp-cavity. The diameter of the calcigerous
tubes is apooth of an inch : their interspaces are equal to between
three or four of their diameters.
In general they do not divide until within a short distance from
the periphery of the tooth, near which they subdivide frequently ;
the terminal branches of the different layers decussate each other.
The tubes at the base of the tooth divide nearer their origin and more
frequently ; which, with the large oblique branches, and the stronger
undulations of the main tubes occasions the interwoven appearance
represented at PI. 65 b ., fig. 3.
The pulp-cavity in old teeth becomes occupied by a coarse hone,