The gradual transition from the simple structure of the compact dentine to the osteo dentine of the anchylosed base of the tooth was not known to Cuvier ; otherwise he could not have supposed that the crown and the base of the tooth of the Mosasaurus were formed by vital processes of so dissimilar a nature as to forbid him considering them as parts of one and the same body. Cuvier had originally described the expanded base of the tooth of the Mosasaur as the root of the tooth ; but afterwards, observing that the corresponding base became anchylosed by ossification of the remains of the pulp to the jaw, he conceived it to be incorrect to regard it as a part of a body which he believed to be an inorganic product, and the result of excretion. " The tooth," he observes, in correcting his first The dentition of the Cycl. nigroluteus is exemplified in the lower jaw, fig. 570. In the tipper jaw, the single premaxillary bone has depressions for twelve teeth, of which only the alternate ones are usually in place ; they are of very small size, with the fang com pressed laterally, and the crown antero posteriorly, so as to resemble a true incisor In form, the summit sloping to an edge from behind forwards, with the middle of the cut ting surface a little produced. Each superior maxillary bone has depressions for fourteen teeth ; they quickly increase in size, and exchange their conical for a sub-hemispherical crown ; the eighth to the thirteenth inclusive are the largest teeth ; they are set obliquely, and pretty close together. In the lower jaw there are two small incisors, at the anterior part of each premandibular bone correspond ing with those of the premaxillary ; these are succeeded by five or six conical teeth, and the rest correspond in size and form with the tuberculate molars of the upper jaw.
All the teeth are attached, after the Pleuro dont type, by their base and outer margin to shallow depressions on the outer side of the external alveolar parapet.
The germs of the successional teeth, c. fig. 570., are developed at the inner side of the base of their. predecessors, a, which they excavate, undermine, and displace in the usual manner.
genera of this family of lizards, e. g. Istiurus, Lophyrus, Calotes, and Otocryptis, have the teeth soldered, like those of Mosasaurus, to the summit of the alveolar ridge, and thence are called " Acrodonts " in all these lizards the maxillary and mandibular teeth may be divided into anterior laniary, and posterior molary teeth. In most of the Iguanians the teeth are lodged in a common shallow oblique alveolar groove, and are soldered to excavations on the inner surface of the outer wall of the groove : these are called Pleurodonts. Most of them possess pterygoid as well as maxillary teeth : but the following genera, Hyperanodon, lepis, Phtynosonia, and Callisaurus, are excep tions.
In the Pleurodont Iguanians, the teeth never present the true laniary form ; and if simply conical, as at the extremes of the maxillary series, the cone is more or less obtuse ; but, in general, it is expanded, more or less trilobate, or dentated along the mar gin of the crown.
The Amblyrh yn gilts, a genus which is some what remarkable for the marine habits of at least one of the species (Aniblyrhynchus ater), whose diet is sea-weed*, has the tricuspid structure well developed in the posterior teeth.
The typical genus of the present family of Saurians (Iguana tuberculata), is characterised by the crenate or dentated margin of the crown of the maxillary and premandibular teeth, a few of the anterior small ones ex cepted. The pterygoid teeth are arranged
in two or three irregular rows, resembling somewhat the " dents en cardes " of fishes. In the full-grown Iguana tuberculata there are from forty-seven to forty-nine teeth in both upper and lower jaws. The number is less in young subjects. The double row of pterygoid teeth are in close order on each side.
In the horned 'Iguana (Metopoceros cor nutus), there are about fifty-six teeth in the upper and lower jaws, of which the four first are conical and slightly recurved. The twelve succeeding teeth are somewhat larger in size, with more compressed and expanded crowns ; the rest are triangular, compressed, with dentated margins. The inner surface of the crown of the tooth is simply convex and smooth ; the outer surface traversed by a me dian, longitudinal, broad, obtuse ridge. There is a single row of small teeth implanted in each pterygoid bone. No Iguanian lizard has teeth on the palatine bones.
The teeth of the Iguanodon, though re sembling those of the Iguana, do not present an exact magnified image of them, but differ in the greater relative thickness of the crown, its more complicated external surface, and, still more essentially, in a modification of the internal structure, by which the Iguanodon equally deviates from every other known reptile.
As in the Iguana, the base of the tooth is elongated, contracted, and subcylindrical ; the crown expanded, and smoothly convex on the inner side. When first formed, it is acu minated, compressed, its sloping sides ser rated, and its external surface traversed by a median longitudinal ridge, and coated by a layer of enamel, but, beyond this point, the description of the tooth of the Iguanodon indicates characters peculiar to that genus. In most of the teeth that have hitherto been found, three longitudinal ridges (fig. 571.) traverse the outer surface of the crown, one on each side of the median primitive ridge ; these are separated from each other, and from the serrated margins of the crown, by four wide and smooth longitudinal grooves. The relative width of these grooves varies in different teeth ; sometimes a fourth small lon gitudinal ridge is developed on the outer side of the crown. The marginal serrations, which, at first sight, appear to be simple notches, as in the Iguana, present, under a low magnifying power, the form of trans verse ridges, themselves notched, so as to resemble the mammilated margins of the un worn plates of the elephant's grinder : slight grooves lead from the interspaces of these notches upon the sides of the marginal ridges. These ridges, or dentations, do not extend beyond the expanded part of the crown : the longitudinal ridges are continued further down, especially the median ones, which do not subside till the fang of the tooth begins to assume its subcylindrical form. The tooth at first increases both in breadth and thick ness ; it then diminishes in breadth, but its thickness goes on increasing ; in the larger and fully formed teeth, the fang de creases in every diameter, and sometimes tapers almost to a point. The smooth un broken surface of such fangs indicates that they did not adhere to the inner side of the maxillm, as in the Iguana, but were placed in separate alveoli, as in the Cro codile and Megalosaur ; such support would appear, indeed, to be indispensable to teeth so worn by mastication as those of the Igu anodon. A fracture of this tooth shows that the pulp was not entirely solidified, but that its cavity had continued open at the thickest part of the tooth.