The enamel which invests the harder dentine, forming the outer side of the tooth, presents the same peculiar dirty brown colour, when viewed by transmitted light, as in most other teeth : very minute and scarcely per ceptible undulating fibres, running vertically to the surface of the tooth, is the only struc ture I have been able to detect in it.
The cement is simply and minutely cellular upon the crown of the tooth, but it exhibits the radiated cells at the base of the tooth.
The remains of the pulp in the contracted cavity of the completely formed tooth are converted into a dense but true osseous substance, characterised by minute elliptical radiated cells, whose long axis is parallel with the plane of the concentric lamellae, which surround the few and contracted medullary canals in this substance.
The microscopical examination of the struc ture of the Iguanodon's teeth thus contributes additional evidence of the perfection of their adaptation to the offices to which their more obvious characters had indicated them to have been destined.
To preserve a trenchant edge, a partial coating of enamel is applied; and, that the thick body of the tooth might be worn away in •11. more regularly oblique plane, the dentine is rendered softer as it recedes from the enameled edge by the simple contrivance of arresting the calcifying process along certain tracts of the inner wall of the tooth. When attrition has at length exhausted the enamel and the tooth is limited to its functions as a grinder, a third substance has been prepared in the ossified remnant of the pulp to add to the efficiency of the dental instrument in its final capacity. And if the following reflections were natural and just after a review of the external characters of the dental organs of the Iguanodon, their truth and beauty be come still more manifest as our knowledge of their subject becomes more particular and exact : " In this curious piece of animal mechanism we find a varied adjustment of all parts and proportions of the tooth, to the exercise of peculiar functions, attended by compensations adapted to shifting conditions of the in strument, during different stages of its con sumption. And we must estimate the works of nature by a different standard from that which we apply to the productions of human art, if we can view such examples of me chanical contrivance, united with so much economy of expenditure, and with such anti cipated adaptations to varying conditions in their application, without feeling a profound conviction that all this adjustment has re sulted from design and high intelligence."# Varanians.—In the great Crocodilian Moni tor (Varanus crocodilinzes), the large fixed com pressed teeth, of which there may be about seven in each tipper maxillary hone and six in each premandibular, are anchylosed by the whole of their base and by an oblique surface leading upwards on the outer side of the tooth to a slight depression on the oblique alveolar surface, as in the Var. striates. The base of the tooth is finely striated, the lines being produced by inflected folds of the external cement, as in the Ichthyosaur and Labyrinthodon, but they are short and straight, as in those of the former genus. The alveolar channel or groove has scarcely any depth ; but the anchylosed base of the tooth is applied to an oblique surface, terminating in a sharp edge, from which the outer side of the free crown of the tooth is directly continued.
The great Varanus, like the variegated species manifests its affinity to the Crocodilians in the number of successive teeth which are in progress of growth to replace each other ; but from the position in which the germs of the successional teeth are developed, the more advanced teeth in this species, as in the Var. variegates, do not exhibit the excavations that characterise the same parts of the teeth of the Enaliosaurs and Crocodiles.
Thecodonts. —We have seen that among the inferior or squamate Saurians there are two leading modifications in the mode of attach ment of the teeth, the base of which may be either anchylosed to the summit of an alveolar ridge, or to the bottom of an alveolar groove, and supported by its lateral wall. These modifications are indicated respectively by the terms " Acrodont " and " A third mode of fixation is presented by some extinct Saurians, which, in other parts of their organisation, adhere to the squamate or Lacertine division of the order, the teeth being implanted in sockets, either loosely or confluent with the bony walls of the cavity ; these I have termed the " Thecodont"* La certians the most ancient of all Saurians belong to this group ; viz. the Thuringian Monitor, or Protorosaurus, and the Palceo saurus of the dolomitic conglomerates near Bristol. The compressed Varanian form of tooth, with trenchant and finely dentated margins, which characterised the ancient Pa lceosaur and Chadeiodon, is continued in the comparatively more recent and gigantic species of terrestrial lizard, of which the remains were discovered by Dr. Buckland in the oolite of Stonesfield, by whom the peculiarities of the jaws and teeth have been accurately and gra phically described in the following words :— " From these remains we learn that the animal was a reptile, closely allied to some of our modern lizards; and viewing the teeth as instruments for providing food to a carni vorous creature of enormous magnitude, they appear to have been admirably adapted to the destructive office for which they have been designed. Their form and mechanism will be best explained by reference to the figures. • " The outer margin of the jaw rises nearly an inch above its inner margin, forming a continuous lateral parapet to support the teeth on the exterior side, where the greatest support was necessary, whilst the inner margin throws up a series of triangular plates of bone forming a zigzag buttress along the interior of the alveoli. From the centre of each triangular plate, a bony partition crosses to the outer parapet, thus completing the successive alveoli. The new teeth are seen in the angle between each triangular plate, rising in reserve to supply the loss of older teeth, as often as progressive growth, or ac cidental fracture, may render such renewal necessary, and thus affording an exuberant provision for a rapid succession and resto ration of these most essential implements. They were formed in distinct cavities, by the side of the old teeth, towards' the interior surface of the jaw, and probably expelled them by the usual process of pressure and absorption, insinuating themselves into the cavities thus left vacant. This contrivance for the renewal of teeth is strictly analogous to that which takes place in the dentition of many species of existing lizards.