Finally, the dental groove is divided by complete partitions II, and a separate socket is formed for each tooth ; and this stage of developement is attained in the highest or gan'sed reptiles, the crocodiles (fig. 573.).
This may be four-fold, and a single tooth may be composed of dentin; cement, enamel, and bone; but the dentine and cement are present in the teeth of all reptiles.
In the Batrachians and Ophidians a thin layer of cement invests the central body of dentine, and, as usual, follows any in flections or sinuosities that may characterise the dentine. Besides the outer coat of cement, which is thickest at the base of the teeth, a generally thin coat of enamel defends the crown of the tooth in most Saurians, and the last remains of the pulp are not unfrequently converted into a coarse bone, both in the teeth which are anchylosed to the jaw, and in some teeth, as those of the Ichthyosaur, which remain free. The only modification of the dentine, which could at all entitle it to be regarded in the light of a new or distinct substance, is that which is peculiar in the present class to the teeth of the Iguanodon, and which will be described in the following section.
varieties of dental structure are few in the reptiles as compared with either fishes or mammals, and its most com plicated condition arises from interblending of the dentinal and other substances rather than from modifications of the tissues themselves. In the teeth of most reptiles the intimate structure of the dentine corresponds with that which has been described as the type of the tissue, e. g. the hard or unvascular dentine, and which is the prevailing modification in Mammalia, viz., the radiation of a system of minute plasmatic tubes from a single pulp cavity, at right angles to the external surface of the tooth. The most essential modification of this structure is the intermingling of cylin drical processes of the pulp-cavity in the form of medullary canals, with the finer tubular structure.* Another modification is that in which the dentine maintains its normal struc ture, but is folded inwardly upon itself, so as to produce a deep longitudinal indentation on one side of the tooth ; it is the expansion of the bottom of such a longitudinal deep fold that forms the central canal of the venom fang of the serpent; but a glance at fig. 568.
will show that, notwithstanding the singularly modified disposition of the dentine (b), its structure remains unaltered ; and although the pulp-cavity (p) is reduced to the form of a crescentic fissure, the dentinal tubes continue to radiate from it according to the usual law. By a similar inflection of ninny vertical longi tudinal folds of the external cement and ex ternal surface of the tooth at regular intervals around the entire circumference of the tooth, and by a corresponding extension of radiated processes of the pulp-cavity and dentine into the interspaces of such inflected and con verging folds, a modification of dental struc ture is established in certain extinct reptiles, which, by the various sinuosities of the inter blended folds of cement and processes of dentine, with the partial dilatations of the radiated pulp-cavity, produces the compli cated structure which is described at p. 868. and figured in cut 552. But this compli cation is nevertheless referable to a modi fication of form or arrangement of the dental tissues, rather than of the structure of the tissues themselves: the calcigerous tubes in each sinuous lobe of dentine, in the most complex tooth of the Lahyrinthodon, exhibit the same general disposition and course as in the fang of the serpent and in the still more simple tooth of the Saurian.
Development. —The teeth of reptiles are never completed, as in certain fishes, at the first or papillary stage ; but the pulp sinks into a follicle, and becomes inclosed by a cap sule; and in certain reptiles this becomes more or less surrounded by bone ; but the process of development never offers the erup tive stage, in the sense in which this is usually understood, as signifying the extrica tion of the young tooth from a closed alveolus.
The completion of a tooth, with the extinct exception of the Dicynodont Reptiles, is soon followed by preparation for its removal and succession : the faculty of developing new tooth-germs seems to be unlimited in the pre sent class, and the phenomena of dental deca dence and replacement are manifested at every period of life ; the number of teeth is gene rally the same in each successive series, and the difference of size presented by the teeth of different and distant series is considerable.