From the real nature of the poison-canal it follows that the transverse section of the tooth varies in form in different parts of the tooth ; at the base it is oblong, with a large pulp-cavity of a corresponding form, with an entering notch at the anterior surface; farther on the transverse section presents the form of a horse-shoe, and the pulp-cavity that of a crescent, the horns of which extend into the sides of the deep cavity of the poison fang : a little beyond this part the section of the tooth itself is crescentic, with the horns obtuse and in contact, so as to circumscribe the poison-canal ; and along the whole of the middle four-sixths of the tooth, the section, of which a magnified view is given in fig. 568., mences a little beyond the base of the tooth, where its nature is readily appreciated, as the poison-duct there rests in a slight groove or shows the dentine of the fang inclosing the poison-canal, and having its own centre or pulp-canal ( p,p), in the form of a crescentic fissure, situated close to the concave border of the inflected surface of the tooth. The pulp cavity disappears, and the poison-canal again resumes the form of a groove near the apex of the fang, and terminates on the anterior surface in an elongated fissure.
The venom-fangs of the viper, rattle-snake, and the are coated only with a thin layer of a subtransparent and minutely cellular cement. The disposition of the den tinal tubes is obedient to the general law of verticality to the external surface of the tooth ; it is represented as seen in the transverse sec tion from the middle of the fang in fig. 563. Since the inflected surface of the tooth can be exposed to no other pressure than that of the turgescent duct with which it is in con tact, the tubes which proceed to the surface d, while maintaining their normal relation of the right angle to it, are extremely short ; and the layer of dentine separating the poison tube from the pulp-cavity is proportionally thin. The calcigerous tubes that radiate from the opposite side of the pulp-cavity to the exposed surface b of the tooth are dispropor tionally long.
The teeth of Ophidians are developed and completed in that part which forms the ori ginal seat of the tooth-germs in all animals ; viz. the mucous membrane or gum covering, the alveolar border of the dentigerous bones. This germ presents the same lax tissue, and is as abundantly developed, as in the Pike, Lophius, and many other fishes ; in which it likewise serves as the nidus and locality the complete development of the teeth. The primitive dental papillm in the common harm less snake very soon sinks into the substance of the gum, and becomes inclosed by a cap sule. As soon as the deposition of the cal careous salts commences in the apex of the papilla the capsule covering that part becomes ossified and adherent to the dentine, and the tooth begins to pierce and emerge from the gum before its mould, the pulp, is half com pleted. Fresh layers of cells are successively
added to the base of the pulp, and converted, by their confluence and calcification, into the tubular dentine, until the full size of the tooth is attained, when its situation in the gum is gradually changed, and its base becomes surface of the pulp ; and the base of the groove of the loose, growing, poison-fling is brought into the same relation with the duct of the poison gland as the displaced fang, which has been severed from the duct.
Saurians. — The existing species of lizards differ from those of the crocodile in the anchy losed condition of the teeth, which present few modifications of importance ; those that yield most fruit to physiology, and which have most expanded our ideas of the extent of the re sources of Nature and the exceptional devi ations from what was deemed the rule of structure in the Saurian dentition, have been discovered by the study of the fossil teeth of extinct forms of the order. Amongst these the most extraordinary in respect of their dental system have been recently discovered in a form ation in South Africa, which seems nearly as ancient as our own coal-seams. I have called them " Dicynodonts,"* from their dentition being reduced to one long and large canine tooth on each side of the upper jaw. As these teeth give, at first sight, a character to the jaws like that which the long poison-fangs give, when erected, to the jaws of the rattle snake, I shall briefly notice their characters before entering upon the description of the more normal Saurian dentition.
Fig. 569. gives a reduced side view of the skull of the species of Dicynodon called D. la certiceps. The cranial cavity (8, 8) is extremely contracted, as in all the cold-blooded quad rupeds : it is bounded on each side by wide and deep temporal foss (1) indicating power anchylosed to the shallow cavity of the alve olar surface of the bone.
In the posterior part of the large mucous sheath of the poison-fang, the successors of this tooth are always to be found in different stages of development ; the pulp is at first a simple papilla, and when it has sunk into the gum the succeeding portion presents a depression along its inferior surface, as it lies horizontally, with the apex directed back wards; the capsule adheres to this inflected ful muscles for the action of the lower jaw. The orbits (o) are large and round ; the nos trils (n) are divided by the junction of the nasal bones (15) with the premaxillaries (22), as in lizards ; there is not a single median external nostril, as in Chelonian and Croco dilian reptiles. The alveolar border of the lower jaw and of the premaxillary part of the * From air, two, and xs,iaopr, the name given by Hippocrates to the canine teeth, and signifying the same idea as their common English denomination.