Three types of teeth, connected by every intermediate step, are distinguished: the solid, the grooved, and the canaliculated or tubular, so-called "perforated"; the third, as we shall explain, being only a further modification of the second. In the grooved tooth, a sulcus runs along the anterior or outer surface, its object being to convey into the wound the secretion of a poison gland. It varies in depth according to the species, and may be so slight as to escape detection without a very strong magnifying glass. In some the sulcus may be very deep and wide, forming a canal round which the tooth folds to the extent of its borders nearly meeting; from this condition the so-called "perforated" fang is derived through the complete fusion of the borders of the tooth, and the obliteration of the line of union except at each extremity. The structure of such a fang may be best understood by imagining a tooth, lined all round with the same layer of dentine and enamel, being flattened out in a vertical plane and then folded over, the outer edges coalescing on the front median line in such a way that the inner wall of the tooth is in reality the anterior surface, and the outer wall the posterior surface, of the ordinary tooth. Grooved teeth, with open canal, are situated either at the anterior extremity (Proteroglyphs) or at the posterior extremity (Opisthoglyphs) of the maxillary bone, usually followed or preceded by a series of solid teeth, which in some cases may likewise show a more or less distinct groove. Such may also be present on the teeth of the lower jaw, as in the European _Coelopeltis_, in some specimens of which a faint groove is visible on the outer side with the aid of a strong lens. The tubular fangs of the Viperidæ are inserted on the posterior extremity of the much abbreviated and erectile maxillary bone, which bears no other teeth. The Proteroglyphs (Cobras, Coral-snakes, Sea-snakes) and the Solenoglyphs (Vipers, Pit-vipers, Rattlesnakes) may be regarded as the diverging extremes in the development of the poison apparatus, both culminating in forms with tubular fangs, the former as derived directly from the Aglyphs (harmless snakes), the latter from the Opisthoglyphs, likewise evolved out of the Aglyphs. That the insertion of the poison fangs of the Viperidæ is really on the posterior extremity of the maxillary bone is evident from the condition of the bone in its recumbent position, especially in the African Viper, _Causus_, which in several respects departs less markedly from the Colubrid type than our European Vipers.