In the caudal region, elongate transverse processes take the place of ribs, and the hæmapophyses are paired, one on each side of the hæmal canal. In the Rattlesnakes the seven or eight last vertebræ are enlarged and fused into one.
No snake shows any rudiments of the pectoral arch, but remains of the pelvic are found in the Typhlopidæ, the Glauconiidæ, the Boidæ, and the Ilysiidæ. In the first these vestiges are reduced to a single bone (ilium?) on each side; in the second they consist of ilium, pubis, and ischium, the latter forming a ventral symphysis, and a rudimentary femur; whilst in the third there is a long ilium, attached to the lower branch of the first bifurcate transverse process of the lumbar vertebræ, bearing three short bones, the longest of which, regarded as the femur, terminates in a claw-like spur which, in males at least, usually appears externally on each side of the vent.
CHAPTER V
DENTITION
In the most generalized snakes—those which show the nearest approach to lizards—teeth are present not only on the rami of both jaws, but also on the premaxillary bone, on the palatines, and on the pterygoids. A reduction of the dentition takes place in various genera, in which the teeth of either the upper or the lower jaw, and of the palatines or pterygoids, or both, may be absent, and the premaxillary is devoid of teeth in the great majority, including all European representatives, of the Ophidia.
In the egg-eating snakes of the genera Dasypeltis and Elachistodon the dentition is very much reduced, in accordance with the peculiar régime, and this deficiency is compensated by the development on some of the anterior thoracic vertebræ of long, tooth-like processes (hypapophyses) directed forwards, and capped with a remarkably dense, vitreous tissue simulating enamel, the function of these tooth-like processes being to break the shell of the egg within the gullet, where none of its contents are lost, the shell being afterwards rejected through the mouth in the form of a pellet.
With the exception of the worm-like Typhlopidæ, which are provided with a few teeth in the upper jaw only, European snakes have teeth on the maxillary, palatine, pterygoid, and dentary bones. Unless the maxillary be strongly abbreviated and modified in connexion with the poison apparatus, as in the Viperidæ, the teeth in the jaws as well as on the palate form single longitudinal series; they are elongate, conical, with or without a sharp posterior edge, more or less recurved, acutely pointed, sometimes needle-like, and directed backwards, as behoves their function, which, in addition to attack and defence, is to prevent the retrogression of the prey in the act of prehension and deglutition. A notable exception occurs in the genus Iguanognathus, from Sumatra, all the teeth having spatulate crowns ribbed along the outer side. Unfortunately, nothing is known as to the food of this remarkable snake. The teeth are coated with a thin layer of enamel. It was held, for a time, that the glossy outer coating was only due to a denser structure of the dentine. As in all living Reptiles with the exception of the Crocodiles, the teeth are not implanted in true sockets, but simply ankylosed to the bone on which, when detached, their slightly enlarged base, or rather the bony tissue on which it rests, leaves a shallow impression, or pseudo-socket. In the process of biting or feeding, some of the teeth are frequently lost, and are readily replaced by others lying in reserve in the gum at the inner side, and becoming fixed to the bone soon after a vacancy occurs. Such replacement teeth, of different grades of development, form several series, so that in a snake like our common Tropidonotus the mouth may contain four times as many teeth as are functional, without reckoning different earlier stages of tooth germs which escape ordinary observation, being placed vertically one above the other.
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 Cœlopeltis, in some specimens of which a faint groove is visible on the outer side with the aid of a strong lens.