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 _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.
The poison fangs of the Viperidæ appear to be movable, folding in the
mouth when at rest, and erected, or even thrust forward, when ready to
act. This, however, is simply due to the mobility of the maxillary bone,
to which they are ankylosed as in all other snakes. There are normally
two equally-developed fangs, close together and side by side, to each
maxillary, followed by several replacement fangs loosely attached behind
them, usually in two series of four. When the two fangs are _in situ_,
they of course both function in the act of biting, although only one is
in relation with the single poison duct; often, however, there is only
one fang in position, either the right or the left, the place of the
other being indicated by a shallow socket which will soon be filled by
one of the posterior reserve fangs moving forward and becoming ankylosed
to the bone. Snake-charmers who extract the poison fangs of the snakes
they use for their performances have therefore to renew the operation
frequently, unless they amputate the bone on which the fangs are
inserted, an injury which the creature does not long survive.
The dentition of the snakes in which the maxillary bone is not movable
vertically falls under three divisions: the Aglyphs, in which the teeth
are all solid; the Opisthoglyphs, in which one or more (usually two) of
the hindermost teeth are provided with a groove; and the Proteroglyphs,
in which grooved or canaliculated teeth are situated in front, followed
or not by solid teeth. Beyond these three principal divisions, the
dentition furnishes important characters for the classification,
although that importance has sometimes been over-estimated. The
maxillary teeth may be equal in length (Isodonts), or the anterior the
longer (Lycodonts), or the posterior the longer, increasing gradually in
size (Coryphodonts) or abruptly, without (Syncranterians) or with a
diastema, or break, in front of them (Diacranterians). These categories
are, however, so completely connected as to preclude their use in
taxonomy beyond helping to define genera. The number of maxillary teeth
and the relative proportions and disposition of the mandibular teeth
also afford useful generic characters.
The European genera may be arranged as follows, according to the
dentition:
I. Teeth few, disposed in a transverse series in the upper jaw only
_Typhlops._
II. Teeth in both jaws and on the palatines and pterygoids.
_A._ A series of solid teeth along the maxillary; no grooved teeth.
1. Anterior maxillary and mandibular teeth longest; 9 or 10
maxillary teeth
_Eryx._
2. Maxillary teeth equal, or increasing in size posteriorly.
_a._ Mandibular teeth 17 to 30; maxillary teeth 15 to 22.
Posterior maxillary teeth longest; mandibular teeth subequal, more than
20
_Tropidonotus._
Posterior maxillary teeth longest; mandibular teeth not more than 20,
posterior smallest
_Zamenis._
Maxillary teeth subequal; mandibular teeth 20 to 25, posterior smallest
_Coluber._
_b._ Mandibular teeth 14 or 15, subequal; maxillary teeth 12 to
15.
Maxillary teeth increasing in size
_Coronella._
Maxillary teeth subequal
_Contia._
_B._ One or two enlarged grooved fangs behind the series of solid
maxillary teeth.
14 to 17 subequal solid maxillary teeth, forming a continuous series;
21 to 23 mandibular teeth, anterior strongly enlarged
_Coelopeltis._
9 to 11 solid maxillary teeth, fourth and fifth or fifth and sixth
enlarged, followed by an interspace; sixth mandibular tooth fang-like,
followed by an interspace
_Macroprotodon._
9 or 10 solid maxillary teeth, forming a continuous series, decreasing
in length posteriorly; anterior mandibular teeth strongly enlarged
_Tarbophis._
_C._ Maxillary with only two large canaliculated fangs side by side,
one of which may be missing; anterior mandibular teeth longest
_Vipera_, _Ancistrodon_.
[Illustration: FIG. 12--MAXILLARY AND MANDIBLE OF--(_a_) _Tarbophis
fallax_; (_b_) _Coelopeltis monspessulana_; (_c_) _Macroprotodon
cucullatus_. (From British Museum Catalogue of Snakes)]
In counting the teeth for the purpose of using this key, care must be
taken to ascertain the full number, as it frequently happens that one or
more are missing; but their place is indicated by the shallow pits in
which their base was implanted, the overlooking of which might convey
the impression of a hiatus such as is characteristic of certain
genera--_Macroprotodon_, for instance. Needless to say, the loose teeth
which are in reserve on the inner side of the jaws or behind the tubular
fangs are not taken into consideration, the numbers given being those of
functional teeth only. Although as a rule the teeth can be counted
easily, on a specimen preserved in spirit, by simply pushing aside the
lips and gums with the finger, it is sometimes necessary to remove and
clean the bones of the jaws, an operation which does not require much
skill.
------------------------------------------------------------------------