Opisthoglypha, with one or two enlarged, grooved. poison fangs at the rear of the upper jaw.
These two groups, the one harmless, the other venomous, but, on account of the position of the poison fangs, rarely dangerous to man, almost grade into one another, a few species being known in which the posterior teeth are only slightly enlarged and some times grooved and sometimes not; they also show a curious parallelism. The Aglyplia are again sub-divisible into four "sub families." (a) Dipsadinae.—Arboreal forms with the anterior teeth in both jaws enlarged and without a groove beneath the chin. This groove, present in almost all other colubrids, is part of the mechanism which allows of the two halves of the lower jaw being pushed apart in swallowing bulky objects and its absence in the Dipsads is connected with the feeding habits of the group; they appear to feed exclusively on slugs. Amblycephalus and Haplo peltura occur in south-east Asia and the Malay Archipelago and Dipsas and its allies are confined to Central and tropical South America.
(b) Acrochordiwie.—Aquatic snakes with valvular nostrils on the upper surface of the snout ; they are heavy-bodied animals frequenting fresh-waters and estuaries in south-east Asia and Central America, and feeding on fish. In a few the enlarged ventral scales are scarcely distinguishable and the common Indo-Malayan Chersydrus has the body laterally compressed and equipped with a fold along the lower surface ; this fold, simulating a median fin, the compressed body and the absence of ventral shields bespeak an existence spent wholly in water. The less specialised forms with cylindrical bodies and retaining the ventral scutes are still capable of rapid movement on land.
(c) Dasypeltinae (Rhaciodontinae).—Terrestrial snakes with only a few small teeth but with downwardly directed projec tions, which penetrate the oesophagus, on some of the vertebrae of the neck; a few species only, in tropical and South Africa. These snakes are highly modified egg-eaters ; the mouth is enor mously distensible and the egg is swallowed whole; when, however, it comes in contact with the vertebral processes in the throat its progress is arrested, and by compression of the muscles of the neck, and backward and forward sawing motions, the shell is cut through and collapses ; the contents of the egg run down into the stomach and the shell is rejected.
(d) Colubrinae.—Typical snakes, with well-developed teeth in both jaws, with a dental groove and with, as a rule, lateral nostrils. This sub-family is cosmopolitan and its members may be more or less aquatic, arboreal, terrestrial or cryptozoic ; as a rule they are oviparous, but some produce fully developed young. Perhaps the most thoroughly aquatic species is Glypholycus bicolor which inhabits Lake Tanganyika and in the general shape and the position of the nostrils superficially resembles the Acro chordime. The common English Grass snake (Natrix natrix)
is a member of a widely distributed genus whose members, though not strictly aquatic, are water loving and whose food consists principally of frogs and fish ; N. natrix, widely distributed through northern Europe, is very variable in colour, but is usually a grey ish-green with some black markings on the back, traces of an incomplete yellow or orange collar on the neck and chequered black and white beneath ; it frequents damp and marshy locali ties and feeds principally on frogs. Its European relative N. viperinus, very common in Spain and Portugal, is more aquatic still and feeds almost exclusively on and the Indian Keel Backs (N. piscator) and the North American Moccasins (N. fasciatus) have similar habits. Allied to Natrix are the N. Ameri can Garter snakes (Thamnophis), a group of likewise semi-aquatic species. The strictly terrestrial genera, very numerous throughout most of the world except the Australian region, feed chiefly on small mammals, birds, toads and lizards. The Rat snakes or Racers of Europe and N. America (Coluber) the Chicken snakes (Elaphe) and the Indian Rat snakes (Ptyas) are all externally of very similar build and either kill their prey by constriction or eat it alive. Coronella, another similar genus, is represented in Britain by the Smooth snake (C. austriaca), a species frequently confused with the viper but readily distinguished by its smooth scales and the enlarged plates on the top of the head ; the species of this genus feed largely upon lizards, and the distribution of the Smooth snake in England corresponds exactly with that of the Sand lizard. Other terrestrial genera with specialised feeding habits are the King snakes (Lampropeltis) of N. America, and the African Lycophidion. The species of the first-mentioned genus live almost exclusively on other snakes, poisonous or harmless, the victim being killed by constriction ; some "mimic" Coral snakes in their colour pattern which consists of broad reddish bands edged with black and separated by narrower yellowish or buff interspaces. Lycophidion is characterised by enlarged teeth on the front of both jaws, the "canine" teeth being also directed outwards, and this is probably an adaptation to enable them to hold the smooth, hard-scaled skinks which form their principal diet. The American Hog-nosed snakes (Heterodon) are remarkable for their superficial similarity to vipers, due to the broad, flat head and stout body; if annoyed they puff themselves up and hiss in true viperine manner, but if this fails to intimidate the enemy they roll over on their backs and "sham dead." Tree snakes are greatly elongate, slender creatures and are represented by such genera as Dendrophis of south-east Asia and Australia and Leptophis of South America; they feed principally on lizards and young birds.