4. Uropeltidae.—Harmless burrowing snakes similar to those of the preceding family, but without any vestiges of hind limbs and with the tail ending in an enlarged scale which is either rugose or produced into two short points. The "earth-snakes" of Ceylon and the hills of southern India are all burrowers in loose earth in damp, forested regions and feed almost exclusively upon earth worms; those whose breeding habits are known are all ovo viviparous.
5. Xenopeltidae.—This family contains but a single species, Xenopeltis unicolor of south-eastern Asia, a handsome snake with highly iridescent black or dark brown scales, each of which has a lighter edge ; the head is small, not sharply marked off from the rest of the body and covered with enlarged shields; there are no traces of hind limbs, teeth are present in both jaws, the tail is short and the belly is covered with transversely enlarged scutes ; total length about 3 feet.
6. Boidae.—Non-poisonous, often large, snakes with claw-like rudiments of hind-limbs visible on either side of the vent, en larged scutes on the belly and teeth in both jaws. This family, which is represented throughout the tropical and sub-tropical regions of the world, is sub-divided into two groups, the Pythons, which are most numerous in the Old World, and the true Boas, whose headquarters are in tropical America with only a few scattered forms in other parts of the globe. These two sub divisions are not recognisable by any external characters, the Pythoninae only differing from the Boinae by the presence of an extra bone (the supraorbital) in the skull.
(a) Pythoninae: see PYTHON.
(b) Boinae.—The majority of Boas are arboreal or semi arboreal, and correlated with this habit the tail is usually more or less prehensile ; all kill their prey (chiefly small mammals and birds) by constriction, and all those whose breeding habits are known are ovoviviparous, in contrast with the Pythons which usually lay eggs. Though the name "Boa Constrictor" is fre quently used to designate any large snake it is only strictly ap plicable to a single South American species, Constrictor con strictor, which is not particularly large, attaining a total length of only about i r feet. The genus Boa contains a number of species in tropical America, including the West Indies, and two in Mada gascar; Corallus, another very similar genus, has a similar dis tribution. The largest species is the Anaconda (Eunectes mu rinus), which inhabits the forests of the Amazon basin and is said to reach a length of 3o f t., though actually, specimens of
even 25 ft. seem to be very rare. The animal is largely aquatic and is usually found along the banks of rivers or in swampy regions; it is dark green in colour with numerous, sharply defined, round, black spots and is highly iridescent. In the Old World, Enygrus occurs in the islands around New Guinea and on those of Polynesia as far east as Fiji; all the species have prehensile tails. Eryx, with about seven species, is an assemblage of small, sand-loving forms in north Africa, south-east Europe and south west Asia ; none of them greatly exceed 3 ft. in length and all have small heads, merging imperceptibly into their bodies, and short, scarcely prehensile tails.
7. Colubridae.—This family comprises the great majority of snakes and its members show great diversity of habits and adapta tions. In the restricted sense in which the name is here used the family may be defined as follows :—typical snakes, without any rudiments of limbs, with the head usually covered by enlarged scales, with enlarged ventral scutes, with teeth in both jaws and poison fangs, if present, situated on the hinder end of the upper jaw and preceded by smaller, solid teeth; distributed over all the temperate and tropical regions. Every kind of terrain is tenanted, from dense tropical forests to deserts and high moun tains, and so uniform are the modifications connected with their different habits that, from its appearance, alone it is possible to draw a fairly safe conclusion as to a snake's mode of life. Bur rowing forms are invariably small with relatively short tails, with small heads merging imperceptibly into the body and with greatly reduced eyes; desert dwellers are usually rough-scaled, with pallid or sombre colouring ; terrestrial species have cylin drical bodies, a distinct neck and moderately long tails ; arboreal snakes are greatly elongate with whip-like tails, long, pointed snouts, very large eyes and a more or less compressed body ; thoroughly aquatic forms have the nostrils on the top of the snout and fitted with valves which can be closed while the animal is under water and, as a rule, the enlarged ventral scutes, so necessary for locomotion on land, are reduced. Two groups of Colubrids can be recognised thus :— Aglypha, without any grooved teeth.