Opisthoglypha.—The back-fanged snakes are divisible into three sub-families which exactly parallel the last three divisions of the preceding group.
(a) Homalopsinae Aquatic snakes with valvular nostrils situ ated on the top of the snout, and often with the ventral scales much reduced. They are confined to the fresh waters and estuaries of south-east Asia, Papua and North Australia and are all ovo viviparous.
(b) Elachistodontinae.—The single snake of this sub-family, known only from a few specimens from Bengal, is the exact Opisthoglyphous analogue of the Aglyphous Dasypeltis ; it is an egg-eater, with reduced dentition and with vertebral processes penetrating the oesophagus.
(c) Boiginae (Dipsadomorphinae).—This sub-family parallels the Colubrinae ; its members are cosmopolitan and adapted to the same types of habitats, though in this group arboreal forms pre dominate. Among these may be mentioned a few which are remarkable for their habits or appearance ; the slender, bodily form and elongate head is exaggerated in some, such as Dryophis mycterizans of the Indian Region or Langaha of Madagascar, by the production of the tip of the snout into a pointed or leaf-like dermal appendage. Many of these tree-snakes are brilliantly green to match the foliage, others brown to harmonise with the branches, but some show brilliant colour patterns which, though conspicuous when removed from their natural surroundings, are almost invisible in the dappled light and shade of the bushes which the animals frequent. Chrysopelea ornata of India and Malaya is often black with yellow dots on the centre of each scale and a series of red-centred yellow, tetra-petalous flowers along the back. This species is also famous as the Flying snake, a name it has earned by its ability to "glide" to a limited extent; to accomplish this feat the body is held straight and rigid, the ribs outwards to their full extent and the belly drawn in so that a considerable concave surface is produced which checks the fall of the animal and enables it, in an emergency, to descend with safety from a considerable height. Some members of the group, for instance the Boomslang (Dispholidus typos) of South Africa, have the peculiar habit of expanding the neck to form a vertical "hood" when annoyed. Unlike the horizontal hood of the cobras, which is pro duced by an outward pressure of the ribs, this vertical "hood" is brought about by inflation of the wind-pipe. In snakes the cartilag inous rings of the trachea are incomplete along the side nearest the backbone, and a strip of tissue between their free ends is very distensible, so that, when air from the lungs is forced into the wind-pipe, this elastic tissue stretches and the neck is inflated.
Terrestrial genera are represented in Europe by Coelopeltis, one of the largest snakes of the continent, in the Mediterranean countries. Erythrolamprus aesculapii of Tropical America is a very variable species whose many colour varieties mimic Coral snakes ; sometimes the groove, on the fangs may be absent. Africa
has many terrestrial forms, of which the commonest are the Sand snakes (Psammophis), and also several burrowing genera, e.g., Miodon of West Africa and Aparallactus from the east and south. Some burrowing forms, notably Apostolepis in South America, exhibit "directive mark" coloration; the body is yellow, sometimes with longitudinal black stripes, and the head and stumpy tail are black with a few lighter dots, so that the similarity of the two ends is remarkable.
Only a few Opisthoglyphs can be considered as dangerous to man ; the situation of the fangs renders it difficult for any but the very large species to bring them into play except on the small est objects. Should, however, a wound be inflicted by one of the larger species, such as the Boomslang, the consequences are apt to be dangerous ; the venom is largely haemolytic in its action and resembles that of the vipers rather than that of the cobras and their allies.
8. Micruridae (Elapidae).—Sometimes regarded as Colubrids, these snakes are "proteroglyphous," i.e., have fixed poison fangs in the front of the upper jaw and they always have well-developed ventral shields. Apart from the Cobras (q.v.), this family con tains other genera in the same regions, the true Coral snakes of Central and South America and a number of forms in Australia. Some of the best known of the latter are the Black snake (Pseu dechis), which reaches a length of 6 ft. and can erect a small cobra-like hood, the Tiger snake (Notechis) and the Death Adder (Acanthophis); the latter has acquired its appellation of "Adder," which usually means viper, on account of its broad, flat head, which lacks the large shields of the other Micruridae, and its thick set body. In the Indian region, the larger species are the Kraits (Bungarus) whose bite, though not so deadly as that of the Cobras, is responsible for a heavy mortality annually. The dreaded Mamba (Dendraspis angusticeps) of tropical and South Africa also belongs to this family, and is one of the few arboreal species in it. Two varieties are commonly recognised, a black, which is really a dark olive, and a green, and as no very small black ones have ever been recorded and the green variety is usually the smaller of the two, it seems probable that the "varieties" are growth stages of the one species ; the green phase is almost entirely arboreal but the black is frequently terrestrial. The Coral snakes (Micrurus or Daps) are the only representatives of this family in America; they are distributed all over the hotter parts of that continent and are largely burrowing and cryptozoic forms. Dolio phis, another genus of burrowing animals from the Malayan region, is remarkable for the enormous development of the poison glands, which extend over the anterior third of the body, and for its habit, when annoyed, of raising the tail to show the red colour of its lower surface.