With the exception of the species of _Atractaspis_, these snakes are all ovoviviparous. The majority are terrestrial; a few lead a semi-aquatic existence, while others are arboreal. Their distribution includes Europe, Asia, Africa (with the exception of Madagascar), and North and South America. They do not exist in Australia. They are divided into two _Sub-families_:-- A. The _Viperinæ_, in which the head, which is very broad and covered with little plates and scales, has no pit between the nose and the eyes; B. The _Crotalinæ_ (κρὁταλον, a rattle), in which the head is incompletely covered with scales, and exhibits a deep pit on each side, between the eye and the nostril. * * * * * Among snakes, the characters that serve as a basis for the determination of genera and species are the general shape of the body, especially that of the head, the arrangement of the cephalic scales, the cranial skeleton, and the dentition. * * * * * _Cranial Skeleton._--The cranium is composed of a certain number of bones, the homologues of which are found in the mammalian skeleton; but the bones are complex, and subject to modifications according to the structure and habitat of each species. The special arrangement of the bones of the face is above all characteristic of the poisonous snakes. Those forming the upper jaw, the palate and the mandibles or “inter-maxillaries” are movable upon each other and on the cranium. The upper and lower maxillaries are united by an extensile ligament and articulated with the tympanic bone, which permits the mouth to be opened very widely when the animal swallows its prey. * * * * * _Dentition._--The _non-poisonous_ snakes have two rows of teeth in the upper jaw--one external, the _maxillary_, usually composed of from 35-40 small, backwardly curved teeth; the other internal, the _palatine_, which only numbers from 20-22 teeth, having the same curvature (fig. 1, A). [Illustration: FIG. 1.--A, Cranial skeleton of one of the non-poisonous _Colubridæ_ (_Ptyas mucosus_); B, cranial skeleton of one of the poisonous _Colubridæ_ (_Naja tripudians_); C, cranial skeleton of one of the poisonous _Colubridæ_ (_Bungarus fasciatus_); D, cranial skeleton of one of the _Viperidæ_ (_Vipera russellii_); E, cranial skeleton of one of the _Viperidæ Crotalinæ_ (_Crotalus durissus_); F, cranial skeleton of one of the _Colubridæ Hydrophiinæ_ (_Hydrophis pelamis_).] In the _poisonous_ snakes the maxillary bones are shorter, and the _outer_ row is represented by a single long and tubular or grooved tooth (the fang), fused with the maxillary bone, which is itself movable (fig. 1, B, C, D, E, F).