Burrowing-snakes live chiefly underground, and often have the visual organ atrophied in consequence, as in _Typhlops_; all the Typhlopidæ, Glauconiidæ, and Uropeltidæ, belong to this category; the Viperid _Atractaspis_ is also a burrowing type. Tree-snakes spend the greater part of their life on bushes or trees. _Corallus_ among the Boidæ, _Dendrophis_ and _Dendraspis_ among the Colubridæ, _Atheris_ and various species of _Lachesis_ among the Viperidæ, may be quoted as examples. Of Water-snakes, some are exclusively aquatic, like the marine Hydrophiinæ and the typical Acrochordinæ (_Acrochordus_, _Chersydrus_) and Homalopsinæ (_Hipistes_, _Herpeton_). _Chersydrus_ and _Hipistes_ occur in the sea as well as in fresh water. Many species of _Tropidonotus_ (_T. tessellatus_ and _T. viperinus_ in Europe), as well as the genera _Helicops_, _Grayia_, _Boulengerina_, etc., among the Colubridæ, _Eunectes_ among the Boidæ, _Ancistrodon piscivorus_ among the Viperidæ, are chiefly but not exclusively aquatic. Our _Tropidonotus natrix_ stands between the Ground-snakes and the Water-snakes; Boas and Pythons are as much Water-snakes as Tree-snakes. As shown by these and many other examples which might be given, a division into categories cannot always be applied with precision, nor does it convey an expression of the natural relationships of the species, as was believed by many systematists of the last century, who appealed to such adaptations for the definition of families. A vertical pupil denotes more or less nocturnal habits. Nevertheless our European Vipers, which are provided with such a contractile pupil, are far from exclusively nocturnal, delighting to bask in the sun, and pairing and feeding in the day-time. The Boidæ appear to be more nocturnal, but no snake is known to be absolutely so, and the two species of _Coluber_ which have been found living in perfect darkness in limestone caves in the Malay Peninsula and China, where they feed chiefly on bats, occur also outside the caves, and probably never breed in them. It is often stated in books that the organs of locomotion for the exceedingly elongate body of snakes are the ribs, and these creatures have even been compared to Centipedes. This statement is no doubt true to a certain extent for slow locomotion on uneven ground, when the ribs and the corresponding ventral shields afford a point of support; but it does not account for the rapid movements, as when a snake darts like an arrow in pursuit of its prey or to escape from an enemy. Besides, the winding motions are not different from those of a Slow-worm or Glass-snake, in which, encased as they are in a bony armour, the ribs cannot come into play at all. The action of the muscles alone is quite sufficient to account for the reptation of snakes, without the ribs having to play an essential part.