The chief differences between snakes and lizards have already been given and need not be repeated, save very briefly. Snakes are always functionally legless, though some have vestiges of the hind pair; the brain-case is wholly bony; the upper temporal bar is wanting; the lower jaws are united in front by ligaments only, like those of the mosasaurs; the vertebrae are greatly increased in number, and always have the additional zygosphenal articulations like those of _Clidastes_ and _Mosasaurus_ and some lizards; there is but one lung, and the eyes are always without free eyelids. But these characters are really not very important, since every one of them is found in the lizards or mosasaurs, except the complete ossification of the brain-case, and even this is partly ossified in the mosasaurs. It is rather the presence of all these characters which distinguishes a snake from a lizard. The number of living snakes is nearly as great as that of the living lizards, and their distribution over the earth is very similar. Snakes are for the most part strictly terrestrial in habit. Some live more or less among trees, and some live in the water, though with but few exceptions all are fully capable of rapid progression upon land. They are almost invariably carnivorous in habit, swallowing their prey whole, and usually alive, as has been described. Some poison their prey or crush it to death before swallowing it. Some feed upon eggs which are swallowed whole and then crushed in their stomachs by projecting bones from the under side of the vertebrae developed for that purpose. In size snakes vary from a few inches in length to twenty-five or more feet, no known extinct forms being larger than the living anacondas and boas. In geological history the earliest remains known date from the latter part of the Cretaceous, and it is quite probable that they have a briefer history than that of the lizards of which they are the descendants. Venomous serpents are known only from comparatively recent geological times, and it is probable that venomosity is the latest and final specialization of importance in the reptilian class. [Illustration: FIG. 81.—_Hydrus bicolor_; sea-snake. (From Brehm)]