It is true that the great majority of lizards have four legs, while the snakes are always functionally legless, but there are some lizards, like the glass snakes and the amphisbaenas, or slow lizards, which are quite legless and there are some snakes which have small but functionless hind legs. As usual, more important differences are found in the skull. The brain-case in all snakes is surrounded on all sides by bone, for the better protection of the brain, with the head resting quite prone on the ground. The brain of the lizards, for the most part, is protected on the sides and in front by a simple membrane. Nearly all lizards have movable eyelids, while snakes do not; snakes have a single lung, and a protrusible tongue, which very few lizards possess; and the lower jaws in front are united in the snakes by a ligament only. Notwithstanding these differences, the snakes and lizards are closely related animals, and must have come from a common ancestry; among all reptiles the known geological history of the snakes is shortest. Lizards, on the other hand, have a very high antiquity, beginning, as we now know, at least as long ago as early Triassic times. They still have many primitive characters in their structure and are the least advanced type of reptiles now living, with the exception of the tuatera. Their remains are seldom found in the rocks, probably because they have always been so strictly terrestrial in habit, for the most part seldom frequenting even the vicinity of the water. The true lizards now living number about eighteen hundred kinds, classified into about twenty families, divided among four chief groups, of which the chameleons, the amphisbaenas, our common lizards, and the monitors are representatives.