LIZARD. Any member of the reptilian order Lacertilia. These animals differ from turtles and tortoises in the ab sence of carapace and plastron and from crocodiles and the Tuatera in never having more than one bony arch across the temporal region. As a rule they may be distinguished from snakes by the presence of limbs but there are limbless lizards which resemble snakes in so many respects that no single character or group of simple external characters serves to differentiate them ; the two orders are undoubtedly closely allied and they are the two dominant groups of reptiles of the present day.
Lizards, of which about 2,50o species are at present known, are a cosmopolitan group, their northern and southern limits approaching those regions where the subsoil is permanently frozen; it is in the tropics, however, that the greatest number, both of individuals and species, is to be found. None are thoroughly aquatic though many take readily to water, and none are efficient fliers but they have invaded almost every other type of environment. Correlated with the environmental conditions and with their habits they exhibit great variety in size, shape and structure, the structural modifications occurring chiefly in the skin, limbs, tail, tongue, teeth and digestive tract. The skin is usually beset with horny scales, which are sometimes underlain by bony plates (osteoderms), and this horny outer covering is shed at irregular intervals, not in one piece as is usual among snakes, but piecemeal; the scales themselves may be small and granular, flattened and overlapping or irregularly enlarged to form tubercles or spines. Limbs may be well developed with five separate clawed digits on each, or entirely absent and an uninterrupted transition from one condition to the other can be traced; in addition, the digits may have fringes of scales or be webbed in desert-dwelling species, be equipped with adhesive pads in climbing forms or be bound together to form opposable bundles. The tail varies considerably in shape and relative size and is often extremely fragile; fracture occurs in definite cleavage planes which pass through the middle of the vertebrae and the lost portion is regenerated, but on a simpler plan, without true vertebrae and usually with a more primitive type of scalation.
If the fracture is incomplete a new tail is often produced from the wound and in this way double, and sometimes triple tails may be produced. The ability to shed the tail has a definite value since the detached member retains its muscular irritability for a short time and its wriggling movements may distract the pursuer's attention for the short time necessary to enable the lizard to make its escape. The tongue is usually covered with papillae which may be scale-like or arranged in parallel rows; in the majority of lizards it is flattened and only slightly notched in front but in some it is rounded, bifid and retractile into a basal sheath, like that of a snake. Teeth are generally all alike, conical and pointed but rarely they are flattened, grooved or ser rated; occasionally they are differentiated into "incisors," "ca nines," and "molars" though the teeth so named are not homolo gous with the same teeth in mammals.
Most lizards reproduce by means of eggs which are left to hatch where they are deposited; they are elliptical, pointed at both ends and enclosed in a leathery or calcareous shell. In different families the practice of retaining the eggs within the body of the mother until the young are ready to be hatched (ovo-viviparity) has been developed but in one or two skinks the egg-shell is entirely lost and the developing embryo receives nourishment directly from the mother through a placenta (vivi parity).
The species existing at the present day (a few fossil forms are known) are grouped into 21 families, based chiefly on features of the skeleton which are discussed in the general article on REPTILES.
I. Gekkonidae.—Geckoes, q.v. Small, soft-skinned lizards with the eye covered by a transparent watch-glass-like covering and often with adhesive lamellae beneath the digits ; cosmopolitan in distribution ; all reproducing by means of hard-shelled eggs.