The third order of Reptiles is that of the Lacertilia, comprising all those animals which are commonly known as Lizards, together with some serpentiform animals, such as the Blind-worms. The Lacertilia are distinguished by the following characters:

As a general rule, there are two pairs of well-developed limbs, but there may be only one pair, or all the limbs may be absent. A scapular arch is always present, whatever the condition of the limbs may be. An exoskeleton, in the form of horny scales like those of the Snakes, is almost always present. The vertebra of the dorsal region are procoelous or concave in front, rarely amphi-coelous or concave at both ends. There is a single transverse process at each side, and the heads of the ribs are simple and undivided. There is either no sacrum, or the sacral vertebrae rarely exceed two in number. The teeth are not lodged in distinct sockets (some extinct forms constituting an exception to this statement). The eyes are generally furnished with movable eyelids. The heart consists of two auricles and a ventricle, the latter partially divided by an incomplete partition. There is a urinary bladder, and the aperture of the cloaca is transverse.

As a general rule, the animals included under this order have four well-developed legs (fig. 301), and would therefore be popularly called "Lizards." In some (Chirotes) there are no hind-feet; in some (Pipes) the fore-limbs are wanting; and others (Anguis, Pseudopus, and Amphisbaena) are entirely destitute of limbs, thus coming closely to resemble the true Snakes or Ophidians in external appearance. These serpentiform Lizards, however, can be distinguished from the true Snakes, amongst other characters, more especially by the structure of the jaws. In the Snakes, as before said, the two rami of the lower jaw are loosely united in front by ligaments and muscles, and are attached behind to a movable quadrate bone, which is in turn connected with a movable squamosal, this giving an enormous width of gape to these animals. In the Lizards, however, even in those most like the Snakes, the halves of the lower jaw are firmly united to one another in front; and though the quadrate bone is usually more or less movable, the jaws can in no case be separated to anything like the extent that characterises the Ophidia.

Fig. 301.   Iguana.

Fig. 301. - Iguana.

Another good general character is to be found in the structure of the protective coverings of the eye. In the Snakes, eyelids are wanting, and the eye is simply covered by a layer of epidermis, constituting the so-called "antocular membrane." In almost all the Lizards, on the other hand, including most of the completely snake-like forms, there are movable eyelids, and in few cases is there any structure comparable to the antocular membrane of the true Snakes. Lastly, the typical Lizards all possess a sternum or breast-bone, but this is wanting in some of the snake-like forms, so that it cannot be appealed to as a character by which the Lacertilia can be separated from the Ophidia, Whatever the condition of the limbs may be, there is, however, always a pectoral arch more or less completely developed, even though the pelvic arch should be wanting.

The whole order of the Lacertilia is very often united with the next group of the Crocodilia, under the name of Sauria. The term "Saurian," however, is an exceedingly convenient one to designate all the Reptiles which approach the typical Lizards in external configuration, whatever their exact nature may be; and from this point of view it is often very useful as applied to many fossil forms, the structure of which is only imperfectly known. It is therefore perhaps best to employ this term merely in a loose general sense. All the Lacertilians possess teeth, though these vary considerably in their arrangement. (If the extinct Rhynchosaurus be truly Lacertilian, this genus has apparently no teeth.) The teeth are always simple, sometimes sharp and conical (Monitor), sometimes blade-like with serrated edges (Iguana), sometimes with rounded crushing crowns (Cydodus). Usually the teeth become anchylosed to the jaw, when they may be fixed by their sides to the inner wall of the alveolar border of the jaw ("pleurodont" dentition), or may be attached by their bases to the summit of this border ("acrodont" dentition). In the extinct Protoro-sanria, the teeth are implanted in distinct sockets ("thecodont" dentition).

The Lacertilia are sometimes divided into three sections in accordance with the structure of the tongue. In one group, including the greater number of the members of the order, the tongue is long, protrusible, and forked (Fissilinguia or Leptoglossa), as in the Serpents. In a second group (Brevi-linguia or Pachyglossa), including the Geckos and Agamids, the tongue is thick, fleshy, and not protrusible. Lastly, in a third group (Vermilinguia) are placed the Chameleons, with their long protrusible worm-like tongue, the extremity of which is clubbed. The following are the principal families of the Lacertilians.

The first family of importance is that of the Amphisbaenidae, including a number of serpentiform lizards, in which both pairs of limbs are uniformly absent (except in the Mexican Chirotes, in which the fore-limbs are present). All, however, possess a pectoral arch, and they are further distinguished from the Ophidians by the fact that the rami of the lower jaw are united by suture, so as greatly to restrict the gape. The type-genus, Amfihisbaena, is South American, and comprises apodal snake-like lizards with short blunt tails, having the vent situated nearly at the end of the body.

In the family of the Chalcididae we also have lizards with long snake-like bodies, but minute fore and hind limbs are present. The scales are rectangular, and are arranged in transverse bands which do not overlap. All the members of this group are American. In the nearly allied Zonuridae the limbs may be well developed, rudimentary, or wholly wanting ; there is a longitudinal fold of skin on each side of the body; and the abdominal scales are square or roundish, and disposed in cross bands. The ears are distinct, and the eyes are provided with eyelids. In this group are the footless American Glass-snakes (Ophisaurus), and the Sheltopusiks (Pseudopus) of the Old World, in which the limbs are wanting, or a rudimentary pair of hind-legs is present.