The bones of the roof of the mouth of the Squamata—that is, of the palate—are narrow and long, and are not closely articulated, as in most other reptiles; they often bear teeth, a primitive character. The teeth of all living lizards and snakes are not inserted in sockets, as are those of the crocodiles, but are co-ossified to the margins or sides of the jaws or the bones of the palate. But this is probably not a primitive character; doubtless the teeth of the early lizards were inserted in sockets like those of most other reptiles. The shoulder bones are absent in many and vestigial in some others. When present and fully developed, they comprise the shoulder-blades or scapulae, a single coracoid on each side, the clavicles, and an interclavicle. The vertebrae, except in some lizards, are procoelous, that is, with the body concave in front and convex behind, a peculiar structure that was developed only in crawling animals. In addition to the usual articulations for the union of the vertebrae there are also, in some of the lizards and mosasaurs and all of the snakes, additional ones called the zygosphene and zygantrum, which will be best understood by reference to Fig. 12, p. 28. But little less characteristic than the loose articulation of the lower jaws, so unique in this order of reptiles, is the manner of attachment of the ribs. They are always single-headed, articulating only with the body or lower part of the vertebra. The single-headed ribs of the plesiosaurs articulate with a projection on each side of the arch of the vertebra; those of the turtles to the space between the adjacent vertebrae; nearly all other reptiles have double-headed ribs, articulating in various ways. This character, it is seen, though apparently a simple one, immediately distinguishes a lizard or a snake from all other animals, except the thalattosaurs and protorosaurs. There is much difference of opinion among naturalists as to the proper classification of the different groups of this order of reptiles. Usually it is divided into four suborders, the Lacertilia or lizards; the Dolichosauria or long-necked lizards of the past; the Mosasauria, or extinct swimming lizards; and the Serpentes or Ophidia, the snakes. It matters very little which classification one accepts so long as it is remembered that the first three groups are closely related to each other. LIZARDS Popularly a lizard is any four-legged reptile covered with scales, but such a definition is not strictly correct, since some lizards are legless and some other four-legged reptiles are covered with horny scales, notably the tuatera or _Sphenodon_ of New Zealand, a reptile long classed with lizards, but now known to belong to quite a different order. Bearing in mind those characters given as characters of the order, it will be necessary to mention only those distinguishing the lizards from the snakes. [Illustration: FIG. 65.—Iguana. (By permission of the New York Zoölogical Society)]