The order Squamata, so called because of the dermal covering of overlapping horny scales, comprises the great majority of living reptiles. Although the scaly covering is characteristic of nearly all the members of the order, the most essential differences distinguishing them from other reptiles are, as usual, found in the skeleton, and especially in the skull. The quadrate bone, that to which the lower jaw is articulated on each side, is not wedged in immovably between other bones of the skull, as in all other reptiles, but is, instead, freely articulated with the cranium in such a way that its lower end moves both backward and forward, as well as inward and outward. This freedom of movement has in the past been thought to be due to the loss of a lower temporal arch, a bony bar connecting the lower end of the quadrate with the hind end of the upper jaw, which is very characteristic, for instance, of the Rhynchocephalia. Indeed, because of the many primitive characters which the lizards possess, it has generally been supposed that the order was an early branch of the rhynchocephalian stem. But we are now quite sure that the lizards are as primitive as the Rhynchocephalia, and that their origin, as an independent branch of the reptilian stem, goes quite as far if not farther back—quite sure that the ancestors of the lizards never had a lower temporal arcade and two temporal vacuities, but that the looseness of the quadrate bone has been due to the gradual loss of a bone which covered the whole side of the skull until only the upper part of it was left. In other words, the ancestral skull of the Squamata must have been like that of _Araeoscelis_, more fully described under the Protorosauria, a group than which there is perhaps none more closely allied to the Squamata.