The genera of the typical family of the squamate Lacertians are arranged in two sub-families, the chief characteristics of which are derived from the dental system.
In the first group, the teeth are solid, or without any permanent internal cavity, and are very firmly anchylosed by their base to the alveolar groove upon the inner side of the jaw ; so that the extremity of the tooth is slightly directed outwards. The species which present this character are called Pleo donts.
In the second group, the teeth are ex cavated, or retain the pulp-cavity, and are less firmly fixed to the jaws, being applied vertically, like piles or buttresses, against the outer alveolar parapet, but not adhering by their base. This group is called Ccelodonts.
The Monitor Lizard of S. America is an example of the Pleodont group, in which the premaxillary teeth are ten in number. The maxillary teeth vary from ten to fifteen on each side, and increase in size as they are placed farther back : the hindmost teeth are tricuspid in young individuals, and present the form of simple tubercles in the old Mo nitors. The mandibular teeth, fifteen to eighteen in number in each ramus, correspond in size and form with those above. In the Ccelodont group, the "Swift lizards" (Tachy dromus) have the pterygoid bones armed with minute teeth. The teeth on both upper and lower jaws are of larger size, and the hinder ones are tricuspid. The true lizards (La certa) have two kinds of teeth quoad form ; the anterior small, conical, and recurved ; the posterior larger, and bi- or tri-cuspid. Some species have also pterygoid teeth ; as the common Lacerta agilis.
In the Gigantic fossil Monitor of Maes tricht, the teeth combine the Pleodont with the Acrodont# characters.
The true affinities of the Mosasaur, which was at least twenty-four feet in length, and the remains of which characterise the chalk formations, were first determined by Cuvier, who places it in the Lacertian group of Saurians, between the Iguanm and Monitors. Its dentition exhibits in an eminent degree the Acrodont character ; the teeth being sup ported on expanded conical bases anchylosed to the summit of the alveolar ridge of the jaw : no existing Saurian exactly parallels this mode of attachment of the teeth, either in regard to the breadth of the alveolar border, or in the relative size of the osseous cones to the teeth which they support. A
shallow socket is left where the tooth and its supporting base are shed. The form of the teeth is likewise different from that hitherto observed in any existing Saurian : the crown is pyramidal, with the outer side nearly plane, or slightly convex, and separated by two sharp ridges from the remaining sur face, which forms a half-cone. All the teeth account of the Mosasaurus, " has no true root, but it adheres to that pulp which has secreted it, and It is further held in connection with it by the remains of the capsule which has furnished the enamel, and which, by becoming ossified also, and uniting itself to the maxillary bone and the ossified pulp, implants or rivets the tooth with addi tional force." The necessity under which Cuvier felt himself compelled to regard the crown and the base of the tooth of the Mosasaur as two distinct parts, is at once banished by the recognition of the principle, that the processes of calcification are essentially the same at every part of a tooth, whether it be free or anchylosed ; and that they are modified only, as 1 have shown in my Memoir on the Formation of the Teeth of the Shark*, ac cording to the density of the part to be pro duced.
Scincoid Lizards.—Most of these smooth scaled lizards have small mouths and slender sharp teeth, fitted best for insect food ; they are usually confined to the upper and lower jaws ; but the medicinal Scink of ancient pharmacy (Scincus officinalis) has four or five small obtuse teeth upon each pterygoid hone. The chief exception to the typical dentition of the present family is made by the large scincoid lizards of Australia, which, on that account, have received the generic name of Cycicdus.t are slightly recurved, and their peripheral surface is smooth. The teeth are implanted upon the premaxillary, maxillary, and pre mandibulary bones ; a series of similarly shaped but much smaller teeth are placed upon the pterygoid bones.