TEETH. 



891 



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 Laccrta agllis. 



In the Gigantic fossil Monitor of Maes- 

 tricht, the teeth combine the Pleodont with 

 the Acrodont* characters. 



The true affinities of the Mosasanr, 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 Iguanae and Monitors. 

 Its dentition exhibits in an eminent degree 

 the Acrodont character; the teeth being sup- 

 ported on expanded conical bases ancliylosed 

 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 Mosasaurns, " lias no true 

 root, but it adheres strongly 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 

 ancliylosed ; 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 officmalis) has four or five 

 small obtuse teeth upon each pterygoid bone. 

 The chief exception to the typicaf dentition 

 of the present family is made by the large 

 scincoid lizards of Australia, which, on that 

 account, have received the generic name of 

 Cycludus.^ 



Fig. 570. 



Lower jaw and teeth of Cydodus nigroluteus. 



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. 



The gradual transition from the simple 

 structure of the compact dentine totheosteo- 

 dentine of the anchylosed base of the tooth was 

 not known to Cuvier; otherwise he could not 

 have supposed that the crown and the base 

 of the tooth of the Mosasaurus were formed by 

 vital processes of so dissimilar a nature as to 

 forbid him considering them as parts of one 

 and the same body. Cuvier had originally 

 described the expanded base of the tooth of 

 the Mosasaur as the root of the tooth ; but 

 afterwards, observing that the corresponding 

 base became anchylosed by ossification of the 

 remains of the pulp to the jaw, he conceived 

 it to be incorrect to regard it as a part of a 

 body which he believed to be an inorganic 

 product, and the result of excretion. " The 

 tooth," he observes, in correcting his first 



* Oclontography, pp. 241. 258. 



The dentition of the Cycl. nigroluteus is 

 exemplified in the lower jaw, fig. 570. In the 

 upper jaw, the single premaxillary bone has 

 depressions for twelve teeth, of which only 

 the alternate ones are usually in place ; they 

 are of very small size, with the fang com- 

 pressed laterally, and the crown antero- 

 posteriorly, so as to resemble a true incisor 

 in form, the summit sloping to an edge from 

 behind forwards, with the middle of the cut- 

 ting surface a little produced. Each superior 

 maxillary bone has depressions for fourteen 

 teeth ; they quickly increase in size, and 

 exchange their conical for a sub-hemispherical 

 crown; the eighth to the thirteenth inclusive 

 are the largest teeth ; they are set obliquely, 

 and pretty close together. In the lower jaw 

 there are two small incisors, at the anterior 

 part of each premandibular bone correspond- 

 ing with those of the premaxillary ; these are 

 succeeded by five or six conical teeth, and 

 the rest correspond in size and form with the 

 tuberculate molars of the upper jaw. 



* Compte Rendu de 1'Academie des Sciences, 

 Decembre, 1839. 



f Hound-toothed : xvxins, round ; eSoi^, tooth. 



