404 ANATOMY OF VERTEBRATES. 



transverse ridges, themselves notched, so as to resemble the mam- 

 millated margins of the unworn plates of the elephant's grinder. 

 These ridges or dentations do not extend beyond the expanded 

 part of the crown : the longitudinal ridges are continued farther 

 down, especially the median ones, which do not subside till the 

 fang of the tooth begins to assume its subcylindrical form. 



At the earlier stages of abrasion, a sharp edge is maintained at 

 the external part of the tooth by means of the enamel which 

 covers, and is restricted to, that surface of the crown. The 

 prominent ridges upon that surface give a sinuous contour to the 

 middle of the cutting edge, whilst its sides are jagged by the 

 lateral serrations. The dentine next the enamel is harder than 

 the vaso-dentine of the opposite half of the crown. When the 

 crow T n is worn aw T ay beyond the enamel, it presents a broad and 

 nearly horizontal grinding surface, and another dental substance 

 is brought into use to give an inequality to that surface ; this 

 is the ossified remnant of the pulp, which, being firmer than 

 the surrounding dentine, forms a slight transverse ridge in the 

 middle of the grinding surface. The tooth in this stage has 

 exchanged the functions of an incisor for that of a molar, and is 

 prepared to give the final compression, or comminution, to the 

 coarsely divided vegetable matters, such as might be afforded by 

 the Clathrarice and similar fossil plants, which are found buried 

 with the Iguanodon. 



In the Crocodilian Monitor ( Varanus bivittatus) the large fixed 

 compressed teeth, of which there may be about seven in each 

 upper maxillary bone and six in each premandibular, are anchy- 

 losed by the whole of their base and by an oblique surface leading 

 upwards on the outer side of the tooth to a slight depression on 

 the alveolar surface. The base of the tooth is finely striated, the 

 lines being produced by inflected folds of the external cement, as 

 in the Ichthyosaur and Labyrinthodon, but they are short and 

 straight, as in those of the former genus. The great Varanus, 

 like the variegated species, manifests its affinity to the Crocodilians 

 in the number of successive teeth which are in progress of growth 

 to replace each other ; but, from the position in which the germs 

 of the successional teeth are developed, the more advanced teeth 

 in this species, as in the Varanus variegatus, do not exhibit the 

 excavations that characterise the same parts of the teeth of the 

 Enaliosaurs and Crocodiles. 



In some extinct Saurians, which, in other parts of their organi- 

 sation, adhere to the Lacertine division of the order, the teeth w T ere 

 implanted in sockets, either loosely or confluent with the bony walls 



