294 FOSSIL REPTILES. 



of the nearest tooth in place, where it is partly enclosed. The 

 new tooth is then certainly in the old, but not entirely en- 

 veloped by it. 



In whichever way the new tooth comes, the time arrives when 

 its growth completely displaces the old one. It produces on its 

 ossified base, a sort of necrosis, or dry gangrene, which breaks 

 its adherence to the jaw, and makes it fall. This is not a 

 spontaneous rupture like that of the old antlers of stags, which 

 fall before the new ones have pushed forth. The agency of the 

 new tooth always goes for something. There can, therefore, 

 be no difficulty in distinguishing the teeth of lizards from those 

 of crocodiles, nor even in distinguishing, to a certain degree, 

 the teeth of one genus of lizards from those of another. 



In the monitors there is no internal alveolar edge, and the 

 new teeth grow in the thick part of the gum, between the bases 

 of the teeth in place, or at the internal face of their bases. They 

 are easily discovered by detaching the gum. These teeth are 

 conical. In the majority of species they are moreover pointed, 

 compressed laterally, and a little hooked. The aquatic monitor 

 of Egypt, and one or two other African species, have only the 

 hinder teeth in straight obtuse cones, or even entirely rounded 

 and blunted at the top. The species with trenchant teeth have 

 the edge very finely crenulated, but the crenulations are some- 

 times visible only with a convex lens. These teeth are not 

 very numerous, not more than a dozen or fifteen on each side, 

 and none in the palate. Neither have the teguixin, ameiva, 

 draccena, and others of this subgenus peculiar to America, any 

 teeth in the palate. Many others also want these palatine 

 teeth. 



The subgenera which have these teeth are the lizards proper, 

 the iguanas, the polychrus, the anolis, and many skinks. There 

 are other variations respecting the form of the teeth in lizards, 

 into which it is impossible, in a sketch of this kind, to enter 

 minutely. 



We have already seen that the hyo'id bone is sim{)le in the 



