TEETH OF REPTILES. 



407 



fourth tooth in the lower jaw passes into a groove in the margin 

 of the upper jaw when the mouth is closed. 



In the Alligators and Crocodiles the teeth are more unequal in 

 size, and less regular in arrangement, and more diversified in 

 form, than in the Gavials : witness the strong thick conical 

 laniary teeth as contrasted with the blunt mammillate summits of 

 the posterior teeth in the Alligator, fig. 275. The teeth of the 

 Gavial are subequal, most of them present the form of a crown, 

 shown in fig. 274, long, slender, pointed, subcompressed from 

 before backward, with a trenchant edge on the right and left 

 sides, between which a few faint longitudinal ridges traverse the 

 basal part of the enamelled crown. 



In the black Alligator of Guiana the first fourteen teeth 

 of the lower jaw are implanted in distinct sockets, the re- 

 maining posterior teeth are lodged close to- 

 gether in a continuous groove, in which the 

 divisions for sockets are faintly indicated by 

 vertical ridges, as in the jaws of the Ichthyo- 

 saurs. A thin compact floor of bone separates 

 this groove, and the sockets anterior to it, from 

 the large cavity of the ramus of the jaw ; it is 

 pierced by bloodvessels for the supply of the 

 pulps of the growing teeth and the vascular 

 dentiparous membrane which lines the alveolar 

 cavities. 



The tooth-germ is developed from the mem- 

 brane covering; the angle between the floor and 



O O 



the inner wall of the socket. It becomes in 

 this situation completely enveloped by its cap- 

 sule, and an enamel-organ is formed at the 

 inner surface of the capsule before the young 

 tooth penetrates the interior of the pulp-cavity 

 of its predecessor. 



The matrix of the young growing tooth Teetll ln clifferent stages of 

 affects, by its pressure, the inner wall of the 

 socket, as shown in fig. 275, and forms for itself 

 a shallow recess : at the same time it attacks 

 the side of the base of the contained tooth ; 

 then, gaining a more extensive attachment by 

 its basis and increased size, it penetrates the large pulp-cavity 

 of the previously formed tooth, either by a circular or semi- 

 circular perforation. The size of the calcified part of the 

 tooth-matrix which has produced the corresponding absorption of 



**J! e 



^e base partly absorbed 



by the pressure of b, the 



succession^ tooth ;beiow 



which is figured c, the 



serm of the next tooth 



to follow, v. 



