390 



ANATOMY OF VERTEBRATES. 



TfCLli of Mcuoiiomc ; fc prcmaxiJlniy, I vomerine, 

 X raaxlllaTy 



by the various sinuosities of the interblended folds of cement and 

 processes of dentine, with the partial dilatations of the radiated 

 pulp-cavity, produces the complicated structure which is described 

 at p. 365, and figured in cut 244. 



Developement. — The teeth of Eeptiles are never completed at the 



first or pajiillary stage ; the pulp 

 265 sinks into a follicle, and becomes 



inclosed by a capsule ; and in 

 certain reptiles this becomes 

 more or less surrounded by bone ; 

 but the ' eruptive ' stage, in the 

 sense in which this is usually 

 understood, as signifying the ex- 

 trication of the young tooth from 

 a closed alveolus, is not exem- 

 plified in recent Reptiles, and 

 was rare in extinct ones.' 



The completion of a tooth, 

 with the exception of the Dicy- 

 nodont Reptiles, is soon fol- 

 lowed by preparation for its 

 removal and succession ; the faculty of developing new tooth- 

 germs seems to be unlimited in the present class, and the phe- 

 nomena of dental decadence and replacement are manifested at 

 every period of life ; the number of teeth is generally the same in 

 each successive series, and the diflPerence of size presented by the 

 teeth of different and distant series in the same individual is 

 considerable. 



The new germ is always developed, in the first instance, at 

 the side of the base of the old tooth, never in the cavity 

 of the base ; the crocodiles form no exception to this rule. 

 The poison-fangs of serpents succeed each other from behind 

 forwards ; in almost every other instance the germ of the succes- 

 sional tooth is developed at the inner side of the base of its 

 predecessor. In the Frog the dental germ makes its appear- 

 ance in the form of a papilla developed from the bottom and 

 towards the outer side of a small fissure in the mucous membrane 

 or gum that fills up the shallow groove at the inner side of the 

 alveolar parapet and its adherent teeth : the papilla is soon 

 enveloped l)y a capsular process of the surrounding membrane. 

 vVs the tooth ac(|uires hardness and size, it presses against the 



cxLin. p. 178, pi. ix., liy. 4, (/. 



