398 RODENTS. 



enamel is situated upon the anterior and outer part of the incisor, 

 forming, by the abrasion of the dentine, the cutting edge of the 

 tooth ; its fibres are larger than in the Kangaroo, and their transverse 

 striae more plainly visible, the enamel is covered by a layer of 

 cement continued from the thicker layer upon the back part of 

 the tooth. 



The tubular structure of the dentine of the molars is figured 

 in PI. 103, fig. 2, d d, as seen in part of a section near the base 

 of the crown ; in this is likewise shown the lower boundary of the 

 investment of enamel, {e e,) and the continuation of the coronal 

 layer of cement, (c c,) from the fang, upon the enamel of the crown. 

 There are no parallel tubuli nor vascular canals in the cement ; 

 but the radiating tubuli from the calcigerous cells are very numerous, 

 and form rich plexuses in the interspaces. The outer part of the 

 cement is dense and devoid of radiated cells. 



CHAPTER VI 



TEETH OF RODENTIA. 



158. — ^We have traced in the Marsupialia a progressive diminu- 

 tion in the number and size of those teeth which intervene between 

 the anterior incisors and the true molars, until the dentition assumed 

 in both jaws the condition expressed by the formula of the genus 

 Phascolomys, p. 393. This condition of one large curved incisor 

 on each side, separated by a wide interval from a short series 

 of molars, characterizes the whole order of Rodents : the rule 

 being proved by the single exceptional family {Leporidce) of Hares, 

 Rabbits, and Pikas, or tail-less Hares of Siberia, which retain a 

 second minute incisor behind each of the normal ' dentes scal- 

 prarii' of the upper jaw. 



The incisors(l) are always regularly curved, the upper ones des- 



(1) PI. 104, figs. 2 & 3, i. 



