404 RODENTS. 



to a simple depression bounded by a raised circular margin, as in the 

 great Cape-mole : that margin being formed by true enamel, but in 

 the Sloths by hard dentine. 



159. Structure. — The proportions and relative positions of the den- 

 tine, enamel and cement which enter into the composition of the scal- 

 priform incisors have already been mentioned ; I have observed the 

 modifications of the enamel, alluded to at p. 399, in the Rat, the Water- 

 Vole, the Beaver, the Agouti and the Hare, and believe it to be 

 common to the Rodentia : the difference in the tubular structure of 

 the anterior and posterior halves of the dentine is more conspicuous 

 in the Hare, Vole and Beaver than in the Agouti. 



In the Agouti a longitudinal slice from the middle of the upper 

 incisor shows the conical pulp-cavity extending through one half of 

 the tooth, and a linear tract continued from its apex to the gnawing 

 surface of the tooth : from this line, the end of which, in most 

 Rodents, is perceptible on that surface, the calcigerous tubes diverge ; 

 those passing to the anterior margin of the tooth describe a sigmoid 

 curve, first convex towards the cutting end, which may be called 

 the crown, then concave : the tubes which proceed to the opposite 

 margin describe a stronger and shorter curve convex towards the 

 crown, and then proceed straighter and more transversely to their 

 termination : the origin of these tubes is obscured by the cut ends 

 of those which were proceeding towards the sides of the tooth : 

 and which are chiefly formed by the posterior half of the pulp. The 

 tubuli are laiuuth of an inch in diameter and are very closely arranged, 

 being separated by intervals of little more than their own diameter : 

 so that the dentine is unusually firm and compact. The exterior 

 clear and denser enamel forms about one- third the thickness of that 

 layer and is coated by thin and well-defined brown-coloured cement. 



The fibres composing the inner and more opake part of the 

 enamel proceed obliquely, but almost transversely across that sub- 

 stance, with a gentle curve in the opposite direction to the last curve 

 of the contiguous dentinal tubes ; viz. with the convexity towards the 

 crown : the fibres of the peripheral layer of the enamel make a slight 

 bend towards the crown : these enamel-fibres are as thick as two 

 of the dentinal tubes with their interspace : their ends are lost 



