RODENTS. 405 



in the clear peripheral substance to which the distinct apparently 

 structureless brown layer of cement is attached, and to which the 

 colour of the convex surface of the incisor is due. 



In a transverse section of the incisor of the Water Vole (Arvicola 

 amphibia), the thin colourless and apparently structureless layer of 

 cement covering the concave surface of the tooth is distinctly 

 traceable from the dentine at the sides of the tooth upon the ante- 

 rior plate of enamel ; and seems to be intimately blended with its 

 exterior surface. The layer of cement becomes thinner at the 

 margin of the enamel, where it is continued from the dentine upon 

 that part, but soon increases in thickness, acquiring the bright 

 brown tint, and separated by a well-defined line from the outer clear 

 layer of enamel. 



The delicate transverse or concentric lines of this substance are 

 seen more distinctly in the transverse than in the longitudinal sections 

 of the incisor. The faint outlines of the dentinal cells, and the 

 traces of the concentric layers of the clear substance of the dentine 

 are likewise best seen in the transverse section of the incisor. No 

 vascular canals are continued from the pulp-cavity into the dentine of 

 the incisors of the Water- Vole or Agouti. 



In the incisors of the Hare and Rabbit the pulp-cavity terminates 

 in a transversely extended fissure : the dentinal tubes proceeding to 

 the anterior enamelled surface after forming the curve, convex towards 

 the crown, proceed in a straighter transverse course to the enamelled 

 surface than in the Agouti. The tubes which pass to the opposite 

 or posterior surface of the tooth are less numerous, less parallel and 

 less closely packed together : they send out more and larger branches, 

 which decussate each other in an elegant arborescent manner. The 

 dentinal tubes of the Hare, figured by Retzius (loc. cit. tab. v. fig. 

 2 a and h) appear to have been taken from the inner and less dense 

 half of the dentine. 



In the Beaver, as in the Hare, the tubes which are characterised 

 by the great number and size of their branches, are confined to the 

 posterior half of the incisor ;(l) the tubes of the anterior half of the 

 dentine(2) in the Beaver's incisor are more numerous, more parallel 



CD PI. 106, dK (2) lb. rf. 



