406 RODENTS. 



and less ramified, and thus produce a greater degree of density and 

 resistance, which must favour the oblique wearing and the mainte- 

 nance of the sharpness of the chisel-shaped extremity of the tooth. 

 Short medullary canals (PI. ]06,vv), are continued from the end 

 of the pulp-cavity into the central tract of the dentine. 



The difference of structure of the anterior and posterior parts 

 of the enamel-plate is very clearly shown in the Beaver's incisor. 

 The fibres of that half of the enamel (I) next the dentine are nearly 

 transverse, their diameter is equal to two dentinal tubes and their 

 interspace ; they describe a gentle sigmoid curve, and are distinctly, 

 almost coarsely, defined : the fibres of the peripheral half of the 

 enamel(2) are more minute, more oblique, and less distinctly defined ; 

 the substance being clearer and denser. 



In a transverse section of the incisor the distinction between the 

 two layers of enamel is still more obvious, and the fibres of the inner 

 half being cut across, give the appearance of fine decussating oblique 

 lines ; while those of the outer half run transversely to the surface, 

 and are crossed by the traces of the concentric layers. The coloured 

 exterior layer(3) is as distinctly defined as in the incisors of the Vole ; 

 the fine fibres of the outer layer of enamel appear to penetrate it, 

 but its continuation with the thin indubitable layer of cement upon 

 the lateral and posterior parts of the incisor lead me to conclude that 

 it is due to the calcification of the capsule, and that it is essentially 

 what Mr. F. Cuvier supposed it to be, cement, but in too small a quan- 

 tity to permit the development of the calcigerous cells. In a trans- 

 verse section of a Beaver's incisor in my microscopical collection the 

 cement is distinctly continued, retaining its peculiar colour, from 

 the enamel, beyond the lateral boundary of that substance upon the 

 dentine, for a short distance. Retzius' denial (loc. cit. p. 63) of the 

 presence of any cortical substance or cement upon the Beaver's 

 incisor appears be founded on a non-recognition of that substance 

 where it is so thin as to be devoid of radiated or Purkingian cells. 



In the molar teeth the enamel presents a uniform structure 

 throughout its thickness, which is like that of the denser outer 

 layer of the enamel of the incisor. It is distinctly coated by a layer 

 of structureless dentine as thin as the coloured layer upon the enamel 



(0 PI. 106, e. (2). lb. e'. (3) lb. c 



