THE TEETH OF RODENTIA. 371 



tinal tubes at that part of the Rodent's incisor which has 

 come into use are much smaller than those near to its 

 growing base, thereby proving that they have undergone a 

 diminution in calibre at a time subsequent to their original 

 formation. Near to the surface actually in wear they 

 become cut off from the pulp cavity by the conversion of 

 what remains of the pulp into a laminated granular mass, 

 so that the dentine exposed on the surface of a Rodent's 

 tooth must be devoid of sensitiveness, and the contents of 

 the dentinal tubes must have presumably undergone some 

 change. But what the nature of the change in the contents 

 of dentinal tubes which have ceased to be in continuity with 

 a vascular living pulp may be, there are, so far as I know, 

 no observations to indicate. 



As was shown by my father (Phil. Trans. 1850), the 

 enamel of Rodents is peculiar, and some little diversity in 

 the arrangement of the prisms exists in different families of 

 the order, their character being in many cases so marked, 

 that it is often possible to correctly refer a tooth to a par- 

 ticular family of Rodents after simple inspection of its 

 enamel. 



In general terms it may be said that the enamel is divided 

 into two portions, an outer and an inner portion (this is 

 true of all save the hares and rabbits), and that the enamel 

 prisms pursue different courses in these two portions. 



Thus in the enamel of the beaver, in the inner half, nearest 

 to the dentine, the prisms of contiguous layers cross each 

 other at right angles, whereas in the outer portion they are 

 all parallel with one another. 



In the genera Sciurus, Pteromys, Tamias, and Spernio- 

 philus the enamel fibres, as seen in longitudinal section, 

 start from the dentine at right angles to its surface; in 

 Castor they incline upwards at an angle of GO , but preserve 

 the distinction between the outer and inner layers very 

 distinctly. 



B B 2 



