408 RODENTS. 



The transverse section of the molar of the Water- Vole {Arvicola) 

 (PI. 168, fig. 3) shows only the latter form : that of the Beaver (PI. 

 107, fig. 2) exhibits chiefly the islands with a single promontory 

 of enamel. 



The transverse section of the crown of the molar of Lagostomus 

 displays not fewer than five islands of enamel, which hard sub- 

 stance is so thick that it enters more abundantly into the com- 

 position of the tooth than the dentine itself. The dentine of the 

 crowm closely adheres to the surface of all the inflected folds and 

 cavities of enamel as well as to that of the outer investing layer ; and 

 in the complex molars, where the folds are numerous, the layer of 

 dentine is in many parts thinner than that of the enamel. As the 

 pulp-cavity sends narrower or wider fissures into each of the processes 

 of dentine it assumes a very complicated form when seen in trans- 

 verse section, as in the molar of the Beaver, PI. 107, fig. 2, w^here the 

 dark fissures in the substance of the tooth indicate the spaces which 

 contained the vascular pulp {v v). The calcigerous tubes maintain at 

 most parts of this complicated dentine {d d) their general course at right 

 angles between the pulp-cavity and the enamel with the usual graceful 

 flexuous sigmoid curves : but in some places, near the angles of the 

 folds of dentine, the curves are stronger and less regular. The 

 secondary minute undulations with the terminal bifurcations, anasto- 

 moses and loops of the dentinal tubuli are shown at PI. 109, fig. 2. 

 The cavities of the enamel-folds which open upon the exterior of the 

 tooth are filled with cement ; and where this is accumulated in sufii- 

 cient quantity, as in the Beaver's molar (PI. 107, fig. 2, c) or in that of 

 the Water- Vole (PI. 108, fig. 3, cj, it presents not only the calcigerous 

 cells but also medullary canals. Prof. Erdl has described and figured 

 the concentric layers of the walls of these canals, and the minute dendri- 

 tic tubuli which radiate from their cavities, and establish a communica- 

 tion between them and the calcigerous cells ; doubtless for the con- 

 veyance of the nutritious colourless plasma transuded from the blood- 

 vessels. The calcigerous cells are larger, more numerous and more 

 angular in the cement of the Beaver's tooth than in ordinary bone. 



The pulp, after the formation of a certain thickness of tubular 

 dentine becomes converted into osseo-dentine in both the rooted and 



