RODENTS. 409 



rootless molars of the Rodents. This fourth substance is exhibited 

 at in the magnified transverse section of the Water- Vole's molar, 

 (PL 108, fig. 3), which shows the four different dental tissues, viz. 

 cement c, enamel e, dentine d and osseo-dentine o, entering in more 

 equal proportions into the formation of the crown, than has, hitherto, 

 been demonstrated in any other tooth. When the crown is worn by 

 mastication down to the place of the section figured, the four subs- 

 tances appear in the same proportions on the grinding surface, 

 contributing to its efficiency as a triturating organ by the inequalities 

 consequent on their various degrees of density and resistance to the 

 abrading forces. 



The transverse section of the molar of the Hare, (PI. 109, 

 fig. 1), shows the single, but deep, lateral fold of enamel (e) extend- 

 ing almost across the entire breadth of the tooth, and followed 

 as usual by the included layer of cement (c) . In the Capybara the fold 

 is carried quite across, and divides the anterior molars into two 

 portions of a prismatic form, with the base turned outwards, and 

 penetrated by a shorter fold of enamel : the posterior molar in the 

 lower jaw is divided into ten or more portions of which only the ante- 

 rior one is prismatic and indented on the outer side, the rest being 

 simple, compressed plates, having a microscopic structure very 

 similar to that in the lobes of the molar of the Hare. In this tooth 

 the primary curvatures of the dentinal tubes are more flexuous 

 than usual : the concentric layers of the clear basal substance may be 

 more clearly perceived than in most other Rodents : the quantity of 

 osseo-dentine which fills the remains of the pulp-cavity is very small. 

 The enamel varies in its thickness both in the outer surface and the 

 inflected fold in the Hare : this fold is elegantly undulated through a 

 greater part of its extent. 



It is peculiar to some of the Rodents with rootless molars to 

 have the sockets of these long curved teeth open at both extremities, 

 so that, in the dry skull, the base of the tooth protrudes as well as 

 the grinding surface : the matrix in such instances adheres to the 

 periosteum which covered the portion of bone absorbed from the 

 bottom of the alveolus. The Jumping-Hare {Helamys capensis), when 

 full-grown, offers a good example of this curious structure. 



