RODENTS. 407 



of the incisor, and which, as it gradually augments in thickness to 

 enter and fill the inflected folds of enamel acquires the radiated cells 

 and also the medullary canals. This thin exterior layer is not repre- 

 sented in Prof. Erdl's figures of the microscopic structure of the 

 molars of Rodentia, which, from their accuracy in other respects I 

 have reproduced in Plates 107 — 109. The size of the calcigerous 

 tubes is unavoidably exaggerated from the limited scale of the 

 drawings ; but their course is correctly given. 



The structure of the rooted molar teeth with simple crowns is 

 correspondingly simple : the dentine consists of calcigerous tubes 

 radiating in the body of the tooth from a single pulp-cavity, with the 

 primary curves rather stronger than usual, especially in the tubules 

 which ascend vertically from the summit of the pulp-cavity. At the 

 base of the crown, where the pulp-cavity divides in order to be 

 continued into the fangs, the tubuli radiate from each division almost 

 horizontally and a section at this part displays their entire course, as 

 shewn in PI. 107, fig. 1, in a molar tooth of the common Squirrel 

 {Sciurus vulgaris) . 



The section of the crown of the molar tooth of the Rat, taken 

 through the middle, shows the external coat of enamel, a small 

 portion of the cement and enamel at the bottom of the fissures and 

 depressions down which these substances are continued from the 

 summit of the crown, and the disposition of the calcigerous tubes in 

 relation to those depressions and to the external coat of enamel : 

 the third molar — the lowest and largest in PI. 105, fig, 10, 3 — shows 

 the crown naturally worn down almost to the degree corresponding 

 with the section magnified in PI. 108, fig. 1. 



The vertical section of the molar of the Flying Squirrel, the 

 crown of which has been worn down in a less degree, shows better 

 the course of the dentinal tubuli relatively to the inflected cups 

 of enamel upon the grinding surface. These cups or depressions, 

 (PI. 108, fig. 3, e, e, e). are filled by a brown-coloured cement (c, c, c). 

 In a transverse section of the crown the inflected vertical folds of 

 enamel appear, as on the naturally abraded surface, in the form of 

 islands including the cement, whilst the lateral folds of enamel project 

 like promontories into the substance of the crown. 



