402 RODENTS. 



by a long undivided continuation of the crown ; when the force begins 

 to be exhausted the matrix is simplified by the suppression of the 

 enamel organ, and the dentinal pulp continues to be reproduced only at 

 certain points of the base of the crown, which, by their elongation, 

 constitute the fangs. The Beaver and other Rodents in the second 

 category of the order, according to the implantation of the molar teeth, 

 exemplify the above condition ; but in the Capybara, Dolichotis, (PI. 

 104, figs. 2 & 3), and other Rodents with rootless molars, the re- 

 production of the molar like that of the incisor teeth, appears to 

 continue throughout the animal's existence. The rootless and per- 

 petually growing molars are always more or less curved ; they derive 

 from this form the same advantage as the incisors, in the relief of 

 the delicate tissues of the active vascular matrix from the effects 

 of the pressure which would otherwise have been transmitted more 

 directly from the grinding surface. 



The complexity of the structure of the crown of the molar teeth, 

 and the quantity of enamel and cement interblended with the dentine, 

 are greatest in the rootless molars of the strictly herbivorous Rodents. 

 The crowns of the rooted molars of the omnivorous Rats and 

 Mice are almost as simple as the tuberculate molars of the Bear 

 or of the Human Subject, which they appear to typify. They 

 are at first tuberculate, as shown in PI. 105, fig. 9 ; when the 

 summits of the tubercles are worn off, the inequality of the grinding 

 surface is for a time maintained by the deeper transverse folds of 

 enamel, the margins of which are separated by alternate valleys 

 of dentine and cement, as shown in PI. 105, fig. 10, 3 a; but 

 these folds sinking only to a slight depth are in time obliterated, 

 and the grinding surface is reduced to a smooth field of dentine 

 with a simple border of enamel, as shown in the upper tooth of 

 figure 10, 3. A similar change in the grinding surface, consequent 

 on age and use, is shown in the molars of the Souslik, or Ground 

 Squirrel, PL 105, fig. 3, b, and in those of the Gerbille, fig. 8, and 

 is common to all that possess roots. 



Examples of various forms assumed by the inflected folds 

 of enamel in the molars of the Rodentia are given from the works 

 of the Cuviers in PI. 105. It will be seen that these folds have 



