298 ANATOMY OF VERTEBllx\TES. 



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 at first tuberculate. 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 ; 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. Examples of various forms assumed by the inflected 

 folds of enamel in the molars of the Rodentia are given in the 

 works of the Cuviers and other naturalists. 1 These folds have a 

 general tendency to a transverse direction across the crown of the 

 tooth (vol. ii. fig. 236, p. 370) : the joint of the lower jaw almost 

 restricts it to horizontal movements to and fro, in the direction of 

 the axis of the head, during the act of mastication. When the 

 folds of enamel dip in vertically from the summit to a greater or 

 less depth into the substance of the crown of the tooth, as in those 

 molars which have roots, the configuration of the grinding sur- 

 face varies with the degree of abrasion; but in the rootless 

 molars, where the folds of enamel extend inward from the 

 entire length of the sides of the tooth, the characteristic con- 

 figuration of the grinding surface is maintained without varia- 

 tion, as in the Guinea-pig, the Capybara, and the Patagonian 

 Cavy. 



The whole exterior of the molar teeth of the Rodentia is covered 

 by cement, and the external interspaces of the enamel-folds are 

 filled with the same substance. In the Chinchillidce and the 

 Capybara, where the folds of enamel extend quite across the 

 body of the tooth, and insulate as many plates of dentine, these 

 detached portions are held together by the cement. Such folds 

 of enamel are usually parallel, as in the large posterior lower 

 molar of the Capybara, which, in shape and structure, offers a 

 very close and interesting resemblance to the molars of the Asiatic 

 Elephant. The modification observed in the Voles (Arvicola) calls 

 to mind the molars of the African Elephant. The partial folds 

 and islands of enamel in the molars of the Porcupine and Agouti 

 foreshow the structure of the teeth of the Rhinoceros. The 

 opposite lateral inflections of enamel in the molars of the Gerbille 

 and Cape Mole-rat, represent the structure of the molars of the 

 Hippopotamus. The double crescentic folds in the Jerboa sketch 



1 cxx". and cxxi". 



