RODENTS. 403 



a general tendency to a transverse direction across the crown of the 

 tooth. Baron Cuvier has pointed out the concomitant modification 

 of the shape of the joint of the lower jaw, which 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 surface 

 varies with the degree of abrasion, of which examples have already 

 been cited ; but in the rootless molars where the folds of enamel 

 extend inwards from the entire length of the sides of the tooth, the 

 characteristic configuration of the grinding surface is maintained 

 without variation, as in the Guinea-pig, (fig. 16), the Capybara, 

 (fig. 17), and the Patagonian Cavy, (PI. 104, figs. 2 & 3). 



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 Chinchillida 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 partial folds and islands of enamel in the molars of the 

 Porcupine (fig. 13), and Agouti, (fig. 14), typify the structure of 

 the teeth of the Rhinoceros : the opposite lateral inflections of enamel 

 in the molars of the Gerbille, (fig. 8), and Cape Mole-rat, (fig. 11), 

 represent the structure of the molars of the Hippopotamus : the 

 double crescentic folds in the Jerboa (fig. 7), sketch out, as it were, 

 the characteristic structure of the molars of the Anoplothere and 

 Ruminantia. 



Although, as has been shown, the molar teeth in many Rodents 

 are rootless and of unlimited growth as in the Edentata, in none 

 is enamel absent or vascular dentine, as the chief constituent of 

 the tooth, present : these essential differences characterise the molars 

 of those Rodents which by use have their grinding surface reduced 



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