314 EODENTIA. 



SPALACID^. 

 Genus Rhizomys, Gray. 



The genus Hhizomt/s forms a small natural group, allied, on the one hand, to 

 Arvicola, and, on the other, to Siphneus. 



The dentition of these genera helongs to three types : in Rhizomys, the molar 

 teeth are always rooted, even hef ore they have appeared externally, and this character 

 is retained through life, so that the crown, as the tooth is not a persistent grower, 

 is the suhject of considerable modification, depending on the extent to which the 

 folds of enamel are worn down. In early stages, the anterior molar of JR,, cliinensis^ 

 Gray, presents as many as four distinct folds, one internal and three external ; hut as 

 age advances, and use wears them away, some portions of the folds form little islets 

 of enamel, depending on differences in their vertical distribution ; but in the well- 

 worn teeth of old animals all traces of these folds and islets disappear, and the tooth 

 becomes cylindrical, with a smooth, slightly concave crown, surrounded by a raised 

 border of hard enamel. The dental conformation is much the same in all the species. 

 In Arvicola the molars are occasionally rooted, but in some of the species they are 

 unfurnished with roots in youth, to gain them, however, with advancing years, so 

 that this group serves to connect Ehizomys with its permanently rooted teeth, to the 

 allied genus Siphneus, in which the teeth are continuous growers. 



However, there is a general type of dental structure wliich closely affines 

 Mhizomys to Arvicola and JSiphneus, the molar teeth of all these groups being 

 resolvable to one type, but varying in the number and extent of the enamel folds ; 

 but, in the two last, the folds are more open and simple than in BUzomys. Yet if 

 the teeth of R, chinensis are compared with those of Siphneus armandii, it will 

 be found that, although the former appear wonderfully complex, they can be 

 reduced to the same structural type characteristic of the latter. In R. chinensis, in 

 which the crowns of the molars are slightly worn, the plan of structure can be 

 clearly traced to the presence of generally one fold on the inside of each molar, 

 and three external folds on the first molar, two or three on the second, and one 

 external fold on the third. According as the tooth is worn, these elementary folds, 

 which are, so to speak, closely pressed together, form wavy outlines, at first more 

 complicated, but ultimately, as the enamel folds rub down by use, becoming a simple 

 circle of enamel. In Siphneus armandii, there is no compHcation of the elementary 

 folds, all the molar teeth conforming to one plan, viz.^ one internal and two external 

 folds ; that is to say, that the first molar has one external fold less than in R. chinen- 

 sis, while the others agree in structure with the second and third molars of that 

 species. A similar uniformity of structure pervades the inferior molars in both 

 genera. The only difi'erence presented by Arvicola is, that the folds are more open 

 and deep, and more numerous on the inner than on the outer side of the tooth, 



