Its Situation in the Order Rodentia. 229 



domen, gray ; ears rather small, obtusely rounded at the tip ; vi- 

 brissas long, black gray at the base ; a few black bristles above 

 the eyes ; eyes moderate ; anterior foot with the intermediate toes 

 longest, equal, exterior toe shortest, but nearly equal to the inner 

 one ; thumb tubercle small ; posterior feet, with the three inter- 

 mediate toes, subequal; the exterior and interior are rather 

 shorter, and the latter shortest ; nails robust, black ; tail thick 

 at base, gradually tapering to the tip, imbricated with scales, hair 

 short, sparse, rigid. 



As this animal exhibits the character of flat crowned teeth, alto- 

 gether destitute of radicles, combined with robust and absolutely 

 perfect clavicles, it is by the latter character at once excluded 

 from Cuvier's second division of the Rodentia, which comprehends 

 Lepus, Lagomys, and Hystrix, together with the several genera 

 formed on the demolition of the Linneaen genus Cavia. 



Of the two divisions, therefore, into which the Rodentia have 

 been separated, from the consideration of the presence or absence 

 of perfect clavicles, the new genus Isodon unquestionably belongs 

 to the first, and maybe grouped with Arvicola, Fiber,* and Geory- 

 chus. It corresponds with the former, in the entire and prismatic 

 form of the teeth. But in these genera each jaw is furnished but 

 with six teeth, which is a smaller number by two than exist in our 

 animal ; and as this numerical character is undoubtedly essential, 

 we are justified in regarding it as distinct from either. In nume- 

 rical dentition, however, Isodon agrees with Castor, Bathyergus, 

 and Helamys ; but without resorting to a detail of other discre- 

 pancies, the manner in which the folds of enamel are arranged in 

 its teeth, very sufficiently distinguishes it from either. 



* Illiger enumerates four molares to each series in the jaw of Fiber; but, 

 after ample examination, I agree with Daubenton and Cuvier, in asserting, 

 that but three exist, of these, the grinding surface of the anterior one in the 

 lower jaw is as long as the two others taken together, and is divided into nine 

 triangular prisms, of which the anterior and posterior ones extend the whole 

 width of the tooth, whilst the seven other prisms are smaller and alternate; in 

 neither of the other molares, of either jaw, does the number of prisms exceed 

 five. Each of these molares is certainly divided into two parts at base, 

 though it is true these roots are not solid. 



