132 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



different significance from that which Llliger attached to it. We hold that a 

 name once shown to be a synonym of a previous name is, to all intents and 

 purposes, dead, and cannot be revived for use in another connection. This 

 decision, which, we think, will be granted to be sound and just by most nat- 

 uralists, has obliged ns to propose a new name for the genus of which rutilus 

 is the type; for we cannot find that it has ever received a distinctive appel- 

 lation, and it is well worthy of full generic rank. 



Diagnosis. — Generally like Arvicola proper in dentition, but molars each 

 2-rooted ; middle lower molar like the last one, with three transverse trian- 

 gles seriatim, instead of alternating lateral ones ; lateral triangles of front 

 lower molar rather opposite than alternate. Inward folds of enamel generally 

 not perfectly fused along middle of tooth. Generally like Arvicola proper in 

 external form, but ears distinctly overtopping the fur ; closely pilous. Bony 

 palate ending posteriorly with a straight transverse edge opposite the middle 

 molar, the whole space between the last molars thus left open. Coronoid 

 process of under jaw short, its apex far below the level of the condyle. 



Type. — Mus rutilus, Pallas. 



This interesting genus is truly arvicoline, not only in general external 

 form, but in the prismatic molars and the structure of their crowns; at the 

 same time, it shows decided affinities with the Murines proper, and seems to form 

 a connecting link between them and Arvicolince. In external form, the con- 

 spicuous ears are the chief departure from typical Arvicola and an approach 

 toward murine forms ; for exserted ears are rare in Arvicolince, if not entirely 

 confined to this genus and Synaptomys ; while in Murines they are the rule 

 without signal exception. The ears of Evotomys do not reach the develop- 

 ment witnessed in Hesperomys proper, but are almost as large as in Onycho- 

 mys, and quite as large as in Oryzomys. The resemblance, in external 

 form, to Synaptomys, is so close that we cannot point out any reliable dis- 

 tinctions; but Synaj)tomys is instantly distinguished by its peculiar cranial 

 and dental features, as elsewhere detailed, these being, in fact, almost exactly 

 as in the Lemmings (Myodes). 



The most remarkable indication of murine affinity is seen in the rooted 

 molars. The molars of Murince are rooted, perhaps without a single excep- 

 tion ; while the molars of Arvicolince are normally rootless, with probably the 

 single exception of the present genus Evotomys. But there is this difference 

 in the mode of rooting between Evotomys and true murine forms: In Evoio- 



