132 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
different significance from that which Illger 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 us to propose a new name for the genus of which ruéius 
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. 
Dracnosis.—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. 
Typr.—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 Arvicoline. 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 Arvicoline, if not entirely 
confined to this genus and Synaptomys; while in Murine 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 Synaplomys is instantly distinguished by its peculiar cranial 
and dental features, as elsewhere detailed, these being, in fact, almost exactly 
as in the Lemmings (Wyodes). 
The most remarkable indication of murine affinity is seen in the rooted 
molars. The molars of Murine are rooted, perhaps without a single excep- 
tion; while the molars of Arvicoline 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 Evoto- 
