MURID“H—SIGMODONTES—OCHETODON. 121 
Sigmodont Murine with grooved upper incisors : 
Reithrodon—Form stout, leporine. Size very large. Tail half as long 
as the trunk. 
Ochetodon—Form slender, murine. Size very small. Tail averaging 
as long as the trunk. 
With typical examples before us of all but one of the described species 
of Ochetodon, we are able to notice the genus with entire precision. 
Ochetodon comprises the smallest Murines of North America; the small- 
est mammals of this continent, excepting some of the Sorictde. In general 
appearance, they are hardly distinguishable on sight from ungrown house- 
mice, they conform to the latter so closely in size, proportions, and color. 
The teeth, however, at once distinguish them from Mus; the molars being 
sigmodont, as in all Murine indigenous to the New World, and almost exactly 
as in North American Hesperomys, while the sulcate incisors are sui generis. 
The remarkable suleation of the upper incisors is unique among North 
American Murine, though recurring in the arvicoline genus Synaptomys. (It 
is much as in Zapus, which latter, however, is the type of a family apart 
from Muride.) The grooves are deep and conspicuous, and nearly as broad 
as the prominent face of the tooth on either side; they are median in situa- 
tion, run the whole length of the tooth, and terminate in a notch, so that the 
conjoined ends of the pair of incisors present four points instead of a straight 
bifid edge. The anterior face of each incisor is a prominent rounded ridge 
on either side of the groove; but the face, as a whole, is so much beveled off 
externally that, when the tooth is viewed in lateral profile, one of these ridges 
is entirely in front of the other, and the tooth appears double by the amount 
of separation that the groove affords. As usual in Murine, each incisor is 
deeper antero-posteriorly than it is wide transversely ; but the incisors differ 
noticeably from those of Hesperomys, &c., in their great curvature, which is 
sufficient to cause their apices to fall behind a perpendicular let down from the 
tip of the nasal bones. 
The under incisors are simple, and, with the entire molar series, much as 
in Hesperomys. But there seems to be a difference in the rooting of the mo- 
lars. In all the Hesperomys examined, the anterior upper molar, at least, 
invariably showed us three roots, making as many distinct perforations of the 
alveolus: two exteriorly, in a line with each other; and one interior, midway 
