MURID^— SIGMODONTES— OUHETODON. 121 



Sigmodont Murines 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 Soricida. 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 Murines indigenous to the New World, and almost exactly 

 as in North American Hesperomys, while the silicate incisors are sui generis. 



The remarkable sulcation of the upper incisors is unique among North 

 American Murines, though recurring in the arvicoline genus Synaptomys. (It 

 is much as in Zapus, which latter, however, is the type of a family apart 

 from MuridcB.) 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 gi-oove affords. As usual in Murince, 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 



