MURIDiE— 8IGMODONTES— OCHETODON. 



121 



Sigmodont Murina: rcilh grooved tipper incisors : 



Size very largo. Tail Imlf ns long 

 Size very sinnll. Tail avernging 



Reithrodon. — Form stout, leporine. 



as tlie trunk. 

 Ochetodon. — Form slender, murine. 



as long as the trunk. 



With typical examples before us of all but one of the described species 

 of Ochetodon, we are oble to notice the genus with entire precision. 



Oc/ietodon com])ri8es the smallest Murines of North America ; the small- 

 est mammals of this continent, excepting some of the Soricidee. 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, ot once distinguish them from Mus ; the molars being 

 sigmodont, as in all Murina indigenous to the New World, and almost exactly 

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



The remarkable sulcation of the upper incisors is unique among North 

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

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

 from Murida) The grooves are deep and conspicuous, and nearly as broad 

 08 the prominent face of the tooth on either si»le ; 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 Murina, each incisor is 

 deeper antero-|)osteriorIy 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 full 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 tooting 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 otber ; and oao interior, midway 



