new Neotropical Muridge. 347 



orbital region very narrow, narrower than the muzzle, its 

 edues square, but not beaded ; interparietal bone broad, 

 ribbon-like, its antero-posterior about one third its trans- 

 verse diameter ; walls of infraorbital foramina as in Sigmodon, 

 the anterior edge cut back underneath a prominent projecting 

 point ; palatine foramina very large, but the dividing septum 

 also very large, nearly filling them up; posterior part of 

 palate with two deep excavations between Ei^ and ^li^, divided 

 by a mesial ridge, and strongly recalling the same part in 

 Microtus ; pterygoid fossaj broad and shallow. Lower jaw 

 very short and high, but the coronoid processes reduced to 

 low blunt triangular elevations. 



Dentition. — Upper incisors extremely broad, smooth, and 

 slightly concave in front so far as the internal five sixths of 

 their surface is concerned, but along their outer edge there is 

 a sharply cut groove or bevel, most visible from the side, 

 whence it suggests the bevelled grooving of Gerhillus or 

 Zapus ; the inner border of the groove forms an anteriorly 

 projecting ridge, internal to which the face of the tooth 

 gradually curves inwards to meet its fellow of the opposite 

 side. (To suppose the inner half of a Zapus incisor flattened 

 in front and enormously broadened, while the outer half 

 retains its natural size and position, would give the best idea 

 of these peculiarly grooved incisors.) Molars more like those 

 of Sigmodon than of any other Cricetine, but they are even 

 more antero-posteriorly compressed, with the result that the 

 grinding-surfaces somewhat recall those of Microtus, with 

 separated enamel loops and islands. On the whole, how- 

 ever, the loops appear to correspond to those of Sigmodon, 

 except that ^i-? is more lengthened and there are four pro- 

 jecting angles on its outer side instead of three. Lower 

 incisors broad and flat in front, and curved very rapidly 

 upwards, so as to produce a very short diastema. 



The above description will show how widely different this 

 peculiar little animal is from any hitherto described. Both in 

 external and cranial characters it has a curious resemblance 

 to Otomys, on which I have based its name. Its long fur, 

 large round ears, anteriorly expanded nasals, and narrow 

 interorbital region are all points in the resemblance, although 

 of course there can be no possible connexion between the two 

 genera. Its really nearest ally is probably Sigmodon ; but 

 even this is very different in many characters, notably in the 

 general form of the skull and the structure of the incisors. 

 The Sigmodon-like Rhithrodon described by me in 1880 ^ 



* P. Z. S. 1880, p. 091. 



