29 



utic. liii. 

 Lonjjitudo ab apice rostri usque ad caudai basin 6 8 



cuiidie 5 6 



ab apice rostri ad marginera oculi . 8 



ab apice rostri ad basin auris .... 1 4§ 



tarsi digitorumque 1 2^ 



auris 8^ 



Latitude auris 8^ 



Hub. Rio Negro. 



This species may be readily distinguished from M. xanthopygus 

 by the greater proportionate length of its tail. 



Two species of small Rodents were next characterized as consti- 

 tuting examples of a new genus, for which Mr. Waterhouse proposed 

 the name of 



Reithrodon.* 



" Denies primores f ; inferioribus acutis, gracilibus, et antic&lagvi- 

 bus ; superioribus gracilibus, antice longitudinalit^r sulcatis. 

 Molares utrinque f radicati ; primo maximo, ultimo minimo : primo 

 superiore plicas vitreas duas extern^ et intern^ alternatim ex- 

 hibente ; secundo, et tertio, plicas duas extern^, interne unam : 

 primo inferiore plicas vitreas tres extern^, duas interne ; se- 

 cundo, plicas duas extern^, unam intemfe ; tertio unam extern^ 

 et intern^, exhibentibus. 

 Artus insequales : antipedes 4-dactyli, cum pollice exiguo unguiculato : 



pedes post ici 5-dactyli, digitis externis et intemis brevissimis. 

 Ungues parvuli et debiles. Tarsi subtiis pilosi. 

 Cauda mediocris, pilis brevibus adpressis instructa. 

 Caput magnum, froUte convexo : oculis magnis : auribus mediocribus, 

 " In the present genus, the incisors, compared with those of the 

 true rats, are rather smaller in proportion, and those of the upper 

 jaw also differ in' having a longitudinal groove, a character which 

 exists in Euryotis (Brants), Gerbillus, Otomys (Smith), Dendromys, 

 and some other genera, but not combined with molars similar in 

 structure to those above described, nor yet with similar external 

 characters. In other respects the incisors resemble those of the 

 genus Mus; that is to say, those of the lower jaw are long, slender, 

 and pointed, and those of the upper are deep from front to back, and 

 somewhat flattened at the sides and in front. The molars gradually 

 decrease in size from the front to the last posterior tooth. The 

 folds of enamel penetrate deeply into the crowns of these teeth, so 

 that those from one side are in contact with those of the other ; these 

 folds of enamel are each nearly opposed to the salient angles of the 

 opposite side. 



" In the two species of this genus with which I am acquainted the 

 fur is long, very soft, and consists of hairs of two lengths. The 



* PeiSpos, a channel ; 080:1, a tooth. 



