29 



unc. liii. 



Longitudo ab apice rostri usque ad caudae basin G 8 



cuudcc 5 6 



ab apice rostri ad marginėm oculi . O 8 



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



tarsi diyitorumque 1 2^ 



auris O 85 



Latitudo auris O 81 



llab. Rio Negro. 



This species may be readily distinguished from M. xanthopygus 

 by the greater proportionate length of its tail. 



T\vo species of small Rodents were next characterized as consti- 

 tuting examples of a new genus, for \vhich Mr. Waterhouse proposcd 

 the name of 



Reithrodon.* 



" Dantės primores f; inferioribus acutis, gracilibus, et anties laevi- 

 bus ; superioribus gracilibus, antice longitudinalitėr sulcatis. 

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

 superiore plicas ^•itreas duas externė et inteme alternatim ex- 

 hibente ; secundo, et tertio, plicas duas externė, interne unam : 

 primo inferiore plicas vitreas tres externė, duas interne ; se- 

 cundo, plicas duas externė, unam interne ; tertio unam extern6 

 et interne, exhįbentibus. 

 Artus in£equales : antipedes 4-dact)'li, cum poUice exiguo unguiculato : 



pedes postici 5-dactyli, digitis externis et intemis bre^'issimis. 

 Ungues parvuli et debiles. Tarsi subtiis pilosi. 

 Cauda mediocris, pilis bre^abus adpressis instructa. 

 Caput magnum, fronte convexo : oculis magnis : auribus mediocribus. 

 " In the present genus, the incisors, compared ■nath 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 vvhich 

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

 and some other genera, but not combiued \vith 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 

 8omewhat tlattened at the sides and in front. The molars graduaUy 

 decrease iu sizc from the front to the lašt posterior tooth. The 

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

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

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

 opiiosite side. 



" In the two species of this genus -ttith vvhich I am acąuaintcd the 

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



* Viiėott. a channul ; OSo», a tooth. 



