182 '■ Mr. O. Tliomas on 



XVI. — Notes on Argentine, Patagonian, and Cape Horn 

 Muridse. By Oldfield Thomas. 



(Published by permission of the Trustees of the British Museum.) 



The Mice of the Hesfeeomys laucsa Group. 



Of the small grey field-mice of the Hesperomys laucha group 

 there is happily no doubt about the identification of the 

 original species, Azara's " Laucha/' as Mr. Perrens sent 

 from Goya, Corrientes, specimens exactly agreeing with 

 Azara's description. 



But I am now convinced that the larger form occurring 

 in the same region — the largest of the genus as now re- 

 stricted^cannot be identified with Rengger's Mus callosus, 

 as I had hitherto supposed, and needs afresh determination. 

 Rengger's animal has too long a tail and too long ears, and, 

 owing to the extreme insufficiency of the description, it 

 should probably he put aside as unidentifiable. Perhaps 

 it was a common rat — to which Rengger compared it — or 

 possibly a Reithrodon, a genus in which the pads of the feet 

 are more conspicuous than in Hesperomys. In any case, 

 however, the doubt is so great that a guesswork identification 

 is of no use to anyone. 



The Goya Hesperomys seems to represent a Parana race of 

 the Cordova H. venustus, and may be described as follows : — • 



Hesperomys venustus callidus, subsp. n. 



Fur not so long as in true venustus, hairs of back about 

 9 mm. in length. Genei'al colour above greyish, very much 

 the colour of Mus musculus, darker " mouse grey " ante- 

 riorly, more drabby posteriorly, but not so drab as in true 

 venustus. Flanks with a slight wash of huffy, decidedly less 

 marked than in venustus. Under surface pale grey, the 

 hairs slaty basally, white or greyish white terminally. Ears 

 large, appearing larger than in true venustus, rounded, brown 

 anteriorly, greyish posteriorly, the usual postauricular white 

 patch not very conspicuous. Hands and feet white. Tail 

 shorter than head and body, thinly haired, pale brownish 

 above, greyish white on sides and below. Mammae usually 

 3—2=10. 



Skull stoutly built, with broadly divergent supraorbital 

 edges, which form well-marked ledges over the orbits, but 

 not upwardly projecting beads ; the ledges in continuati(ni 

 with distinct ridges running across tlje parietals. 



