474 Mr. O. Thomas on new 
Skull apparently without marked characteristics. 
Dimensions of the type :— : 
Head and body 139 mm.; tail 99; hind foot 30:5; 
ear 25. 
Skull: greatest length 36°8 ; condylo-incisive length 33°6 ; 
zygomatic breadth 19°7; nasals 16:6; breadth of brain- 
case 15°6 ; palatal foramina 9°8 ; upper molar series 6°8, 
Hab. Otro Cerro, Catamarca. Alt. 3000 m. 
Type. Adult male. B.M. no. 20. 3.17.23. Original 
“number 808. Collected 13th November, 1919, by E. Budin. 
Presented by Oldfield Thomas. Seven specimens. 
The species of Rethrodon are all exceedingly closely 
allied, so far as the characters of their skulls and teeth are 
concerned, these being almost the same in all. But the 
geographical forms may be distinguished by colour and by 
the degree of hairiness of the soles, the southern cuniculoddes, 
hatchert, and jflammarum having the plantar hairs coming 
forward to the anterior pair of sole-pads, the intermediate 
auritus, Desm. (with which my pamparum is probably syno- 
nymous), having the main mass of the hairs ceasing at the 
posterior pair of pads, but with a certain number of hairs 
between the pads. Finally, the more northern forms from 
Uruguay, Corrientes, and now that of Catamarca all have the 
pad region naked. 
In colour all but cawrinus have the belly mostly buffy, but 
in auritus of Buenos Ayres Province the thighs are white in 
front. 
The type-locality of auritus was the Pampas south of 
Buenos Ayres, and in this region there seems to be no animal 
but a Retthrodon which can be fitted to Azara’s description 
of his “ rat oreillard,’’ the basis of Desmarest’s Mus auritus. 
My Retthrodon cuniculoides pampanus would therefore be a 
synonym of aurdtus, unless a form with less buffy on the 
belly proves to occur in the open country just south of 
Buenos Ayres itself. 
But in the less dry south-eastern corner of Buenos Ayres 
Province there isa duller form, due no doubt to more saturate 
conditions, which may be distinguished as 
Reithrodon auritus marinus, subsp. n. 
Essential characters of true auritus, but duller throughout, 
less buffy, the ears almost uniformly dark, without bright 
buffy metentote, the under surface wholly dull greyish washed 
with buffy, the inguinal region and front of thighs not white 
as In auritus. 
