Jrom the Delta of the Parana, 97 
slender, finely scaled, brown above, whitish below, the usual 
narrow dark line perceptible along the lower side. 
Skull of about the same size as in O. longicaudatus, mar- 
kedly larger than in O. flavescens. The interorbital region 
broader than in the first-named, its edges sharply square, but 
not ridged. Zygomatic plate rather less projected forward. 
Palatal foramina ending just opposite the front edge of m’. 
Incisors directed backwards towards the throat, their angle 
with the tooth-row about 65°. Molars rather larger than in 
longicaudatus. 
Dimensions of the type (measured in flesh) :— 
Head and body 105 mm.; tail 125; hind foot 28; 
ear 17. 
Skull: greatest length 27-5 ; condylo-incisive length 23:2 ; 
zygomatic breadth 14:4 ; nasals 10°2 ; interorbital breadth 4; 
breadth of brain-case 12°5; palatilar length 11°5; palatal 
foramina 6; upper molar series 4. 
Type. Adult male. B.M. no. 17.6.1.23. Original 
number 2846. Collected 17th February, 1917. 
This second species of Oryzomys is an unexpected dis- 
covery, for all over Uruguay and Argentina (apart from the 
far south) only one species—O. flavescens or a member of that 
group—has hitherto been known, and O. delticola would seem 
to be quite isolated in the present locality. 
It would appear to be most nearly allied to the Chilian 
O. longicaudatus, and thus bears the same geographical 
relationship to that animal that the Parana coypu does to the 
Chilian one. 
4. Oryzomys flavescens, Waterh. 
&. 2825; anda @? in spirit. 
This, the only non-peculiar mouse in the delta, is widely 
distributed over Uruguay and Middle Argentina. Specimens 
vary very much in coloration, the under surface of some being 
strongly buffy and of others white, but extremes and all inter- 
mediate phases are found in the same localities. 
5. Akodon arenicola hunteri, subsp. n. 
6. 2821, 2822, 2833, 2837, 2841, 2847; 9. 2823, 2838; 
and one @ in spirit. 
Like true arenicola in colour and other characters, but 
averaging decidedly larger, the hind foot 21-22 mm. in length, 
while in arenicola it is ordinarily 18-19, rarely reaching 20; 
Ann. & Mag. N. Hist. Ser. 8. Vol. xx. 7 
