478 On new Species of Reithrodon ce. from Argentina. 
blackish, little browner on the rump, sides dark slaty grey, 
abruptly changing halfway down to the greyish white (some- 
times tinged with buffy) which covers the whole of the under 
surface and inner sides of the limbs, though the hairs are 
everywhere slaty greyish at base. Crown black. Ears 
thickly hairy, dark brown, the proectote blackish. Upper 
surface of hands and feet whitish, often with darker meta- 
podials. Tail heavily haired, the hairs below longer than 
those above, so as to form a swimming fringe; blackish 
brown above, lighter brown, sometimes even dull whitish 
below. 
Dimensions of the type:— 
Head and body 168 mm.; tail 154 ; hind foot 36 (with 
claws 39, therefore barely 14 inch English, and decidedly 
less than 14 inch French or German) ; ear 22°5. 
Skull: greatest length 39; condylo-incisive length 37:2 ; 
zygomatic breadth 19; nasals 15; interorbital breadth 6 ; 
breadth of brain-case 16; zygomatic plate 3:7; palatilar 
length 17°3; palatal foramina 8°8; upper molar series 674. 
Hab. Islands of Parana delta. Type from Isla Ella, other 
specimens from Los Cisnes, Rio Carabelas. 
Type. Adult male. B.M. no. 17.6.1.6. Original num- 
ber 2843. Collected 15th February, 1917, by R. Kemp. 
Presented by Oldfield Thomas. Fifteen specimens obtained 
by Mr. Kemp and eight by Mr. H. E. Box. 
Besides its difference in size, Lichtenstein’s animal appears 
to have had nothing like the extent of the whitish on the 
under surface so prominent in S. aquaticus, where the white 
rises nearly halfway up the sides. In tomentosus no white is 
shown in the plate, and the description runs “ Mitte der 
Bauchseite matt aschgrau.” A casual note of my own on 
the type in Berlin says “ belly but little lighter,” so there is 
certainly nothing of the striking bicolor coloration charac- 
teristic of S. aquaticus. 
This is one of the interesting delta animals which, in order 
to carry on at all, have either to swim or climb, for their 
habitat is completely flooded whenever the waters of the 
Ia Plata estuary are banked up by the south-east wind. 
