178 On new Muridee Jrom Paraguay. 
not overhanging the orbits. Zygomatic plate strongly con- 
cave in front. Palatal foramina of about uniform breadth, 
reaching back to the level of the first lamina of m’. 
Incisors rather opisthodont, the incisive index about 61°. 
Molars of normal size and shape, but remarkable for the 
partial or complete suppression of the cross-crotchet between 
the outer cusps, this crotchet being usually considered as 
characteristic of all the Oryzomys series of genera. 
Dimensions of the type :— 
Head and body 106 mm.; tail 106; hind foot 25 ; 
ear 16. 
Skull: greatest length 29; condylo-incisive length 26°6 ; 
zygomatic breadth 15°5; nasals 11°5; interorbital breadth 
3°8; zygomatic plate 3-2; palatilar length 13; palatine 
foramina 6°1; upper molar series 4°8. 
A second speéimen is rather larger—head and body 120 mm., 
skull 30:3. 
Hab. Northern Chaco of Paraguay. Type from Jesema- 
tathla, west of Concepcion. Alt.100m. A second specimen 
from Mision. 
Tyne. Young adult male. B.M. no. 20. 12.18.16. Ori- 
ginal number 22. Coilected 15th August, 1920. 
This very distinct swamp-rat is readily distinguishable 
by its intermediate size, its comparatively short tail, most 
Oryzomys having this organ longer than the head and body, 
its unridged supraorbital edges, concave zygomatic plate, and 
the obsolescence of the cross-crotchets in the molars. Im- 
portant as’ the last character usually is, the animal is so 
essentially an Oryzomys in all other respects that I do not 
think there is any doubt that its proper place is in that genus. 
Possibly even the reduction of the crotchets is a mere abnor- 
mality. 
I have much pleasure in naming the species after its 
discoverer, the Marquis de Wavrin, who is carrying out an 
important exploration in South America, and has presented 
to the British Museum the small mammals he obtained in 
Paraguay. 
Akodon toba, sp. n. 
A large olivaceous species, with whitish belly. 
Size as in some of the largest members of the genus. 
Fur soft and fine, hairs of back about 9 mm. in length. 
General colour above finely ticked olivaceous grey, almost 
exactly as in A. arviculoides montensis of Paraguay, but 
rather less suffused with buffy, so that the general effect is 
