338 Mr. O. Thomas on 
Skull longer than that of P. Simonsz. Nasals narrow, 
evenly rounded behind. Supraorbital ridges normal, not 
especially heavy, not forming postorbital projections, and, as 
usual in this group, not crossing the parietals. Palatal 
foramina open anteriorly, narrowing posteriorly, their com- 
bined outline pear-shaped ; their edges not continued back- 
wards into ridges. Posterior edge of palate widely V-shaped, 
the angle of the V opposite the middle of m’. Basioccipital 
of medium breadth, evenly broadening backwards, narrower 
than in P. Simons?, broader than in P. Roberti. Bulle 
decidedly larger than in P. Simonsz. 
Dimensions of the type (measured in the flesh) :— 
Head and body 245 millim.; tail 195; hind foot, s. u. 53, 
ce. u. 57; ear 27. 
Skull: greatest length 59; basilar length 42°5; greatest 
breadth 27; nasal length 21°5; interorbital breadth 12 ; 
palate length 20°5 ; diastema 12°5; palatal foramina 6°3 x 3; 
length of bulle 11:5; length of upper tooth-series 8°8. 
Hab. Mapiri, Upper Rio Beni, N.W. Bolivia. Altitude 
1000 m. ' 
Type. Male. B.M.no.1.1.1.37. Original number 1208. 
Collected 31st August, 1900, by Perry O. Simons. Hight 
specimens examined. 
Quite a number of forms of Proechimys are closely similar 
to each other externally, and even in the skull only differ 
from each other by some comparatively trifling character, such 
as size and shape of the palatine foramina, breadth of basi- 
occipital, or size of bulle, yet in these they are locally quite 
constant. It seems necessary to call them “ species,” in 
order to define them at all; but as such they are very unsatis- 
factory, for they simply cannot be distinguished with any 
certainty externally. The present animal is closely allied to 
the Bolivian specimens referred by Giinther to P. brevicauda*, 
a Bolivian skull being figured; but the type of that animal 
was from the Huallaga, and its belly is quite differently 
coloured. 
Cavia rufescens pamparum, subsp. n. 
Size of the true C. rufescens; colour of C. aperea. 
General colour above clear greyish, varying to olivaceous 
grey, lined with black ; the head, body, outer side of limbs, 
and the usual collar all of much the same tone. Belly dull 
whitish, the bases of the hairs greyish. Interramia also 
whitish, which colour is continued upwards and backwards 
towards the base of the ear. 
# P, Z, S. 1876, p. 749, 
