Mammals from British Guiana. 151 
brown.”” On the rump the buffy becomes rather clearer. 
Top of head like back; sides of nose with a distinct buffy- 
yellow spot; eyes surrounded by a prominent whitish ring. 
Hairs of ears brown externally, yellowish internally, but 
without marked contrasts. Under surface soiled whitish or 
buffy, not sharply defined, the bases of the hairs slaty, the 
tips either white or buffy, the variation in this respect being 
considerable, as is also the case in S. Alston’; hairs of throat 
and groin whitish to their roots. Front of arms like body, 
inner sides whitish; hands and wrists dull whitish; hind 
limbs similar in colour, but the inner side of the leg is nearly 
or quite naked; fifth hind toe as in S. Alstonz, but little 
longer than first, and falling short of the base of the fourth. 
Tail about as long as the body without the head, well haired, 
brownish above, white below for its whole length. 
Skull quite similar to that of S. Alstont in general struc- 
ture, but shorter and perhaps rather broader in proportion. 
Dimensions of the type (measured in the flesh) :— 
Head and body 133 millim.; tail 95; hind foot, s. u. 25, 
Cue 20; ear 19: 
Skull: greatest length 32; basilar length 26; greatest 
breadth 19; nasals 11°54; interorbital breadth 4:5; palate 
from henselion 13°8; diastema 8:2; palatal foramina 7 x 2°4; 
length of upper molar series 5:3. 
Type. Male. B.M. no. 1. 6. 4. 100. Original number 51. 
Killed October 18, 1900. 
_ This species is readily distinguished by its shorter skull 
and lighter colour from S, Alstonz, to which alone it is allied 
and with which it forms a special group. 
21. Oryzomys navus messorius, subsp. n. 
Six skins and two spirit-specimens. 
Close to Oryzomys (Oligoryzomys) navus, Bangs, of which 
it appears to form a geographical race. 
General colour above grizzled greyish fawn, varying 
towards rufous; rump more rufous than back. Face greyer, 
Ears comparatively short, but little darker than the general 
colour of the head, very different therefore to the blackish 
ears of O. fulvescens and costaricensis. Under surface dull 
whitish, with scarcely a trace of buffy, not sharply defined 
laterally ; the hairs all greyish basally. Hands and feet 
white, ‘T’ail short for this group, thinly haired, brown above, 
lighter beneath ; not so sharply contrasted as in O. fulvescens, 
Skull as usual in this group. 
