152 Mr. O. Thomas on 
Dimensions of the type (measured in the flesh) :— 
Head and body 82 millim.; tail 95; hind foot, s. u. 21, 
c. u. 22; ear 12°5. 
Skull: greatest length 22:5; basilar length 16°4; greatest 
breadth 12; length of nasals 8; interorbital breadth 3:6; 
palatal foramina 4:1 x2; length of upper molar series 3°]. 
Type. Female. B.M. no. 1. 6. 4. 97. Collected No- 
vember 29th. 
This local representative of the Oligoryzomys group seems 
to differ from O. navus, the only form which is not geogra- 
phically very remote, by its shorter tail and slightly different 
colour, and no doubt represents a tenable subspecies. It is 
not improbable that O. fulvescens, O. navus, O. costaricensis, 
and perhaps other described forms will all prove to be local 
subspecies of one widely-spread species. 
22. Zygodontomys stelle, Thos. 
Twelve skins. 240 and 500 feet. October to December. 
Evidently the commonest rat of the region. 
23. Proechimys cayennensis, Desm. 
Six skins of different ages. 240 and 500 feet. November 
and December. 
Without topotypes of P. cayennensis I cannot be certain of 
the determination of these specimens. They are certainly 
very different from the Brazilian forms referred to cayennensis 
by Waterhouse, but geographically are likely enough to be 
Desmarest’s species. ‘They are of the same rufous group as 
P. Cherriet, Uricht, trinitatis, and other north-western species 
of the genus. 
24, Cavia porcellus guiane, subsp. n. 
Four skins of different ages from the Kanuku Mountains, 
7th and 8th December, 600 feet, and one from Berbice, on 
the coast. 
Similar in general characters to the typical Brazilian form, 
but much paler and greyer above and less deeply buffy below. 
Size, character of fur, &c. as in the typical form, General 
colour above grey (near the ‘‘olive-grey” of Ridgway), 
heavily grizzled and lined with black, this latter colour pre- 
dominating on the posterior dorsal area, while the head, neck, 
shoulders, flanks, and rump are of the clearer greyish. ‘The 
light constituent of the general grey varies in different speci- 
mens from whitish towards buffy. Under surface whitish or 
slightly buffy, the bases of the hairs slaty grey ; the usual 
