416 MR. OLDFIELD THOMAS ON A REMARKABLE 
of the animal could be obtained from them, and merely on the 
basis of its small size and its possession of five cheek-teeth it has 
been placed in Microsciurus, a group of true squirrels found in 
Central and North-western South America. 
Now at last the Museum has received from Mrs. McConnell, 
widow of the late Mr. F. V. McConnell, so long and frequent a 
contributor to the Museum collections, three specimens of the 
“Guianan Pigmy Squirrel, one of them with a practically perfect 
skull. 
An examination of this skull shows that instead of being in 
any way related to Microsciwrus or other forms of American 
Sciurine, the Guiana Squirrel is a member of the Nannosciur ne, 
in which it forms a special genus closely related to Vannosciurus. 
This genus may be diagnosed as follows :— 
SCIURILLUS. 
Abstract P.Z.S. 1914, p. 36 (May 12th). 
General structure of skull and number of teeth as in Vanno- 
sciurus, agreeing with that genus and differing from Myosciwrus 
in all the characters recorded by me* as distinguishing these 
genera from each other. An ectopterygoid present, broad, but 
not so long as in Vannosciurus. 
Postorbital processes over posterior root of zygoma.  Inter- 
orbital space as broad as the brain-case. Zygomata very broad 
and strong. Anteorbital foramen small, far in front of the 
teeth, as in Vannosciurus, its opening continued upwards as a 
peculiar curved groove along the front edge of the anteorbital 
fossa. 
5 
Cheek-teeth 4? as in Nannosciurus. Molars low, as in other 
Nannosciurine, their set normal, as in Nannosciurus, the last 
molar not facing outwards as in Myosciurus. Their upstanding 
cusps, both above and below, very little developed. Their surface 
more smoothly basin-shaped, with less evident transverse ridges. 
Type. Sciurus pusitlus Desm.t 
As a genus, Sciurillus is very closely related to Nannosciurus, 
the reduction in the prominent transverse ridges of its molars, 
the peculiar structure of its anteorbital foramina, and its high 
but abruptly truncated ectopterygoids being its chief distin- 
guishing characters. From d/yosciwrus, though both are un- 
doubtedly of the same group, it is more widely separated. 
In presenting this highly interesting case to students of 
geographical distribution, I may point out that the whole of 
North America is full of Squirrels of the other subfamily, the 
Sciurine, and that these have penetrated into South America as 
* Ann. Mag. N. H. (8) iii. p. 474 (1909). 
+ Should any doubt be thrown on the determination of Sciwrus pusilius, the 
genus should be considered as founded on the species represented by the type of 
S. kuhlit. 
