_Skull._--As compared with the Bornean form of _Sciurus notatus_, the skull of _S. lutescens_ is much smaller (greatest length about 45 instead of 50) the rostrum is relatively shorter and broader, and the audital bullæ are less elongate antero-posteriorly. Teeth as in _Sciurus notatus_ except that they are uniformly smaller. _Measurements._--External measurements of type: total length 355; head and body 177; tail vertebræ, 177; hind foot 45 (41). Average and extremes of six specimens from the type locality: total length 356 (329-375); head and body 186 (177-196); tail vertebræ 170 (152-178); hind foot 43.8 (41-45); hind foot without claws 40.7 (39-42). Cranial measurements of type: greatest length 45.4 (50.4)[19]; basal length 39 (43); basilar length 36.4 (41); palatal length 20 (23); palatal width between middle molars 6 (6); greatest length of nasals 13 (14.8); greatest width of both nasals together 6.6 (7); interorbital breadth 15.4 (17); mastoid breadth 21 (21); zygomatic breadth 26 (29); depth of braincase at anterior edge of basi-occipital 16 (16.8); mandible 28 (30); maxillary toothrow (alveoli) 8 (9); mandibular toothrow (alveoli) 8 (9). _Specimens examined._--Seven (one in alcohol), all from the type locality. _Remarks._--This squirrel is recognizable among the members of the _S. notatus_ group by its light colors, and particularly by the pallor of the under parts. In the latter characteristic it is approached by the form inhabiting Pulo Laut, but with this exception it is unique among the fulvous bellied species. The six specimens show no variation worthy of note. SCIURUS SERAIÆ sp. nov. _Type._--Adult male (skin and skull) No. 104660 U. S. National Museum. Collected on Pulo Seraia, South Natuna Islands, May 29, 1900. Original number 415. _Characters._--Most nearly related to the small, pallid, _Sciurus lutescens_ from Sirhassen Island, but upper parts slightly less pale, and under parts and pale side stripe buff-yellow, the former without admixture of gray. _Color._--Upper parts as in _Sciurus lutescens_ except that the pale bands on the hairs are more nearly buff than cream-buff. Tail essentially as in _S. lutescens_ but a shade less pale. Under parts buff-yellow darkening irregularly to dull orange-buff. Dark side stripe broad and well defined. _Skull._--The skull closely agrees with that of _Sciurus lutescens_ in both size and form, though it is perhaps even broader in proportion to its length. Teeth as in _S. lutescens_. _Measurements._--External measurements of type: total length 368; head and body 197; tail vertebræ 171; hind foot 44 (40). Average and extremes of four specimens from the type locality: total length 347 (323-368); head and body 184 (171-197); tail vertebræ 163 (152-171); hind foot 43.7 (43-45); hind foot without claws 40.1 (39.5-41).