Cranial measurements of type: greatest length 45; basal length 38.6; basilar length 36; zygomatic breadth 26.4; least interorbital breadth 17; mandible 28; maxillary toothrow (alveoli) 8.6; mandibular toothrow (alveoli) 8.6. _Specimens examined._--Four, all from the type locality. _Remarks._--As might be expected from the geographic position of the island it inhabits, _Sciurus seraiæ_ differs from the Bornean _S. notatus_ in much the same way as the Sirhassen representative of the group. It is readily distinguishable from the Sirhassen animal by the different color of the under parts. In color _Sciurus seraiæ_ closely resembles _S. abbottii_ of the Tambelan Islands. The latter is, however, a much larger animal, with a longer and relatively narrower skull. SCIURUS RUTILIVENTRIS sp. nov. _Type._--Adult male (skin and skull) No. 104658 U. S. National Museum. Collected on Pulo Midei (Low Island), South Natuna Islands, May 24, 1900. Original number 405. _Characters._--Size slightly greater than that of _Sciurus lutescens_ and _S. seraiæ_, but not equal to that of the Bornean or Bunguran representatives of _S. notatus_. Color above as in _S. seraiæ_. Under parts bright clear orange-rufous. _Color._--Color exactly as in _Sciurus seraiæ_ except that the pale side stripe is light cream-buff and the under parts are bright orange rufous. Tail without trace of red suffusion. _Skull and teeth._--The skull and teeth are a trifle larger than in _Sciurus lutescens_ and _S. seraiæ_, but the difference is scarcely a tangible one. _Measurements._--External measurements of type: Total length 368; head and body 190; tail vertebræ 178; hind foot 45 (41). Average and extremes of seven specimens from the type locality: total length 356 (330-368); head and body 186 (178-190); tail vertebræ 173 (165-184); hind foot 45.5 (43-48); hind foot without claws 42.2 (39.5-45). _Specimens examined._--Seven, all from the type locality. _Remarks._--This squirrel is remarkable among the Natuna members of the _S. notatus_ group for the brilliant color of its under parts. In this respect it surpasses all of the related forms with which I am acquainted. The red color is, however, strictly confined to the body, showing no tendency to spread to the tail as in _S. miniatus_ of the Malay Peninsula. SCIURUS RUBIDIVENTRIS sp. nov. 1894. _Sciurus notatus_ THOMAS and HARTERT, Novitates Zoologicæ, I, p. 659. September, 1894 (part, specimens from Bunguran). 1895. _Sciurus notatus_ THOMAS and HARTERT, Novitates Zoologicæ, II, p. 491. December, 1895 (part, specimens from Bunguran). _Type._--Adult female (skin and skull) No. 104671 U. S. National Museum. Collected on Bunguran Island, North Natunas, June 22, 1900. Original number 498.