_Skull._--In general appearance the skull of _Mus integer_ resembles that of _Mus validus_.[12] It is shorter (greatest length about 51 instead of 55) and the rostrum is relatively broader and deeper. Audital bullæ similar in form to those of _Mus validus_, but the surface less irregular. Region between anterior bases of zygomata broader than in _Mus validus_ so that the arches are more nearly parallel. _Teeth._--The teeth are relatively as well as actually smaller than in _Mus validus_ and the enamel pattern is normal, that is, the posterior upper molar consists of two transverse folds, and an anterior internal tubercle. There is no trace of the supplementary outer tubercles of the corresponding tooth of _Mus validus_. _Measurements._--External measurements of type: total length 463; head and body 235[13] tail vertebræ 228;[13] hind foot 48 (45); ear from meatus 19; ear from crown 15; width of ear 15. In adult male topotype: total length 462; head and body 234;[13] tail vertebræ 228;[13] hind foot 46 (44); ear from meatus 21; ear from crown 16; width of ear 16. Cranial measurements of type: greatest length 52 (55);[14] basal length 45 (48.6); basilar length 41.6 (45.6); palatal length 23 (26); least width of palate between anterior molars 5 (5); diastema 14 (14.6);[15] length of incisive foramen 8 (9); combined breadth of incisive foramina 3 (3.6); length of nasals 21 (22.6); combined breadth of nasals 6 (6.2); zygomatic breadth 25 (28); interorbital breadth 8 (8); mastoid breadth 19 (19); breadth of braincase above roots of zygomata 18.8 (20); depth of braincase at anterior border of basi-occipital 12.8 (15); frontopalatal depth at posterior extremity of nasals 12.8 (13.4); least depth of rostrum immediately behind incisors 10 (10); maxillary toothrow (alveoli) 9.6 (11); width of front upper molar 3 (3); mandible 30 (31); mandibular toothrow (alveoli) 9 (10). _Specimens examined._--Four, three from the type locality, and one from Pulo Lingung. _Remarks._--This rat is probably a near relative of the Bornean _Mus mülleri_ of Thomas.[16] The specimen from Pulo Lingung does not differ appreciably from the others. MUS SABANUS Thomas. 1887. _Mus sabanus_ THOMAS, Ann. and Mag. Nat. Hist., 5th ser., XX, p. 270. October, 1887 (Mt. Kina Balu, Borneo). 1894. _Mus sabanus_ THOMAS and HARTERT, Novitates Zoologicæ, I, p. 658. September, 1894 (Bunguran). Thirteen skins and one extra skull, all from Bunguran. There is little probability that this rat is the same as the true _Mus sabanus_ of Borneo. MUS RAJAH Thomas. 1894. _Mus hellwaldi_ THOMAS and HARTERT, Novitates Zoologicæ, I, p. 658. September, 1894 (Bunguran). 1894. _Mus rajah_ THOMAS, Ann. and Mag. Nat. Hist., 6th ser., XIV, p. 451. December, 1894 (Mount Batu Song, Borneo). 1895. _Mus rajah_ THOMAS, Novitates Zoologicæ, II, p. 26. February, 1895 (Revised determination of Bunguran specimens).