Feet heavy and robust. Thumb short, with a flat blunt nail. Soles and palms naked, the former with six well developed tubercles, the latter with five.
Mammæ, p. 2—2, i. 2—2 = 8.
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.