770 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.xxiv. 
length of incisive foramen, 8.4 (8.4); combined breadth of incisive 
foramina, 2.8 (3.8); length of nasals, 17.4 (18.6); combined breadth of 
nasals, 5 (5); zygomatic breadth, 22 (25); interorbital breadth, 8 (7); 
mastoid breadth, 17.4 (19); breadth of braincase above roots of zygo- 
mata, 17 (16. 8); depth of braincase at anterior border of basioccipital, 
12.2(13.6);fronto-palatal depth at posterior extremity of nasals, 11.8 
(11); least depth of rostrum immediately behind incisors, 9 (9); man- 
dible, 28 (29); maxillary toothrow (alveoli), 7.6 (7.4): mandibular tooth- 
row (alveoli), 7.4 (7). 
Specimens examined. — Twelve, all from Trinkut Island, Nicobars. 
Remarks. — In external characters, the series of twelve skins show 
practically no variation. The 3^ounger specimens are less clear buff 
beneath, but with this exception the color is perfectly constant. The 
skulls are likewise very uniform. Here the variation is confined to 
the slight differences in size usuali}^ correlated with greater or less age. 
The nearl}^ straight lateral outline of the nasal is an unvarying charac- 
teristic of the entire series of skulls. 
Whether Mus hurrus is a direct descendant of the house rat can not 
be decided, but its relationship with this wandering species is not 
remote. The dark color and less spreading zygomata are alone suffi- 
cient to distinguish the Nicobarean animal, however, from any of the 
very numerous specimens of Mus norveglcus that have come under my 
observation. 
MUS BURRULUS, new species. 
Type.—K<\\x\i male (skin and skull), No. 111817, U. S. N. M. Col- 
lected on Car Nicobar, January 25, 1901, by Dr. W. L. Abbott. 
Original number, 865. 
Characters. — Similar to Miis hurrus, but much smaller; hind foot, 38 
(36); greatest length of skull, 41. 
Color. — The color and other external characters so closely resemble 
those of Mus hurrus as to need no detailed description. 
Skull. — The skull though differing from that of Mus hurrus chiefly 
in its smaller size is also distinguishable by its much more slender ros- 
trum, a character which is in no way due to differences of age. The 
outer margin of the nasal is straight, as in the larger animal. 
Teeth. — The teeth are similar to those of Mus hurrus, though per- 
ceptibly smaller throughout. 
Measurements. — External measurements of type: Total length, 357 
mm. ; head and body, 174; tail, 183; hind foot, 38 (36); ear from meatus, 
19; ear from crown, 15; width of ear, 15. 
Cranial measurements of type: Greatest length, 41 mm.; basal 
length, 37.4; basilar length, 34.8; length of nasals, 15; combined breadth 
of nasals, 4; zygomatic breadth, 19.8; interorbital constriction, 6.8; 
maxillary toothrow (alveoli), 7. 
