xo. 1626. MAMMALS FROM EASTERN SUMATRA—LYON. 645 
Skull.—The skull of Mus mandus does not differ from that of J/us 
asper, except in its distinctly larger size. 
Measurements—For external and cranial measurements of the 
type and series see table, page 648. 
Specimens examined.—Six from Pulo Rupat, six from Pulo Padang, 
three from the Kateman River, four from the confluence of the 
Sungei Mandau with the Siak River, and one from the confluence of 
the Gasip and Siak rivers. 
Remarks.—The rats of the Mus asper group in eastern Sumatra 
fall into two well marked groups, a small bright colored form with 
rusty bellies differing in no essentials from d/us asper of the Malay 
Peninsula and the species just described, larger, duller, darker, and 
with clear gray bellies. The mouth of the Gasip River is the only 
locality where both forms appear to have been taken together. Else- 
where in eastern Sumatra only one form has been taken at a given 
locality. The relations of the two forms are not at all clear, but the 
material thus far collected show them to be quite distinct, and no 
intermediate specimens are met with. It is not improbable that both 
forms may belong to the same species which is branching out in two 
different directions in eastern Sumatra, and at various localities one 
form or another is becoming dominant. Rats of this group on the 
Malay Peninsula are quite variable both as to size and as to intensity 
of color of the upperparts and rustiness or grayness of the underparts. 
These variations are most pronounced in specimens from the east 
coast of the Peninsula, but they are by no means so pronounced or 
constant as they are in the Sumatran examples, and many inter- 
mediate conditions are met with, which is not the case with the rats 
from Sumatra. 
MUS LINGENSIS Miller. 
1900. Mus lingensis MILLER, Proc. Washington Acad. Sciences, II, p. 206, 
August 20, 1900. (Type-locality, Linga Island.) 
1903. Mus lingensis MILLER, Proc. U. S. Nat. Mus., XXVI, p. 4638, February 
3, 1903. (Specimens from Tapanuli Bay, western Sumatra.) 
Twelve specimens from Aru Bay, eight skins with skulls and four 
odd skulls. 
For measurements see table, page 648. 
MUS FIRMUS Miller. 
1902. Mus firmus MiLurrR, Proc. Acad. Nat. Sci. Philadelphia, March, 1902, 
p. 155, issued June 11, 1902. (Type-locality, Linga Island.) 
1908. Mus firmus MILter, Proce. U. S. Nat. Mus., X XVI, p. 461, February 3, 
1908. (Specimens from Tapanuli Bay, Sumatra.) 
One specimen, an adult male, from the Siak River 
For measurements see table, page 649. | 
Proc. N. M. vol. xxxiv-—08—42 
