Descriptions of East Asiatic Mammals 141 
The Agile Gibbons, Hylobates agilis, which appear in two 
sometimes intergrading color phases — blackish brown and buffy 
brown — are the only members of the genus in which the set of 
the hair of the forearm lies from the elbow toward the wrist. 
In the dark-colored phase the rump area is conspicuously paler ; 
in the pale phase the underparts may be darker brown than the 
back. In both phases the white brow band is well defined, the 
whiskers white, and the throat buffy. The face is black. There 
is no dark cap as in H. lar entelloides. The Agile Gibbons are 
rather smaller than the White-handed; the length of head and 
body 18% to 19% inches, of the foot 5 to 5% inches. The range 
includes the lower Malay Peninsula and Sumatra. 
The Hoolock Gibbon, Hylobates hoolock, like the Agile 
Gibbon, displays two color phases, which, however, seem not to 
intergrade. In the dark phase the color varies from brownish 
black to black and the underparts are very dark brown. In the 
pale phase the upper parts vary from yellowish to reddish buff, 
clouded with brown on the flanks and limbs. The tips of the 
fingers are usually blackish. The brown undersurface is always 
darker than the upper surface. In the dark phase the whitish 
brow band, sometimes interrupted at the center, is present. The 
cheeks, chin, or even the whole face may be whitish. The pale 
phase has the brow area and a ring around the eye pale, but the 
cheeks often very dark. In females the color is whitish, tinged 
with brown; the chin, chest, hands, and feet are white. The 
length of the head and body varies from 23 to 25 inches, that 
of the hind foot is about 6 inches. The species, originally from 
the Garo Hills, is found up to 4000 feet through Assam, the 
northern part of Burma, the Shan States, and southern Yunnan. 
The Hoolocks are great vocalizers. In the Burmese jungles in 
the early morning they can be heard calling and answering with 
their intensely resonant and penetrating "Whoko! Whoko!" 
They are likely to be found on steep, heavily forested hillsides, 
where difficulty of approach combined with the keenness of 
