In general habits it in no way differs from the other species of _Hylobates_ already described.
In regard to the Bornean specimens of this species, Dr. Anderson makes the following observations: "This species varies from grey to dark yellowish-brown, but the grey tint in certain lights appears pure ashy, and in others of a brownish tint. In some the chest and abdomen are frequently yellow, and this seems to be the character of individuals met with on the west coast of Borneo, while those inhabiting the meridional parts of the island have the hands and fore part of the body of a black-brown or reddish-brown. In both of these varieties there is a yellowish-white superciliary streak. The last of them leads into the varieties of _Hylobates_ from the neighbouring islands of Sulu, to the north-east of Borneo, in which the upper parts of the body are either grey or brownish, the lower part of the back and the loins being a little more clear than the rest." The outer surface of the limbs, the back part of the head, the supercilium, and the sides of the face are more or less pure ashy-grey. "Specimens of this Gibbon obtained by me," writes Mr. Charles Hose, who is well known for his Bornean researches, { 158}"at Claudetown, and now in the British Museum, show that the colouring in different parts of the body must be considered of little importance, as I obtained eleven specimens, five of which were in the same troop and the other six from the same locality, varying in colour as much as it is possible for them to do; some had yellowish backs and black chests, others black backs with yellowish chests, and some were nearly black all over; whilst others were almost a complete silver-grey. I, therefore, come to the conclusion that _H. muelleri_ and _H. leuciscus_ cannot be separated. The peculiar bubbling noise they make is similar. I think it very unlikely that two distinct species should be so constantly found together as they are in Sarawak.
"The natives call the silver-grey variety 'Emplian' or 'Wa-Wa,' and the dark one, 'Emplian arang' (coal), because of its colour."
III. THE WHITE-CHEEKED GIBBON. HYLOBATES LEUCOGENYS.
_Hylobates leucogenys_, Ogilby, P. Z. S., 1840, p. 20; Blyth, J. A. S. Beng., x., p. 838 (1841); Martin, Mammif. Anim., p. 445, _cum fig._ (1841); Is. Geoffr., C. R., xv., p. 717 (1842); id., Arch. Mus., ii., p. 535 (1843); Gray, Cat. Monkeys Brit. Mus., p. 11 (1870); Schl., Mus. Pays-Bas, vii., p. 13 (1876); Scl., P. Z. S., 1877, p. 679, pl. lxx.; Anderson, Zool. Res. Exped. Yun-nan, p. 6 (1878; with synonymy).