Natural History of the Mammalia of India and Ceylon

Genus Hylobates — The Gibbons

Which, with the long arms of the Ourangs and the receding forehead of the Chimpanzee, possess the callosities of the true monkeys, but differ from them in having neither tail nor cheek pouches. They are true bipeds on the ground, applying the sole of the foot flatly, not, as Cuvier and others have remarked of the Ourangs, with the outer edge of the sole only, but flat down, as Blyth, who first mentions it, noticed it, with the thumb or big toe widely separated.