Siamanga syndactyla, Gray, Cat. Monkeys Brit. Mus., p. 9 (1870), id., op. cit., p. 9 (1870); H. O. Forbes, Nat. Wand. East. Arch., p. 129 (1885).
(Plate XXXVIII.)
Characters.—This is the largest species of the genus, measuring more than three feet; it is stouter than H. hoolock, and its hair is entirely glossy black, having no white hairs anywhere; the face is black, as is also the distensible skin of the large bare patch on the throat, which overlies its great laryngeal pouch. The second and middle toes are united by a web as far as the last joint. The hair on the arms and fore-arms converges towards the elbow.
The skulls in most of the species of this genus closely resemble each other; that of the Siamang is distinguished by its larger size, and in having the supra-orbital ridges more developed, while the occipital region is more truncated, and there is at the symphysis of the lower jaw a true, though slight, chin.
PLATE XXXVIII.
THE SIAMANG GIBBON.
The frontal lobes of the brain are broad and much flattened, and not full and rounded as in the Orang. The olfactory bulbs project forward, slightly beyond the frontal lobes of the cerebrum; the occipital lobes are much reduced, while the large cerebellum projects distinctly backwards from below the cerebrum—characters in which this very highly organised member of the genus shows a retrogressive development, thus differing from all the other Man-like Apes, in all of which the cerebrum entirely covers both the olfactory lobes in front, and the cerebellum behind.
The large laryngeal sac, communicating by two openings with the larynx, and formed by the extension of the thyro-hyoid membrane, distinguishes this from all the other Gibbons.
Distribution.—The Siamang is confined to the island of Sumatra. It has been recorded from Malacca and Tenasserim; but some doubt exists as to the accurate determination of the individuals referred to, no really authentic specimen having yet been obtained out of Sumatra.