THE GIBBONS. 167 



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. 



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-hke 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 Su- 

 matra. 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. 



Habits. — The Siamang is gregarious, frequenting the great 

 forest-trees from 200 to 300 feet above the sea up to 3,000 or 

 4,000 feet. 



I made the acquaintance of this species in Southern Sumatra, 

 and during my stay in that island had various opportunities of 

 observing many of them in their homes. It was not uncom- 

 mon to come suddenly on a colony of them both in the forest 

 and among the tall isolated outliers, when they happened to 

 be covered with fruit. The satiated members of the company 



