486 Zoological Society. 
Gibbon has a pretty good chin for a monkey. In this respect it ap- 
pears to approach Man more nearly than the higher Orangs. The 
lower jaw of Man is more uniform in its depth than that of this 
Gibbon: its angle too is not quite so much rounded; the external 
edges of the ascending and horizontal branches do not form quite so 
obtuse an angle at their meeting. 
The question may be asked, What are the effects of age in altering 
the form of the skull in the Gibbons? In answer I will remark, 
first, that the muzzle is elongated and the cranium thrown in a 
more backward position, in consequence of the necessity for length- 
ening the dental edge to receive the second or permanent series of 
teeth. This will be evident by a comparison of the skulls of the 
young White-cheeked and Hoolock Gibbons, figured in Martin’s 
Nat. Hist. Quad., Part 8, with that of the adult Agile Gibbon in the 
Bristol Institution (the subject of this paper) and with Dr. Harlan’s 
plate of that of the adult Hoolock, Trans. Amer. Phil. Soc., ubi supra. 
The latter comparison is very satisfactory on this point, as the spe- 
cimens compared are of the same species. A corresponding elonga- 
tion of the facial parts takes place in the Orangs, as demonstrated by 
Prof. Owen, Zool. Trans. vol.i. or Zool. Proc. 1835, p. 30. A similar 
change is also observable in the human species; the facial angle of 
the infant decreases with age until the second teeth are cut. Secondly, 
with increasing age another change takes place in the greater promi- 
nence of the supraciliary ridges and the margin of the orbit. I appeal 
again to the illustrations of Martin and Harlan, and to the original 
sketch of the Agile Gibbon. A similar development of the cranial 
ridges takes place in the Orang Utan and Chimpanzee with age; 
in the former, in the temporal and sagittal crests ; in the latter, in the 
orbital margin. Thirdly, in the anchylosis of the bones of the cra- 
nium and the face. This would appear to have taken place in the 
skull of the Hoolock figured by the late Dr. Richard Harlan (ubi supra), 
whilst in the immature one figured by Martin (wi supra) the sutures 
are represented. This change is observed to take place in the adult 
Orang, but not in the Chimpanzee. Fourthly, it appears probable, 
from a comparison of the before-mentioned materials, that the infra- 
orbital foramen, and the foramen which gives exit to the dental blood- 
vessel and nerve in the lower jaw, become smaller by age. 
From these observations it will be apparent that the skull of the 
Gibbons, like that of the Orangs, is far more anthropoid in youth 
than in mature age. The prolongation of the muzzle, the retrogres- 
sion of the cranium, the smallness of the facial angle, the develop- 
ment of the orbital ridges, the anchylosis of the bones, and the small- 
ness of the foramina, all distance the aged more than the immature 
Gibbon from the human race. 
Compared with the human skull, the head of this Gibbon is distin- 
guished by its lengthened ovate figure; its narrowness, especially 
behind the orbits ; by the large size and inflated parietes of the orbits ; 
by the want of vertical elevation of the forehead, and the consequent 
position of the brain behind, not above the face; by the great eleva- 
tion of the supraorbital ridges; by the development of the muzzle, 
necessitating the backward position of the zygomatic arches and the 
