13 



in the Baboons and othcr Quadrumana. I have observed it in Pithe- 

 cia Satanus, an American species. It sometimes occurs in the adult 

 huraan cranium. 



The lower jaw is rather lengthened in figure, decidedly more so 

 than in Man, in conseąuence of the production of the muzzle. It is 

 shallowest just below the termination of the molar series, deepening 

 tovvards the symphysis, which is not very retreating, so that the 

 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 horizontai branches do not form quite so 

 obtuse an angle at their meeting. 



The ąuestion 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 throw'n in ą 

 more backward position, in conseąuence 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 skuUs 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 šame 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 ; tlae facial angle of 

 the infant decreases vvith age until the second teeth are cut. Secondly, 

 vvith increasing age another change takes place in the greater promi- 

 nence of the supraciliary ridges and the margiu 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 craniai 

 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 

 orbitai margiu. Thirdly, in the anchylosis of the bones of the cra- 

 nium and the face. This -vv-ould appear to have taken place in the 

 skull of the Hoolock figured by the late Dr. Richard Harlan {ubi supra), 

 \vhilst in the immature one figured by Martin (ubi supra) the suturės 

 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, cind the foramen which gives exit to the dental blood- 

 vessel and nerve in the lower jaw, become smaller by age. 



From these observations it vdll be apparent that the skull of the 

 Gibbons, likę 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 orbitai ridgei, the anchylosis of the bpnes, and the small- 



