Zoological Society. 485 



and of which mention is made in Martin's * Natural History of Qua- 

 drupeds,' Part 8. 



Section I. — Of the Skull. 



The cranium of the Hylobates agilis is elongate and ovate in form, 

 much- contracted behind the orbits, which are very projecting and 

 deep and surmounted by very elevated supraciliary ridges. The 

 muzzle is rounded and broad, so that the face, although considerably 

 prominent, has not attained the lengthened shape of the Baboons or 

 of the adult Orang Utan. The forehead, which is narrow, is but 

 slightly arched above the orbits, so that the whole of the cranium is 

 behind the face. 



A slightly elevated ridge of bone, arising from the supraorbital 

 ridges, which becomes contracted during its passage over the coronal 

 aspect of the skull, and again expands towards the occiput, marks 

 the boundary on either side of the temporal muscles. This elevated 

 medial portion is smooth, whilst the lateral portions of the skull are 

 roughened by muscular attachments. This development is similar 

 to that of the Chimpanzee, whilst in the Orang Utan the sagittal 

 and temporal crests are elevated to an extraordinary extent. 



The supraorbital ridges, we have before remarked, are much-de- 

 veloped. Such is the case in the Chimpanzee, where however they 

 form a junction across the face, which does not take place in the 

 Active Gibbon. The orbits have a very prominent margin, are very 

 large and deep, and are much swelled out externally, so that their 

 outer portion " projects very boldly from the cranium." Sir Thomas 

 Stamford Raffles says of the Siamang, ** The orbits of the eyes are 

 circular and remarkably prominent," Linn. Trans, vol. xiii. p. 242. 

 Such too is the character of the skull of the adult Hoolock figured 

 by Dr. Harlan in the Transactions of the American Philosophical 

 Society, vol. iv. New Series, p. 52. 



The nasal bones make a slight elevation, thus resembling Man 

 more than the Orang Utan or even the Chimpanzee. The osseous 

 opening of the nose is wide and rather large. The figure of the face 

 viewed in front, from between the orbits to the dental edge, resem- 

 bles a wedge whose point is directed downwards. This form is con- 

 trary to that of the Baboons, where the wedge is inverted. The 

 infraorbital canal opens by a single hole, as in Man and the Chim- 

 panzee. This foramen is smaller in the Gibbon than in those animals. 



The outward curvature of the zygomatic arch is not great ; it is 

 placed far more posteriorly than in Man, in consequence of the 

 lengthening of the facial portion of the skull. 



The skull of this Gibbon is anchylosed, externally at least, into 

 one piece. Prof. Owen tells us that the cranial sutures are oblite- 

 rated in the adult Orang Utan, Syndactylous Ape, and frequently 

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

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

 human cranium. 



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

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

 shallowest just below the termination of the molar series, deepening 

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



