Zoological Society. 485 
and of which mention is made in Martin’s ‘ Natural History of Qua- 
drupeds,’ Part 8. 
Section I—Or rue 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 
