AND AFFINITIES OF THE GORILLA. 259 
character to the Human skull, and to which only the Gorilla, in the Ape-tribe, makes 
any approximation. 
No Orang, Chimpanzee, or Gibbon shows any rudiment of mastoid processes ; but 
they are present in the Gorilla, smaller indeed than in Man, but unmistakeable; they 
are, as in Man, cellular, pneumatic, and with a thin outer plate of bone. This fact led 
me, in a former memoir, to express, when, in respect to the Gorilla, I only knew the 
skull, the following inference, viz.—‘‘ from the nearer approach which the Gorilla 
makes to Man, in comparison with the Chimpanzee or Orang, in regard to the mastoid 
processes, that it assumed more nearly and more habitually the upright attitude than 
those inferior anthropoid Apes do'.” This inference has been fully borne out by the 
rest of the skeleton of the Gorilla, subsequently acquired. 
In the Chimpanzee, as in the Orangs, Gibbons, and inferior Simie, the lower surface 
of the long tympanic or auditory process is more or less flat and smooth, developing in 
the Chimpanzee only a slight tubercle, anterior to the stylohyal pit. In the Gorilla the 
auditory process is more or less convex below, and developes a ridge, answering to the 
vaginal process, on the outer side of the carotid canal. The processes posterior and 
internal to the glenoid articular surface are better developed, especially the internal one, 
in the Gorilla than in the Chimpanzee ; the ridge which extends from the ectopterygoid 
along the inner border of the foramen ovale terminates in the Gorilla by an angle or 
process answering to that called “styliform ” or ‘‘ spinous” in Man, but of which there 
is no trace in the Chimpanzee, Orang, or Gibbon. 
The orbits have a full oval form in the Orang; they are almost circular in the 
Chimpanzee and Siamang, more nearly circular and with a more prominent rim in the 
smaller Gibbons; in the Gorilla alone do they present the form which used to be 
deemed peculiar to Man. There is not much physiological significance in some of the 
latter characters, but, on that very account, I deem them more instructive and guiding 
in the actual comparison. 
The occipital foramen is nearer the back part of the cranium, and its plane is more 
sloping, less horizontal, in the Siamang, than in the Chimpanzee and Gorilla. Consider- 
ing the less relative prominence of the fore part of the jaws in the Siamang as compared 
with the Chimpanzee, the occipital character of that Gibbon and of other species of 
Hylobates marks well their inferior position in the quadrumanous scale. 
In the greater relative size of the molars, compared with the incisors, the Gorilla 
makes an important closer step towards Man than does the Chimpanzee. The molar 
teeth are relatively so small in the Siamang, that, notwithstanding the small size of the 
incisors, the proportion of those teeth to the molars is only the same as in the Gorilla: 
in other Gibbons (Hylobates lar), the four lower incisors occupy an extent equal to that 
of the first four molars, in the Chimpanzee equal to that of the first three molars, in 
the Siamang equal to that of the first two molars and rather more than half of the 
? Trans. Zool. Soc. vol. iii. p. 409. 
VOL. V.—PART IV. 2M 
