54 
circumstances tending to produce a stronger variety of Chimpanzee. 
The value of the character from size is established by the concurrence 
of the foregoing more fixed ones. The supraciliary ridges are rela- 
tively as strongly developed and as prominent in the skull of a female 
adult Trogl. niger as in that of the Trogl. Gorilla, and they are as 
angular and rough or uneven in the skull of the adult male Trogl. 
niger as in that of the adult male Trogl. Gorilla. The male Trogl. 
niger shows also the median prominence between the orbits above 
the root of the nose. 
In six skulls of Troglodytes niger Dr. Wyman found that ‘‘ the 
temporal ridges are generally separated from each other by a space 
varying from half an inch to one or two inches, according to age, 
but in none of them is to be seen even a rudiment of the interparietal 
ridge.” In an adult, but by the condition of the teeth, not old 
male Trogl. niger, the temporal ridges have met above the oblite- 
rated suture, and developed the rudiment of an ‘ interparietal ridge,’ 
which would probably have risen above its rudimental state had 
the exercise of the large temporal muscles been longer continued. 
Processes, ridges and crests dependent upon the stimulus of muscular 
action for their development, are the seats of most variety, and the 
least safe or satisfactory osteological marks of specific distinction. 
In the great males of the Tr. Gorilla even a certain range of variety 
is presented by the skulls of the four adult males, which we are now 
able to compare. 
In the one described by Dr. Wyman the interparietal or sagittal 
crest is elevated about 14 inch above the skull, and terminates 
above in a thin and free edge: in the fine male skull figured, 
and in the older male’s skull, the two temporal ridges, though 
touching each other at their base, do not coalesce to form a single 
sagittal crest, but each terminates in a free edge, inclining from its 
fellow, and neither of them rise to half an inch at their highest part, 
three inches behind their point of contact. 
4. The specific character of the zygomatic arches is best shown 
by the depth and convex or angular upper contour of the squamosal 
portion of the arch. 
5. Dr. Wyman has well indicated the characteristic forms of the 
anterior and posterior nares; and the conformity of the four skulls, 
two males and two females, submitted to his able and scientific scru- 
tiny, in this important character, with the three skulls which I have 
described, adds to our confidence in its constancy and value. The 
observed range of variety does not materially affect the well-marked 
difference of form in the posterior nares. Dr. Wyman finds in the 
Tr. niger that ‘‘ the transverse diameter of the orifice exceeds that 
of the vertical, but in the Jr. Gorilla the vertical is twice that of the 
transverse, a condition which results from the elongation downwards 
of the superior maxillary bones.” In one skull of an adult female 
Trogl. niger, in the Bristol Museum, the vertical diameter equals the 
transverse diameter of the posterior nares, and it exceeds it by about 
one-half only in the three skulls of the Tr. Gorilla in the same museum. 
6. With regard to the sixth character, which was pointed out to 
