1886. ] MR. O. THOMAS ON CRANIAL VARIATION. 127 
until the zygomatic spread in A is no less than 74 per cent. of the 
length of the skull, as compared to 59 per cent. in B, the actual breadth 
being nearly one third greater. Secondly, they have pressed upon 
the inner walls of the temporal fossee, and have thereby reduced the 
breadth of the skull at the interorbital constriction to such a degree 
that in A its least width is only 16 millim. (36 per cent. of the basi- 
cranial axis) as compared to 23°5 millim. (57 per cent.) in B. That 
the width at this constriction grows absolutely, as well as relatively, 
less in many mammals has been observed by several authors, but it 
is, I think, by no means sufficiently realized. Thirdly, the great 
biting ‘muscles have pressed upon and thereby constricted the 
posterior narial passage, its outside breadth being only 10°5 millim. 
in A as against 11°9 in Bt Inside the brain-case again, the con- 
striction of its anterior part has reduced the cranial capacity from 
40 c.cm., as it is in B, to 35 in A, the smaller capacity being 
therefore found in the larger and older skull. 
These various differences are shown in the drawing (Pl. XI.) 
better than they can be explained by any amount of description. 
Finally, I would again lay special stress on the fact that B is 
absolutely adult, so far as any definition of adult age can be drawn 
up, and has therefore no sign whatever of immaturity, such as 
would put the species-maker on his guard, and yet that in a skull 
so adult as this such changes may take place in advanced life as to 
alter its whole appearance and proportions, and even its cranial 
capacity. 
To disprove the natural suggestion that the differences above 
noted are due to sex, there comes fortunately the skull C, collected 
at the same time and place, which in its age-characters is intermediate 
between A and B, but is far smaller and more lightly built, and is 
quite obviously the female form corresponding to the other two. 
The following are the measurements of the three skulls for 
comparison :— 
A. d. 1 Ay alll aye 
Peneth*... .. ... STS. RE SEAMPT H ip 107 93 
Greatest breadth . ........ AP RB: 63 56 
Beate, Tent oa. ws ae. 63 60 52 
mee teadth 2.4. tes ace OO 34 ok 
Interorbital constriction... ...... 16 23°5 19 
RASICIAMIAL AXIS? «.... 0. os «0..< 45 4] 35 
Capacity (in cubic centimetres)... 35 40 31 
* Basion to front of premaxillx. 
* Basion to anterior edge of basisphenoid. 
