1886.] MR. O. THOMAS ON CRANIAL VARIATION. 127 



until the zygomatic spread iu 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 fossse, and have thereby reduced the 

 breadth of the skull at the interorbital constriction to such a degree 

 that iu A its least width is only 16 millim. (36 per cent, of the basi- 

 ci'anial 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 B. 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 (PI. 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 di'awn 

 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. cJ. B. cJ. C. $. 



Length 1 112 107 93 



Greatest breadth 83 63 56 



Palate, length 63 60 52 



Palate, breadth 36 34 31 



Interorbital constriction 16 23-5 19 



Basicranial axis ^ 45 41 35 



Capacity (in cubic centimetres). .35 40 31 



^ Basiou to front of premaxillas. 



'^ Basiou to anterior edge of basisphenoid. 



