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Show 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 fossae, 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 B. Inside the brain-case again, the constriction 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. X L) 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. 6. B. o*. C. 2- Length1 112 107 93 Greatest breadth 83 63 56 Palate, length 63 60 52 Palate, breadth 36 34 31 Interorbital constriction 16 23*5 19 Basicrauial axis2 45 41 35 Capacity (in cubic centimetres).. 35 40 31 1 Basion to front of preniaxillse. 2 Basion to anterior edge of basisphenoid. |