EXPLANATION OF PLATE 25 
Crania of Aplodontia rufa phaea, to show some of the changes which take 
place with age; X 0.67. Figs. la, 1b, 1c, no. 20317, Mus. Vert. Zool.; figs. 2a, 
2b, 2c, no. 20306, Mus. Vert. Zool.; figs 3a, 3b, 3c, no. 8974, Mus. Vert. Zool. 
Note in the crania dorsally the disproportionate expansion of the skull, par- 
ticularly posteriorly; the marked reduction in width of the interorbital con- 
striction; and the disappearance of sutures. In the crania ventrally note the 
increase in measurements, the disappearance of sutures, and the disproportionate 
lateral growth of the mastoid processes as compared with the audital tubes; but 
note that the distance across the palate between the fourth premolars remains 
nearly constant. In posterior view note the disproportionately rapid outgrowth 
of the mastoid processes, with consequent disappearance of audital tubes, the 
change in outline of the foramen magnum, and the change in outline of the 
entire posterior aspect of the cranium, due to the development of the lambdoidal 
ridges and the outgrowth of the mastoid processes. 
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