CASTORIDA—CASTOR—CASTOR FIBER. 435 
quence of the increased thickness and density of the bones. The ridges for 
the attachment of muscles also become more strongly developed in old age 
All of the forty-five skulls, of which measurements are given below, had 
attained mature dentition, but the variation in size they present with age is 
very considerable. In the series of sixteen from Arctic America, the varia- 
tion in length ranges from 4.45 to 5.25, and in breadth from 3.15 to 3.85. In 
a series of twelve skulls from the Platte and Upper Missouri Rivers, the 
variation is still greater, ranging in length from 4.40 to 5.65, and from 3.10 
to 415 in breadth. The apparent wide range of individual variation in size 
in the large series of skulls examined is largely due to differences of age, as 
will be seen from an examination of the subjoined table of measurements, in 
which the relative age of the specimens is indicated, as judged by the condi- 
tion of the sutures, the teeth, the relative development of the ridges for mus- 
cular attachment, and the density of the bony structure. In four very old 
skulls from nearly the same locality, the greatest variation is from 5.10 to 
5.25 in length, and 3.50 to 3.85 in breadth. In the three marked in the table 
as “old”, but evidently younger than those marked ‘very old”, the variation 
ranges from 4.50 to 4.90 in length. and from 3.30 to 3.60 in breadth. Those 
evidently much younger, and marked in the table as “‘middle-aged”, eight in 
number, all fall within these extremes, so that the range of purely individual 
variation may amount to fully 20 per cent. of the average. In respect to 
particular elements of the skull, the variation is greater than in general size. 
Nos. 9477 and 7201, with a length of 5.25 each, vary in breadth from 3.50 
to 3.85. Yet the narrower of these two skulls has much the wider nasals, 
which have a breadth of 0.95 against 0.87 in the other; they are, however, 
at the same time shorter, having a length of only 1.72 against 1.90 in the 
other, and hence vary enormously in general form, in the one being short 
and greatly expanded, in the other long, narrow, and of nearly uniform width 
throughout. In the one, the zygomatic arch is short, broad, and heavy ; in 
the other, narrower, longer, and more slender. The interparietal bone in 
these two skulls is as different in shape as can well be imagined. In No. 
7201, the anterior half is triangular, the posterior abruptly expanding, and 
basally greatly widening. In No. 8676, it is more than one-half smaller, and 
is regularly triangular to its base. In other skulls, it is even still narrower ; 
this element of the skull presenting, in different specimens, almost endless 
variations in respect to both size and form The frontals are perhaps still 
more variable, especially in respect to the interparietal portion. This portion 
