CA8TORID.E—CA8TOK— CASTOR FIBER. 



435 



quence of the increased thickness and density of the bones. The ridges liir 

 the attachment of muscles also become more strongly developed in old age 

 All of the forty-five sknlls, of which measurements arc 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 5.2H, and in breadth from 'i.lb to S.H^. 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 4 15 in breadth. The apparent wide range of individual variation in size 

 ill the large series of skulls examined is largely due to differences of age, as 

 will be seen from an examination of the subjoined tai)le 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 re8|>ect to 

 particular elements of the skull, the variation is greater than in general size. 

 Nos. 9477 snd 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, (.ad 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 

 baoally greatly widetiing. 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 

 mor? ' ariable, especially in respect to the interparietal portion. This portion 





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