28 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Size. — The three specimens measure as follows : — 
No. 
Sex. 
Age. 
Total length. 
Tail vertebrae. 
Hind foot. 
Ear. 
4012 
$ 
juv. 
460 
95 
70 
26 
4013 . 
? 
juv. 
470 
100 
75 
28 
4014 
s 
juv. 
465 
103 
70 
28 
I have found great difficulty in obtaining specimens of Arctomys 
monax monax of the right age to make a fair basis for comparison 
with the form occurring on the north shore of Lake Superior. My 
specimens of the latter are all young of the year, which have not yet 
shed the milk premolars, yet the condition of the bones and sutures 
in the skull shows that they are not very immature. A young 
female taken at Wareham, Mass., on August II, 1894, and evidently 
younger than any of the Nepigon specimens measures : total length, 
504; tail vertebrae, 138; hind foot, 80; ear, 34. A young male 
taken at the same locality on August 7, 1893, and very much 
younger than the last measures: total length, 479; tail vertebrae, 
112; hind foot, 74.5 ; ear, 30.5. 
Skull and teeth. — The fact that all three specimens of Arctomys 
from Nepigon are immature makes any comparison of cranial char¬ 
acters between this form and the typical subspecies a matter of 
difficulty and uncertainty. I have, however, found the skulls of 
eight specimens with which a fair comparison can be made. The 
Lake Superior skulls are evidently the oldest of the lot, though none 
of the eleven have shed their milk premolars. In spite of their 
greater age they are slightly smaller than the others, and the rostral 
portions of the skulls are uniformly shorter, narrower, and less deep. 
Until adults of the two races can be compared, it is impossible to 
state with certainty what the true differential cranial characters ares 
I can find no differences in the teeth. The following table show. 
O 
the extent of the differences in size of these skulls. 
