86 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
mal color marks were in the median line, and moreover that they 
occupied regions which in many mammals are normally marked with 
white. 
Table of Ratios and Average Measurements of Microtus pennsylvanicus 
and M. breweri. 
Microtus pennsylvanicus (Ord). 
Locality. 
No. of specimens. 
1 Sex. 
I 
Averages. 
Ratios to 
total 
length. 
Maxima. 
Minima. 
• 
Total length. j 
Tail vertebrae. 
Hind foot. 
Tail vertebrae. 
Hind foot. 
Total length. 
Tail vertebrae. 
Hind foot. 
Total length. 
j Tail vertebrae. 
Hind foot. 
Elizabethtown, N. Y. 
5 
cf 
168.8 
46.4 
20.8 
27.5 
12.3 
174 
48 
22 
165 
45 
20 
(6 C6 
3 
? 
164 
45.7 
21.2 
27.9 
12.9 
166 
51 
21.5 
162 
42 
21 
Burlington, Vt. 
2 
cf 
182 
49.5 
22.7 
27.2 
12.5 
183 
53 
23 
181 
46 
22.4 
(£ U 
4 
? 
170.7 
46.2 
20.7 
27.0 
12.1 
179 
50 
22 
160 
40 
20 
Peterboro, N. Y. 
9 
cf 
180 
48.1 
21.0 
26.9 
11.8 
195 
61.0 
22 
160 
33.5 
20 
a tt 
27 
9 
172.3 
45.7 
20.4 
26.5 
11.8 
185 
53.3 
21.8 
160 
38.4 
19 
North Truro, Mass. 
5 
cf 
169.8 
44.7 
21.3 
26.2 
12.5 
182 
49.5 
21.6 
161.3 
41 
20.8 
ti (6 
3 
? 
180.3 
47.8' 
21.? 
26.5 
11.8 
193 
50.5 
21.6 
167.6 
45 
21 
Martha’s Vineyard, “ 
6 
cf 
172.3 
43.8 
21.9 
25.5 
12.7 
188 
45 
23 
164 
41 
21 
•'< (6 
16 
? 
177.9 
45.1 
21.6 
25.4 
12.1 
192 
51 
23 
160 
39 
20 
Nantucket, “ 
34 
cf 
170.3 
43.9 
21.9 
25.7 
12.9 
191 
51 
23.4 
156 
38 
20.4 
<< it 
29 
cf 
163.3 
41.7 
21.5 
25.5 
13.1 
181 
49 
22.6 
135 
33 
20.4 
Microtus breweri (Baird). 
South Pt. Island, Mass. 5 
cf 
200.8 
49.8 
23.7 
24.8 
11.8 
211 
58 
25 
191 
45 
22.4 
“ “ “ 8 
$ 
186 
47.9 
23.1 
25.6 
12.4 
203 
56 
24 
176 
41 
99 
The skull of Microtus breweri is readily distinguished from that 
of M. pennsylvanicus by its large size and peculiar brain case and 
interparietal. The differences are most accentuated in the older 
mice, the skulls of young individuals being often unidentifiable. 
The specimens figured are all old and of approximately the same 
age. They show the differences in size and form which are charac- 
