MURIDA®—ARVICOLINA —ARVICOLA RIPARIUS. 175 
from Massachusetts, is 4.00 & 1.56 0.80. Occidentalis” alone has pro- 
portionally longer tail and feet than any other of the whole series, but it 
grales through “californicus” to the rest. The difference in the length of 
the foot between “occidentalis” and * californicus” is only 0.07, which of 
course furnishes no character of the slightest import, as we have seen a vari- 
ation of more than twice as much in eastern specimens—enough to more 
than counterbalance the greater relative discrepancy, which is owing to 
“occidentalis” being a smaller individual. According to Baird’s measure- 
ments, the tail of “occidentalis” 
is 2.00, and of californicus 1.50; but there 
appears to be some mistake here, and at any rate the specimens now in our 
hands show no such discrepancy, one being about 13, the other 17. They 
are both prepared in the same style, and apparently by the same person. 
The above will be sufficient, we presume, to establish what we claim: 
that in points of size and shape there is no difference whatever between the 
eastern series and nearly all the western series, and that there is no difference 
in these respects of specific import, or even enough to constitute a geograph- 
ical variety in any of the 145 specimens; and that, therefore, if more than 
one species is to be established, it must be upon something else than size or 
shape. 
To this we should add a word concerning Arvicola “modesta”. We regret 
that we cannot agree with the proposer of this species, that although it is “not 
quite grown, the skull shows that it would not attain to much greater size”, 
To our view, this skull is that of a very young animal, as shown by its size, 
smoothness, fragility, absence of muscular impressions, loose sutures, &e. It 
corresponds in these and all other respects with several specimens of very 
young eastern and western Arvicole measuring two or three inches long; 
while the external dimensions are the same as those of the several young 
ones with which it is associated in the last series given. 
We supplement the foregoing table with one of more detailed measure- 
ments from alcoholic specimens, dismissing it with the remark that it amply 
confirms what has been already deduced. 
