60 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
the maximum at 3.75. Even with this liberal cutting-off of extremes, we 
find Hesperomys leucopus to range from 2.75 to 3.75 in the same locality, and 
establish a variability of a full inch—that is, over twenty-five per cent. of 
the mean length. 
In length of tail-vertebree, one specimen stands 2.10; but this may be 
excluded, and 2.40, of which there are several instances, be accepted as a 
normal adult minimum. The figure 3.40 is probably the normal adult maxi- 
mum. When we take in the pencil of hairs at the tip, we should widen the 
limits a trifle, since this last is a very variable feature. The whole tail, there- 
fore, varies in length at least one inch, and probably a little more, just as we 
should have anticipated from the nature of the case. We have already seen 
that the tail averages 0.25 of an inch shorter than the head and body—that 
is, it, just reaches to half-way between the eyes and the snout, the latter dis- 
tance being 0.50. Now, for its variation of redative length, we have :—In 
several specimens, the tail is a full inch (even after striking off a margin for 
possible error) shorter than the head and body ; in others, the tail is equal to 
or longer than the head and body—sometimes over a fourth of an inch longer. 
So that, as the head-of this species averages a little over an inch in length, it 
follows that the tail of dewcopus may barely exceed the body alone, or it may 
considerably exceed the head and body together. 
It gives us pleasure to find that our results agree very closely with those 
Mr. Allen reached in his valuable paper* on the Mammals of Massachusetts. 
The slight difference comes from the fact that to keep largely within bounds, 
and so to be unquestionably on the sate side, we lopped off a certain 
margin from our extremes, while Mr. Allen presented his. His paragraph is 
well worth quoting in this connection :— 
“The most variable character consists in the relative length and number 
of the caudal vertebrae. About one-fifth of the Massachusetts specimens have 
the tail-vertebree equal to or longer than the head and body together ; occa- 
sionally, a specimen is found in which the tail-vertebree alone exceed this 
length by one-fourth to one-half an inch. At least four-fifths, however, have 
the tail shorter than the head and body, and occasionally one occurs with the 
tail only equal to the body alone. In these latter, the proportional length of 
the tail-vertebree to the length of the head and body is as 68 to 100; in 
the other extreme, or in those with long tails, as 118 to 100. The variation 
“ Bull. Mus. Comp. Zool. i, 227. 
