MURIDA—ARVICOLINA—ARVICOLA RIPARIUS. 161 
but which, especially in connection with the characters of the other teeth, 
proved decisive. The second noticeable modification of the crescent affects 
the posterior part of its convexity, which may develop a little lobe or spur 
simulating a third external triangle; then, as a necessary result, the curve is 
sinuate instead of regularly convex, having a little concavity in one part. 
The third principal modification is in respect of the posterior horn of the 
crescent. This is generally a simple lobe finishing the tooth behind; but it 
may, as one of endless slight changes, produce a sort of curved spur or hook. 
When this fold is pretty long, narrow, and curyed, it looks like a little addi- 
tional crescent lying in the belly of the main one. It is hardly necessary to 
add that none of these modifications have the slightest significance beyond 
indices of individual variability; and we should not have entered into the 
tedious minutiz except to bring this fact prominently forward, and to char- 
acterize the pattern of this important tooth in all its phases. 
We shall open the discussion of A. riparius with the examination of 
nineteen typical specimens from the locality of the original réparius Ord. 
These were all collected at the same time and place, at a season of the year 
when they are necessarily full-grown, and were all prepared by the same 
person, in the same style of skin. Whatever differences, therefore, may be 
found are to be attributed to purely individual variation, of which we thus 
obtain a perfectly reliable equation, free from any disturbing functions, such 
as age, season, climate, locality, &c. Whatever variation in size and propor- 
tion may be found becomes a still purer index, if possible, from the fact that 
nof one of these specimens is distinguishable from any one of the other 
eighteen in’ color; for, as far as color is concerned, they all might have 
belonged to the same litter. In respect of the measurements, we should 
premise that, as great care was taken in preparing the skins, probably no one 
of them differs in total length 0 25 from the original dimension in life, and 
that, if anything, the lengths as given are a trifle under life-size, from shrink- 
age in drying; and, secondly, that the loss in drying of the feet and tail may 
be estimated at about 5 per cent. 
11M 
