MURIDH—ARVICOLINA—ARVICOLA RIPARIUS. 179 
more than half as long again as the upper, averaging about 0.33 (from 0.27 
to 0.38). An interesting point is seen in the eighth and ninth columns: the 
distance from the tip of the lower incisors to the apex of the descending 
process of the jaw is invariably a little greater than that from the same point 
to the back of the condyle. This probably characterizes this section of the 
genus; for in Petymys, at least, and perhaps some other sections, these two 
measurements are equal. The number of western skulls (eight) is too few 
to institute any very critical comparison; they average 1.10 0.64, while the 
eastern ones are 1.06X 0.65; there is every probability that this dispropor- 
tion would disappear with larger series. It is only to be noted further that 
No. 4156 bis (type of occidentalis) is among the largest skulls of the series ; 
tending to show that the present dimensions of the skin (34 inches) are below 
the truth, and, therefore, that the disproportion we found in the length of its 
tail and feet is really not so great as appears from the dried skin. Its dimen- 
sions are in no wise appreciably different from those of “californicus”, No. 4156. 
No. 7363 is the type of 4. “longirostris”, and must be examined in this 
connection ; for, as we have shown, there is nothing in the dimensions, nor, 
as we shall show beyond, is there anything in the coloration that cannot be 
exactly matched in almost any half-dozen samples of riparius. The diagnosis 
is:—“‘Skull 1.08 0.61, or as 1.00:55. Muzzle of skull very long. Distance 
between upper molars and incisors more than one-third the whole length of 
the skull.” But this length, 1.08, is precisely what we found as the average 
of the whole series. The width, 0.61, is within 0.03 of the average. Skull 
No. 7217, from New York, gives precisely these dimensions, 1.08 0.61; 
several others approach it within one or two hundredths. I can appreciate 
but one single discrepancy between this skull and an average one The 
incisors are a little tilted forward, so that their tips fall in advance of a per- 
pendicular from the tips of the nasals, instead of about under the ends of the 
nasals, as usual. This, with the corresponding change in the alveolus, makes 
the distance from the incisors to the molars just appreciably greater than 
usual; thus it is 0.37 in ‘‘longirostris”, and only 0.34 in the New York skull 
just cited. But the molars are not “thrown further back than in other 
species”; for, on taking their bearings from any other points of the skull than 
the incisors, they will be found in the usual position. Examination of a number 
of skulls with reference to the ‘‘set” of the incisors shows that this is very 
variable; their tips may fall behind, below, or in advance of the nasals, and 
