MURID.E— ARVICOLIN^E— ARVICOLA RIPARWS. 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 Pitymys, 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 1.10X0.64, while the 

 eastern ones are 1.06X0-65; there is every probability that this dispropor- 

 tion would disappear with larger series. It is only to be noted further that 

 No. 4156 his (type of occidenUdis) is among the largest skulls of the series; 

 tending to show that the present dimensions of the skin (3£ 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. 415t>. 

 No. rflf is the type of A. "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 1.08X0.61, 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. Hff, from New York, gives precisely these dimensions, 1.08X0-61; 

 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 



