MURIDiK- AIlVlCOLINiE -AKVICOLA BIPARIUS. 



175 



from Massachusetts, is 4.00 X 1 -56X0.^0. " Occidentalis " alone lins pro- 

 portionnlly longer tail and feet than any other of the whole series, but it 

 grades 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 Duird's measure- 

 ments, the tail of "occidentalis" is 2.00, and of californkm LfiO; but there 

 appears to he some mistake here, and at any rate the specimens now in our 

 hands show no such discrepancy, one being about 1|, the other 1^. They 

 are both prepared in the same style, and apparently by the same person. 



The above will be sufllicient, we presume, to establish what wc claim : 

 that in points of size and shape there is no difference whatever between the 

 eastern series and nearly all the western scries, and that there is no difference 

 in these respects of specific import, or even enough to constitute a geogroph- 

 ical variety in any of the Wo spejimens; and that, therefore, if more than 

 one species is to be r!st4ibli8hed, it must be upon something else than size or 

 shape. 



To this we should add a word concerning Arvicola " modesti> ". 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, &c. It 

 corresponds in tliese and all other respects with several specimens of very 

 young eastern and western Arvicola 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 det'-i'^d measure- 

 ments from alcoholic specimens, dismissing it with the rerna ...at it amply 

 confirms what has been alreaily deduced. 







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