172 BULLETIN OF THE 



differs, then, from P. Richardsonii only in its slightly larger size, the pro- 

 portion of length of tail to length of body being essentially the same in both. 

 Some smaller specimens are referred to this from the Upper Missouri, 

 of which measurements are not given. Two of the large specimens are 

 marked males ; the sex of the other is not indicated. To this species 

 is of course referred the long-tailed Carleton House variety mentioned 

 by Richardson, to which, as already observed, Bonaparte gave the name 

 longicauda. 



From the preceding comparisons and remarks the inconstancy and the 

 arbitrary character of the distinctions claimed as specific are fully evi- 

 dent. It appears that short tails by no means always accompany small 

 size, nor long tails large size ; that both occur at the same localities, as well 

 as at points as remote from each other as the most distant localities at 

 which the species has been found, as Hudson's Bay Territory and the 

 Arctic Regions on the one hand and Massachusetts, Pennsylvania, and 

 Illinois on the other ; that between the " species," as characterized by 

 Professor Baird, there is an almost insensible intergradation in all the 

 essential characters, some of the so-called species resting on distinctions 

 that are by no means differences (as P. Richardsonii and P. longicauda ; 

 P. Cicognanii and P. noveboracensis, very nearly) ; finally, that, contrary 

 to the belief of this author, the short-tailed species (P. Cicognanii and P. 

 noveboracensis) have a range to the northwards equal to that of the others, 

 the P. erminea of Richardson being distinctly referable in its proportions to 

 P. Cicognanii. 



Although differing radically with the eminent author of the Report on 

 the Mammals of North America in respect to the number of valid species 

 of this group in America,— the only American zoologist who has given it 

 special attention, — I can but commend the candor he has exhibited in his 

 attempt to clear up the discrepancies of former authors, and to sift the sub- 

 ject of its obscurities, as well as the manner in which he has presented 



his material. 



An examination of numerous specimens from the New England and 

 other Northern States has shown me that the variations in the relative 

 length of the tail to the body are merely analogous to similar individual 

 variations in the squirrels and other small mammals that have this part 

 considerably developed,— a variation not always due merely to the length- 

 ening or shortening of the vertebral segments, but occasionally to an in- 

 creased or diminished number of the vertebra? themselves. Also, that the 

 variation in size so noticeable in specimens from the same locality is in 

 great part sexual, — the males in nearly all species of MusteKdce being 

 considerably larger than the females, — but in many cases to immatu- 

 rity, and somewhat also to the natural individual range in this respect, 



