292 PROCEEDINGS OP THE ACADEMY OF 



methods of measurement is avoided) we observe that the total 

 length varies from 2.90 to 3.60 a difference of 0.70 a fair rate of 

 variability for a species. The tail, as usual, varies more than the 

 bodv, or, in this instance, about an inch, whether the vertebrae alone 

 being considered, or the terminal pencil be taken into account. 

 The relatively shortest tail (vertebras alone), is half an inch 

 longer than the head and body, or, with the hairs, an inch longer. 

 The relatively longest tail (vertebras alone), is an inch and a 

 quarter longer than the head and body ; with the hairs, nearly 

 two inches longer. Much as usual, the rate of relative variability 

 is greater than that of absolute difference in dimension. The 

 soles vary a tenth of an inch in length, or about 9^ per cent, of 

 their average length. Other smaller measurements give corre- 

 sponding results. The rates of variation, which are determined 

 for the present species, are doubtless equally applicable to others 

 of the genus, of which I have no alcoholic specimens. 



Upon the supposition that the other allied species described in 

 this paper are really distinct from P. penicillatus, this has a re- 

 markably restricted geographical distribution. So far as I am 

 aware, there are no specimens extant from beyond the limits of 

 the Colorado Valle}', in that portion of its extent which embraces 

 Arizona and the contiguous part of California on the opposite 

 side of the river. Specimens are rare ; there may be none whatever 

 in Europe. Woodhouse, LeConte, Baird, and myself, are, per- 

 haps, the only naturalists who speak of the species from autop- 

 tical examination. 



The relationships of some allied species being considered under 

 other heads, the only point now arising for discussion relates to 

 the " Cricetodipus parvus" of LeConte, 1. c. The specimen upon 

 which the writer based his remarks is now before me. It is not 

 adult, as supposed, but very young and ungrown, as shown by the 

 unworn state of the teeth ; although the tail is not crested, there 

 arc indications that it would have become so ; the relative propor- 

 tions and coloration are exactly as in P. jyenicillatus, to which 

 I have little hesitation in referring it. In any event it is a true 

 PeroriDathus, arid not Cricetodijms at all, as shown beyond ques- 

 tion by the obviously naked soles and distinctly recognizable 

 lobe of the antitragus. 



