508 



MONOGRAPHS or NORTH AMERICAN ltODKNTIA. 



* Measurements of length and of tail-verteb'ffi from Wood house. In its present state, the tail, as well as can be jndged, 



is about 4.50 long, exclusive of the terminal pencil, which is half an inch more. The dry measurements are not reliable 



Taking the last seven sets of measurements of the foregoing table (all 

 made from sexed specimens in the flesh, and taken by the same person,* so 

 that a source of error which lies in different methods of measurement is 

 avoided), we observe that the total length varies from 2.90 to 3.60 — a differ- 

 ence of 0.70 — a fair rate of variability for a species. The tail, as usual, varies 

 more than the body, or, in this instance, about an inch, whether the vertebrae 

 alone are considered, or the terminal pencil be taken into account. The 

 relatively shortest tail (vertebrae alone) is half an inch longer than the head 

 and body, or, with the hairs, an inch longer. The relatively longest tail 

 (vertebrae 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 9A per cent, of their average length. 

 Other smaller measurements give corresponding results The rates of varia- 

 tion which are determined for the present species are doubtless equally appli- 

 cable 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 remarkably restricted geo- 

 graphical distribution. So far as I am aware, there arc no specimens extant 

 from beyond the limits of the Colorado Valley in that portion of its extent 



"In measuring tails, I take as nearly as possible the true length, by placing one leg of the com- 

 i..Lsses snugly above the autis. The length to which the tail projects beyond the body-hairs, or its osten- 

 sible dimension, is considerably less. 



