1G4 



MONOOltAPUS OF NORTH AMERICAN RODENTIA. 



tlie mean. The nvcrnge length of the nine specimens, as mensurcd fresh, is 

 4.30, and of tlie seven dried ones is 4.75; the result 4.52, ns the true Icngtit 

 of the aninial, is siigiitly in excess of thai deduced from the Philadclphiu 

 series, and precisely what we should expect from the increase in latitude. 

 But lu'tore considering the limits of variability in total length of this series, 

 we must allow f()r the j)robal)le changes produced in stuffing. Thus No. 

 1578, which was 4.10 htng when fresh, measures in its present state only 

 3.75, without obvious "bunching up" of the skin; while No. 105)8, with 

 apparently moderate overstuffing, reaches 5.75, and it doubtless was at least 

 5.00 in life. Making due allowance for these contingencies, it will be quite 

 safe to assnnic one inch in length of head and body as the fair average varia- 

 bility of nuiture individuals living in the same locality. It is beyond doui)t 

 tiiat this capacity for variation will be found exemplitlcd in every considerable 

 series of adult specimens from any locality. 



The tails of the above series average 1.C6 in length ; but this dimension, 

 as we have remarked, is certainly in appreciable t'xeess. We should not 

 place the true average above l.fiO at most, and this would correspond pre- 

 cisely with the slight increase in average size of the Massachusetts over the 

 Philadelphia series; the tail being, as before, one-third as long as the head 

 and l»o(ly. We have, however, a higher rate of variation in the length of the 

 tail, viz, from 1.37 to 1.80 at least, which is almost half an inch. Although 

 this may .wem excessive, and be thought in)t quite reliable, owing to what \>ie 

 said of the measurements as taken, yet we arc sure it is nothing unusual, 

 and, in fact, we d(!monstratc further on, from consideration of more material, 

 n greater variability of the tail than this. The pencil of hairs at the tip is n 

 viTy inconstant feature, depending upon the general condition of the animal 

 as to pelage, and |)crhaps upon season. It averages about a fourth of an 

 inch, and ordinarily ranges between O.IO and 0.30. The measurements of 

 the feet, in tlie above table, arc essentially the same as those of the Phila- 

 dolpiiia series, and need not detain us, though we may gather the fact that 

 increase- in total stature is noi always accompanied by corresponding increase 

 in these members, since No. 101)8, the largest of the specimens, has feet of 

 only average dimensions. 



As corroborative of the two foregoing tables, and ns introducing new 

 data ill dimensions, we shall next present n table of sixteen specimens from 

 sligiitly-separated localities (but of strictly the same geographical range), 

 and separated from the localities already noticed by a considerable interval. 



