MURIDJ3— ARVI0OLIN.E— ARVICOLA RIPARIUS. 



163 



the same relatively: — The average head and body is to the average tail 

 :: 4.35 : 1.50 :: 1 : 0.34; that is, the tail is almost exactly one-third as long 

 as the head and body. The average fore foot is to the average hind foot 

 : : 37 : 80 ; that is, it is a little less than one-half as long. It should be 

 observed, likewise, that the variation in the proportional lengths of parts is 

 considerable, i. e., the longest trunk has not always the longest tail and feet, 

 nor is a long hind foot always accompanied by an equally lengthened fore 

 foot. These calculations, moreover, it should be remembered, really exhibit 

 a minimum of purely individual variation; for the nineteen specimens occurred 

 under the most similar conditions possible. 



We shall next introduce a table of sixteen specimens, all from the same 

 locality, but taken at different seasons, in different stages of growth, and 

 prepared by different persons, for which reasons the series may be held to 

 fairly express an average of -purely individual variability. In color, they are 

 all so much alike, that no tangible difference can be noted, some being merely 

 a shade browner or grayer than others. On an average, they are a trifle 

 browner or brighter than the Philadelphia series. 



Table XL. — Measurements of sixlmi >y» cimer.s cf A. ripaiuus collected hy J. IV. J'. Jt »/,«, at Miidlcboro', Afass. 



From this table it appears that the head, from point of the nose to 

 the occiput, averages just one and a third inches in length, which is slightly 

 less than one-third of the head and body together, and a little less than the 

 average length of tail; its variation is 0.15, or a little over ten per cent, of 



