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THE AMERICAN NATURALIST [Vol.L 



the boundary of the lighter ventral hairs between the front 

 and hind legs. This streak is very reddish in color and 

 adds to the lighter and warmer coloring of the dorsal sur- 

 face. Ventrally, the tips of the hairs are to all extents 

 and purposes unpigmented and this produces a white or 

 nearly white ventral surface. The proximal half of the 

 hair is dark pigmented, a distinction between this type 

 of white area and that found in spotted animals where the 

 hair is white throughout its length. This type of agouti 

 has been given the name of white-bellied agouti (gris a 

 ventre blanc; Cuenot). 



In crosses with gray-bellied agouti the white-bellied 

 form is epistatic. It can be obtained in a homozygous 

 condition and is, as stated by Cuenot and by Morgan, one 

 of a series of four allelomorphic forms of coat pigmen- 



It is the appearance of this epistatic " white-bellied" 

 agouti in two experiments involving gray-bellied agouti 

 and non-agouti mice that I wish to record. 



Experiment A 



In 1913, a cross was made between wild gray-bellied 

 agouti mice and a race of dilute brown mice which I had 

 started in the laboratory of the Bussey Institution. 



The agouti race used was directly descended from wild 

 animals without any out crossing with any domesticated 

 mice. It therefore represented a stock of wild mice 

 raised for several generations in captivity. 



From this stock of agoutis I have recorded 531 young. 

 All of them were gray-bellied agoutis 2 of the type that 

 one might catch wild in any house. A certain amount of 

 variation in intensity of pigmentation occurred among 

 these mice. This variation, however, was not different 

 so far as could be observed from that shown by a consider- 

 able number of specimens of this species from any single 

 locality, judging from skins in the museum of comparative 

 zoology of Harvard University. Among these skins, and 



2 The brown agoutis recorded later in this paper are of the dark or 



"gray- bellied" type. 



