144 THE CHICAGO ACADEMY OF SCIENCES. 
The genitalia of the American stagnalis seem to be almost identical 
with those of the European form as figured by Baudelot, Prasch, Cooke 
and Keferstein. The figure by Baudelot (pl. 4, fig. 1) approaches 
nearest to our race. Cooke! figures the penis-sac as enormously en- 
larged and of a totally different shape from anything found in the 
American form. No variation in the shape of this organ has been 
noted, in the American species, in the different seasons. There would 
seem to be no stable characters in the genitalia by which to separate 
the American from the European varieties of stagnalis. The charac- 
teristic features of the genitalia of stagnalis are the peculiar bulb- 
shaped form of the prostate, the small size of the penis as compared 
with the penis-sac, the great length of the vas deferens, and the position 
and insertion of the retractor muscles of the male organ. 
RANGE (Figure 9): North America from about the 37th (Colo- 
rado) and 41st (Illinois, Ohio’) parallels of north latitude to the Arctic 
Ocean. A glance at the map shows that stagnalis appressa is an in- 
habitant principally of the lake basins extending in a northwesterly 
direction from the great lakes to the Yukon River; it also inhabits a 
second large territory from southern Utah and Colorado northward 
between the Rocky Mountains and the Cascade Mountains, and the 
Sierra Nevadas. These two areas cover the regions of the great lakes 
left by the retreat of the ice sheet and also the Quaternary lake basins 
west of the Rocky Mountains. The absence of stagnalis from the 
waters of the great plains of the Dakotas and Nebraska is noteworthy, 
and indicates that the species is primarily a great lake form. 
Comparing the distribution map with the regional map (figure 1), 
we find that stagnalis occupies the Canadian, Hudsonian, Mackenzian, 
Yukonian, Alaskan, Columbian and a part of the Californian, Colo- 
radoan, Upper Mississippian and the Great Basin regions. It is 
absent from the Labradorian, the eastern part of the Hudsonian and 
the Nova Scotian regions. It is also absent, apparently, from the terri- 
tory west of the Canadian Rocky Mountain chain. Its extension in 
the Mississippian region is confined to the upper part. This species 
is one which prefers cold or temperate climates. Its northwesterly 
extension is strongly suggestive of its Asiatic origin, a fact further 
emphasized by its apparent absence from the northeastern part of 
North America. 
Compared with Merriam’s zone map, stagnalis is found to occupy 
Jop. cit., p. 144, fig. 55. 
1Anthony records stagnalis appressa from Cincinnati, but this was prob- 
ably an error, as it has not been substantiated by specimens, 
