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 (pi. 4, fig. 1) approaches 

 nearest to our race. Cooke 1 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 3?th (Colo- 

 rado) and 41st (Illinois, Ohio 1 ) 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 



iop. cit., p. 144, fig. 55. 



Anthony records stagnalis appressa from Cincinnati, but this was prob- 

 ably an error, as it has not been substantiated by specimens. 



