3C3 



single snail species besides M. transvcrsum from the middle lake in 

 1920. Five of these species were the same as the five already listed 

 from the upper lake on a preceding page. This group, distinguished 

 from the other five by the fact of occurrence above the middle lake 

 in 1923, included one sphaeriid of the genus Musculium, one viviparid, 

 and three species of the sphaeriid genus Pisidium, sometimes called 

 pea-clams in the literature. The five new entrants in the 1923 li.sts, 

 embracing snails whose farthest northward occurrence in 1922 was 

 the middle lake, included three additional species of Pisidium not taken 

 in the upper lake in 1922 or in any part of either the upper, middle, 

 or lower lakes in 1920, and two species of Sphaeriuni, also not taken 

 in the upper lake in the summer of 1922, but one of them recorded 

 from the lower lake in 1930. 



The distinction between these two groups of the less tolerant snails 

 and their relation, in the matter of tolerance to pollution, to the other 

 recent snail groups and to the Viviparidae and Amnicolidae apparently 

 exterminated between 1915 and 1920 is shown in the table that follows. 



