24 SIXTEENTH REPORT. 



Pisidium sp. Several species of Pisidia were obtained in this habitat, 

 but the specimens and records were lost by fire. 



3. Shells of mucky shores. In the most protected places, the water plants, 

 such as white and yellow water-lilies, potamogetons, wild celery, etc., were 

 much more abundant on the shelf and at the edge of the step-oif . In con- 

 sequence, there was often a considerable deposit of fine muck, and the bot- 

 tom conditions of these places more closely approached the pools, than did 

 those of the other portions of the shelf. The step-off was much more gradual 

 in these places; often it was hardly noticeable. 



Clams. 



Anodonta grandis footiana (Lea). Frequent; depth distribution similar to 

 that in preceding habitat. 



Lampsilis luteola (Lam.). Infrequent; ditto preceding. 



Lampsilis nasuta (Say). Rare; ditto preceding species. 



Strophitus edentulus (Say). Very rare; one specimen obtained from about 

 35 cm. of water. 



Primary species. 



Campeloma decisum (Say). Very common; distribution similar to that in 

 preceding habitats. 



Planorhis hicarinatus, var. approaching portagensis Baker. Shelf form. 

 Common; similar in distribution to that in preceding habitat. 



Planorhis campanulatus smithii Baker. Quite common; distribution simi- 

 lar to that in preceding habitat. More numerous than the preceding species 

 in the most mucky places. 



Lymnaea stagnalis perampla Walker, young specimens. Frequent; mostly 

 in water about 55 cm. in depth, and in the most mucky places. 



Sphaerium sulcatum (Lam.). Infrequent; mostly in water about a meter 

 in depth. 



Secondary species. 



Planorhis hicarinatus, var., juvenile. Common; similar in distribution to 

 that in preceding habitat. 



Planorhis campanulatus smithii Baker, juvenile. Common; in similar 

 places to adults. 



Amnicola limosa (Say). Frequent; similar in distribution to those of 

 marly shores ; especially numerous on sticks and debris. 



Valvata tricarinata (Say), and var. confusa Walker. Quite infrequent; 

 similar in distribution to preceding species. 



Planorhis parvus Say. Infrequent; similar in distribution to preceding; 

 altho usually in shallower water along with the next form. 



Physa ancillaria parkeri (Currier), juvenile. Quite rare; similar in distri- 

 bution to those on marly shores. 



Planorhis deflectus Say. Very rare; similar in distribution to Planorhis 

 parvus. 



4. Deep littoral shells, which came up into shallow water to hreed. As has 

 already been mentioned in a former paper (G), there were six primarj^ forms 

 which lived in the deep littoral zones of Douglas Lake, but came up on the 

 shelf to breed. These apparently came up in a regular order during the 

 summer and at quite separate times, except in colder summers, like that of 

 1912, when they came up crowded together in the early part of September, 

 altho apparently in the same order as before. 



Planorhis hicarinatus portagensis Baker. Bred in great abundance, in 

 similar situations to habitat of shelf form; came up usually in midsummer. 



