38 ' JEAN DAWSON 



tage over snails that do not form a mucous epiphragm. The 

 sealing in clay is the result of chance; and depends upon the 

 presence of clay and upon the snail assuming the proper posi- 

 tion for sealing. In the presence of clay, Lymnaea will be one 

 of the last gasteropods to inhabit a pond subject to severe 

 drought, but if peat or sand is present Sphaerium stand a better 

 chance because it burrows into the soil. 



Much growing vegetation, as Hypnum, is very disastrous, both 

 because it prevents the snail from reaching the substratum, and 

 because it decays and thus kills even the buried bivalves. Moss 

 and loose debris ordinarily furnish protection to Sphaerium. 

 Too much erosion of clay is perhaps not conducive to the endur- 

 ance of desiccation because it may bury the snails so deep they 

 cannot get out. Perhaps much of the richness of molluscan life 

 in many ponds that dry up is due to the advantages they offer 

 through having a clay substratum and a moderate amount of 

 vegetation. The majority of snails in the above studied swamp 

 are Lymnaea palustris and Sphaerium. Comparatively few 

 Physa were found. If a pond has no clay in its bed and is sub- 

 ject to drying, Physa is likely to die off with the pond weeds 

 because it but rarely forms a mucous epiphragm. In general 

 the survival of Physa in ponds that dry up is therefore depen- 

 dent upon the presence of a certain amount of clay. 



V. General Discussion and Summary. The physical conditions of 

 the river system are very different from those of the lake and 

 pond, yet the most favorable habitats for Physa have essen- 

 tially the same factors. The following examples serve to illus- 

 trate how these conditions are brought about in stream and 

 lake by widely different agencies. Snails are found protected 

 from the current behind the stones in a stream bed and they 

 also found shelter from the waves behind spits. The factors 

 making up these habitats are essentially the same yet there 

 are not many points of resemblance between a creek and lake 

 or between the modifying agents, stones and spit which are 

 effectual in producing like environments. 



In general, the occurrence of the snail Physa in bodies of 

 water about Ann Arbor, Mich., proved to be very general but 

 very discontinuous. Streams do not offer conditions favorable 

 for Physa. Springs and brooks, especially those not connected 

 with streams, are practically barren, which is due perhaps to 



