130 MOLLUSCA. 



larger ones may form the shallow, sand dune lakes and swamps, as 

 will be described under those habitats. 



7. Shalloiv littoral, sandy, pi^otected. Along protected portions of 

 sandy shoals, Scijyus americaniis has formed marshes of considerable 

 extent. These marshes are protected from the direct force of the waves 

 and currents in various ways. Off some, a point or sandbar has been 

 formed as described under habitat six. These marshes, when small, 

 are often the first stage in the production of the sand pools (or when 

 larger, of the sand dune lakes to be described under habitat nine). 

 Also, the whole south shore of Sand Point and of North Island, and the 

 sandy stretch on Stom^ Island which is protected by the latter, all have 

 a belt of Scirpus americonus along the water's edge and form extensive 

 marshes. Those on Sand Point, east of "The Bayou", and those on 

 Stony Island are, however, rather unfavorable habitats for molluscan 

 life as they are not well enough protected, the former being unprotected 

 from the south (Wild Fowl Bay being too wide for the opposite shore 

 to afford such protection) and the latter from the northwest. Those 

 on North Island are, on the other hand, ver}^ favorable, as the water 

 deepens slowly off shore and the entire south side forms a shallow bay; 

 here the pickerel weed is intermingled with the common sedge, Scirpus 

 americaniis. In all of these localities, the molluscs congregate in great- 

 est numbers where the water is from six to twelve inches deep, and where 

 there is a belt of dead and decaying sedges floating along the shore. 



Better protected marshes, as those along the east end of Wild Fowl 

 Bay, and the similar one in the marshy cut-off between the lower part 

 of the Pigeon River and Saginaw Bay, have a marly bottom and a 

 sparse growth of Charo. These are especially well protected by their 

 sheltered position to the east of large points, by the shallowness of the 

 water and by the outlying barrier beaches characteristic of such posi- 

 tions. In all of these places the molluscs are to be found in large num- 

 bers all over the bottom, especially in ver}" shallow places, where the 

 spires of such large shells as Lymnaea stagnalis appressa often project 

 above the surface of the water. In these places, the water, being so 

 shallow, is quite w^arm. 



Lymnaea reflexa was the most abundant shell in these localities. It 

 varied considerably; the variety imlkeri and many other variations 

 were found.* Lymnaea ohrussa, L. stagnalis appressa, and L. hwnilis, 

 Planorhis deflectus^ P. trivolvis, and P. exacuoiis, Physa heterostrapha and 

 P. gyrina were also obtained, but in lesser numbers, together with a 

 few juvenile specimens of Planorhis truncatus, and the lake forms of 

 Strophitus edentulus, Lampsilis luteola, and L. ventricosa. In addition, 



*Baker, H. Burrington. Variations in Lvmnaea reflexa Say, from Huron County. 12tLi 

 Ann. Rept. Mich. Acad. Sci., 1910, pp. 60-63. 



