MOLLUSCA. 137 



the shore, but the definite mucky zone of the preceding habitat is not 

 present. 



In this locaHty, there were found considerable numbers of Sphaerium 

 sulcatum, and a ventricose form of Amnicola lustrica; small numbers of 

 Valvata tricarinata unicarinata, Planorbis campanulatus minor, and P. 

 -parvus, and single specimens of Pisidium splendidulum, Lymnaea obrus- 

 sa, Amnicola ivalkeri, and of some species of Musculium (too young to 

 indentify). 



17. County drain. This ditch empties into the south-east corner of 

 Rush Lake. It has a clay bottom and is almost choked with Spirogyra, 

 except at the mouth, where the bottom is mucky and covered with a 

 growth of Potamogetons, Ceratophyllum, etc. There is no current in 

 this drain except after heavy rains; but it does not dry up as the heavy, 

 clayey soil is almost impenetrable. Near the mouth of this ditch, Lym- 

 naea stagnalis appressa was quite abundant. In the upper portions, L. 

 polustris, Planorbis trivolvis, and Physa gyrina were present, the first 

 in much greater abundance than the others. 



Habitats of the Swamps. 18. — Annual swamps. In every part of the 

 region, where work was carried on, there was one habitat that was al- 

 ways present and which always showed similar conditions and faunas. 

 This was the temporary swamp. The swamp in the sand dunes, altho 

 it may only be a small hollow protected by a few dogwoods, differs from 

 the larger hollows in the alluvial woods mainly in the number rather 

 than in the kinds of shells. In all cases, the substratum is made up of a 

 thick layer of decaying leaves, etc., the water is shallow when present, 

 and there are extreme seasonal variations between dessication and a 

 flooded condition. The similarity of these conditions apparently domi- 

 nate over the effects of differences in the sub-soils. The formation and 

 surroundings of these swamps vary greatly. In the sand dunes, any 

 hollow capable of holding water in the spring becomes choked with dog- 

 woods (Cornus stolonifera and circinata), whose leaves and roots prevent 

 too rapid seepage and evaporation, and an ideal habitat of this kind is 

 formed. In the long hollows between the ridges, as in the beds of old 

 lakes and swamps, there is a thicker deposit of leaf mould, alders (Abms 

 incana) and perhaps a few ashes and swamp maples {Acer rubrum) help- 

 ing in its production, and the seepage is slower ; here we find the num- 

 ber of shells increasing but the species remaining about the same. 

 There are also the swamps in the low woods on Stony and North Islands, 

 where there is a thin layer of alluvial soil over limestone, and those of 

 similar localities around Rush Lake and Caseville, where the underlying 

 soil is clay. In both of these the trees shading the swamps are about 

 the same; they consist of swamp maples and sugar maples {Acer rubrum 

 and saccharum) and the elm {Ulmus americanus). 



