MICHIGAN ACADEMY OF SCIENCE. 33 



vented rapid absorption and evaporation of the water, so that finally the 

 pervious, sandy soil sucked up the rain almost as fast as it fell. A similar 

 destruction was more apparent in Dickinson County, Michigan, on similar 

 soil (7); hollows, which contained no water in the summer after the dis- 

 astrous fires of 1908, were still choked with the scorched trunks of the dead 

 tamaracks, etc., and still contained large numbers of dead and partially 

 burnt, aquatic shells. 



Primary species. 



Aplexa hypnonwi (L). Quite abundant. 



Succinea retusa Lea. Quite abundant; semi-amphibious. 



Lymnaea palustris (Mueller). Common; local. 



Secondary species. 



Sphaerium occidentale Prime. Very common. 

 Zonitoides nitida (Mueller). Frequent; semi-amphibious. 

 Planorbis umbilicatellus Cockerell. Infrequent. 



b. Discussion of Swamp Forms. 



In Chart IV, the temporary swamps of the same three regions in northern 

 Michigan have been listed, and their molluscan faunas tabulated, after the 

 manner of those of the permanent ponds. The first two pools were in Huron 

 County, the second two in Dickinson County, the fifth in the present region. 

 The first were temporary swamps formed between the sand dunes of Sand 

 Point, the second the richer, forest swamps of the flat, clay-soiled regions 

 around Rush Lake (5). The third was a burnt-over tamarack swamp, 

 which differed considerably from the other swamps, and was listed, in this 

 connection, to call further attention to the destruction of temporary swamps 

 by fire, a matter which has already been discussed. The characteristic shell 

 of such tamarack swamps appeared to be Pisidium roperi. The fourth 

 series of swamps were those formed on the mud flats of AlcKinnon Brook, 

 a small stream, which flowed out of the clay moraines thru the sand}', out- 

 wash regions. 



It will be seen from this chart that Aplexa hypnorum was the most char- 

 acteristic X)f the large molluscs, as Sphaerium occidentale was of the smaller. 

 Two medium-sized species of Lymnaea also appeared to be connected with 

 this habitat: Lymnaea palustris in Huron County and around Douglas 

 Lake, and Lymnaea kirtlandiana, var., in Dickinson County. In addition, 

 small planorbices seemed to be present quite universally, while Musculium 

 truncatum and, to a lesser degree, Musculium partumeium were important 

 swamp forms in Huron County. (Compare 3, Station XXI). 



The temporary swamps around Douglas Lake seemed to be quite typical 

 in regard to their molluscan fauna ; two of the most typical forms were quite 

 numerous, while a third appeared to be common locally, and the small 

 planorbices were represented by Planorbis umhilicatellus. They were quite 

 barren, however, and only contained these widely distributed forms, while 

 the other swamps listed contained some accessory species; this was most 

 noticeable in the rich, hardwood swamps around Rush Lake. 



