MOLLUSCA. 145 



sand. Almost all of North Island and the northern portion of Stony 

 Island are quite heavily wooded, mostly with deciduous trees, so that 

 there is a good carpeting of humus. In fact, the whole appearance of 

 the islands suggests the deciduous woods of the southern part of Michi- 

 gan. 



There is a wide strip all the way around North Island and along the 

 northern and western shores of Stony Island that is high and dry. l)ut 

 a large part of the center of the former and the whole southern two- 

 thirds of the latter is lower, and is covered with pools during the rainy 

 season. The northern portion of this damp area on Stony Island is 

 wooded and is relatively dry during the summer, as is the whole of 

 North Island, but the southern part of the former forms a large swamp 

 wdiich is never dry and is without trees. This part is apparently de- 

 void of molluscan life except in open pools which have been included 

 in the treatment of perennial swamps. Probably this lack of life is due 

 partly to the stagnancy and the oily character of the water, and partly 

 to the fact that the marsh grasses and sedges grow so thickly as to ex- 

 clude the light. 



30. Swampy icoods. As mentioned under the general treatment ol 

 these islands, there are considerable areas near the center of both North 

 and Stony Islands which are damp and quite heavily wooded. The.se 

 woods are mostly made up of deciduous trees, which in these swampy 

 places are mostly small, such as ash, sugar maple and American elm 

 saplings, with a few larger trees among them. There are also a few 

 arbor vitae (Thuja occidentalis) scattered thru them. 



These damp, well-shaded localities, as might be expected, formed 

 excellent habitats for the larger shells, but, on the other hand, few of 

 the more minute species were found. I cannot explain this, but do not 

 think that I could have so entirely overlooked them if they had been, 

 present, as considerable time was spent in searching for them. Their 

 absence seems all the more strange because they were found in con- 

 siderable numbers among the drift on the sandy beaches of North Island- 

 In this habitat, Polygyra albolahris was the most abundant shell, 

 both on Stony and North Islands. On both islands, P. thyroides. P. 

 niultilineata, Pyramidula alternata and Agriolimax agrestis were also ol)- 

 tained quite abundantly. In addition, on Stony Island, a few speci- 

 mens of Polygyra monodon. and on North Island, a few individuals of 

 Pyramidida alternata alba, P. cronkhitei anfhonyi, Zonitoides arborea, 

 and Succiyiea avara major were found. 



31. High icoods. The vn.de strip of high woods around North Is- 

 land, and that along the north and west shores of Stony Island chffer 

 mainly from the preceding habitat in being much higher and drier, antl 

 in containing, on the average, larger and fewer trees. Here, in addition 



19 



