142 MOLLUSCA. 



caying sedges, and under driftwood washed up with them, there are to 

 be found considerable numbers of land shells, especially the smaller 

 ones; these places comprise one of the most favorable land habitats for 

 shells in the sand dunes. 



As these habitats are separated from all humus deposits by consider- 

 able stretches of sandy beach, and the humus deposits near these shores 

 are themselves devoid of a large proportion of the shells found along the 

 beaches, it seems probable that a large proportion of these localities 

 are populated from driftwood, etc., .washed upon the beaches from other 

 more favorable localities, such as the clay region inland and the alluvial 

 deposits on the rocky islands. In most of the places studied, this would 

 mean a journey by w^ater of not much over a mile, and land shells will 

 stand an immersion of several hours. All of these places must neces- 

 sarily be repopulated nearly every spring as the ice and the fall storms 

 must usually sweep these beaches clean. 



In these places, Succinea retusa, S. avara, Pymmidtila cronkhitei 

 anthonyi, Agriolimax campestris, Zonitoides arhorea, Vitrea hammonis, 

 and Carychium exigimm were found in great abundance. Vertigo 

 ovata, Bifidai'ia tapjyaniana, Carychium exile, Zonitoides minuscula. Z. 

 nitida, Vitrea hinneyana, Pblygyra cdholabris, P. a. maratima, P. mono- 

 don, and Cochlicopa hthrica were also obtained, but in lesser numbers. 



These habitats are very short-lived. In addition to the fauna being 

 swept away nearly every winter by the ice and the fall storms, slower 

 changes are constantly affecting all of these shores, as has been discussed 

 under the habitats of Saginaw Bay, so that these conditions are either 

 moving out towards the Bay or in towards land, depending on their 

 position. As they are usually in more or less protected places, the 

 movement is probably outwards, as in such localities there is usually 

 deposition. 



26. Borders of lakes and sivamps. The sand dunes are usually ar- 

 ranged in ridges more or less parallel to the shore. Between these 

 ridges, swamps, marshes, small ponds and even lakes may be formed. 

 Here the deciduous trees, such as the maples (Acer saccharinum and A. 

 ruhrum), the red and white oaks (Quercus rubra and Q. cdba), the ashes, 

 the dogwoods (Cornus stolonifera and C. circinata), and the alders (Alnus 

 incana) predominate, the coniferous trees being in the minority, but 

 represented by the white pine (Pinus strohus), the arbor vitae (Thuja 

 occidentalis) , and the hemlock (Tsuga canadensis), \^dth an occasional 

 clump of tamaracks (Larix laricina), balsam firs (Abies balsamea). or 

 Norway pines (Pinus resinosa). This vegetation of course causes the 

 formation of considerable leaf mould and humus, and a rich soil has also 

 been formed where sedges, etc., have partly or entirely filled in a marsh, 

 pond or lake. Among the fallen logs and decaying leaves in these 



