144 MOLLUSCA. 



favorable land habitat for molluscan life in this region, only excepting 

 the exposed sand beach where no mollusc can live. They are very dry 

 and have little humus, and that little comes mainly from coniferous 

 trees, the principal trees of .this region being white, Norway, and jack 

 pines (Pinus strohus, P. resinosa, and P. divaricata), with lesser numbers 

 of poplars (Populus treniuloides. P. grandidentata. and P. halsamifera), 

 and a scattering growth of red. white, pin, and scarlet oaks (Quercus 

 rubra, Q. alba, Q. palustris, and Q. coccinea). The fallen leaves of these 

 deciduous trees might harbor more snails, despite the dominance of the 

 conifers, except that the mycelia of a dry mould attack all of the leaves 

 except those directly exposed; and this, together with the exceeding 

 dryness, effectually prevents all molluscan life except under fallen logs. 

 Here a few dwarfed shells are to be found, which are more numerous, 

 contrary to what might be expected, in the pine woods on the outer- 

 most dunes, perhaps on account of the fact that the mould does not ap- 

 pear to be able to obtain as strong a footing among the pine needles. 

 The sand roads on these ridges form effectual traps for molluscs, and 

 several specimens were found dead and dried up, which had apparently 

 perished in attempting to cross them. (Pis. I, II a, III b, IV b.) 



Polygyra albolabris maratima was by far the most abundant shell on 

 these dunes, both on Sand Point and north of Rush Lake. Polygyra 

 fraterna was also present, but in lesser numbers, in the latter locality, 

 while a single specimen of typical P. albolabris was obtained from under 

 a log in the humus near a cabin on Sand Point. 



29. Inner sand dimes. The inner and more permanent sand dunes 

 have a thicker deposit of humus than the outer shifting ones, so that 

 they are not as dry and barren as the latter. The deciduous trees pre- 

 dominate here, especially the oaks, the same species being present as 

 were found on the outer dunes. Also there is a thicker undergrowth 

 of brake {Pteris aquilina) and a considerable layer of leaf-mould. The 

 ground, however, is dry enough so that most of the snails and slugs are 

 congregated under the decaying logs during the summer, altho they may 

 wander out among the leaves at night and in wetter seasons. They are, 

 nevertheless, much more numerous both in species and individuals than 

 on the sand dunes nearer the Bay. 



This habitat was best represented just north of Rush, Lake. Here 

 were found a number of specimens of Polygyra albolabris maratima, P. 

 fraterna, Pyramidida aUernata, Pallifera dorsalis, and Zonitoides ar- 

 borea. A few albino specimens of the last species were also obtained. 



Habitats of North and Stony Islands. — North and Stony Islands, as 

 has been mentioned under the rocky, littoral habitats, are formed of 

 limestone and covered with a relatively thin deposit of alluvial soil, ex- 

 cept along the north shores where the rock outcrops or is covered with 



