MOLLUSCA. 127 



bare rock is ex]wsc(l. The farther north one goe.s, the hirjier are the 

 outcroppings, until Port Austin is reached, where the rock predomi- 

 nates. This is probably due to the fact that the main currents come 

 down along the shore from the north. The islands, however, being out 

 some distance from the protection of the shore, have as much rock ex- 

 posed as points far to the north of them. The rock forming the points 

 to the north is Marshall sandstone; that forming the outcrops on the 

 two islands studied, North and Stony Islands, is Maxville limestone. 

 (PI. XIV). 



This firm sul)stratum of rock gives a secure footing to ])lants. so that 

 there is often a narrow belt of Scirpus americanus growing in the crev- 

 ices along the edge of the water, even in places where the exposm'e to 

 the wave action must be very severe. The rocks under water are cover- 

 ed with a thin layer of gelatinous, diatomaceous material and marl, 

 while crevices, etc., support tufts of Cladophora and Chara. These 

 plants can, of course, obtain no such footing on the unprotected, sandy 

 stretches, as the sand there is constantly shifting. 



This habitat was preeminently one of univalve molluscs with large 

 feet, as these found here a firm substratum to which they could cling. 

 Physa ancillaria mognalacustris, Lymnaea emarginata ontarioensis, and 

 Goniohasis livescens were present in very large numbers all along the 

 rocky shores of North and Stony Islands ; Goniobasis livescens and Physa 

 ancillaria occurred on the rocks off Oak and Hat Point. In addition, 

 a few specimens of Physa heterostrapha were found off the north shore 

 of North Island, and a thin form of Physa ancillaria magnalacustris on 

 an artificial rock breakwater off Sand Point. 



3. Rock pools. On the northwest shore of Stony Island, the strata 

 of rock are slightly tilted up toward the lake so that the beach is formed 

 of successive terraced outcroppings. When these terraces outcrop 

 near the water's edge, shallow pools are formed behind them; some of 

 which are connected with the Bay all of the time while others are filled 

 only by the waves. The outcropping ledges of rock protect the pools 

 thus formed from the direct action of the waves, and along these ridges 

 there are often dwarfed willows and belts of Scirpus americanus which 

 also help to protect them. A rank growth of Cladophora is character- 

 istic of these spots, and the rocks are covered with a thin layer of gel- 

 atinous material, probably full of diatoms. These pools form a habitat 

 rich in food for molluscs, but with rather adverse physical conditions, 

 despite the protection already referred to. 



Go7iiobasis livescejis. Physa ancillaria magnalacustris, and Lymnaea 

 emarginata ontarioensis were found in these places in considerable 

 a])undance; specimens of Lymnaea humilis and a light-colored, little- 

 malleated form of L. palustris, were also collected, but in lesser numbers. 



