96 MARINE AND FISHERIES 



5 GEORGE v., A. 191 & 



5. At places there are open or somewhat exposed channels with accumulations 

 of clean sand, almost free from organic material. 



6. In addition to the wave-action on the main shore and on the shoals, there 

 is a constant flow of water in and out among the islands, giving conditions of 

 exposure, temperature and oxygenation which are in marked contrast with those 

 of the protected bays and especially with those of the inland ponds. 



In general the species of mollusca exhibit great flexibility in their environ- 

 mental relations. In many cases ecological selection is operative within broad 

 limits, while in a few the environmental type is more or less specific. The chief 

 factors in this selection appear to be (a) exposed or protected situation, (b) depth 

 of water, (c) degree of aeration, (d) character of the bottom, and (e) food conditions. 

 In the Genus Lymnaea, the long-spired forms occur in the stagnant bays, while 

 the short-spired ones inhabit rocky shores. The species of Planorhis also occur 

 in the swampy bays, though P. defiectus extends its range to the exposed rocky 

 islands and the pools occurring on them. Most of the species of Physa are quite 

 general in their distribution, but P. integer prefers the exposed shores. The 

 genera Ancylus, Amnicola and Campeloma and the families Unionidae and Sphae- 

 riidae occur in swampy bays and also, though much less abundantly, in inland 

 ponds, with an extension of this range in Amnicola, the Unionidae and sometimes 

 Campeloma to the sandy channels and of the Sphaeriidae to these channels and to 

 sand or gravel banks in the deeper waters. Goniobasis is found abundantly where 

 there are currents of clear, well-aerated water, in sand channels, on sandy beaches 

 and on the exposed rocky shores. The genus Valvata is a sand-loving one, although 

 of the two species, one, tricarinata is also found plentifully in weedy, muddy bays. 



The inland ponds are exposed to extremes of summer and winter temperature. 

 They are limited in the facilities they afford for migration to the deeper waters. 

 They are poorly provided with means of aeration and are often surfeited with 

 decaying vegetation and so afford an environment not highly favorable to mollus- 

 can life. Few forms occur and these not abundantly. Among them are Ancylus 

 parallelus, Campeloma decision, Amnicola limosa and the Sphaeriidae. 



The protected muddy bays where these conditions are reversed afford a rich 

 molluscan fauna with a wide range of species which includes the Unionidae, the 

 Sphaeriidae, the genera Valvata, Amnicola, Goniobasis, Planorbis, Ancylus, Physa 

 (with the exception of P. integer niagarensis) and the long-spired species of Lym- 

 naea {palustris, columella and haldemani). 



In the weedy sand-runs the same forms occur, with the exception of Planorbis 

 exacuous, P. dilatatus and the Lymnaea mentioned above. In clean sand channels, 

 free from weeds and exposed to currents, the Unionidae, the Sphaeriidae and 

 the genera Campeloma, Valvata and Goniobasis occur. 



The exposed rocky shores which seem to afford a scanty supply of food and an 

 abundance of well-aerated water, yield Lymnaea emarginata canadensis, L. decollata, 

 L. stagnalis sanctamariae, Planorbis defiectus and the various species of the Physa. 



In the shallow island ppols which are well-aerated and have a good supply 

 of food, but which, on the other hand are subjected in some cases to destruction 

 by drought and to severe winter conditions, Planorbis defiectus and Lymnaea 

 palustris abound. 



