100 THE LIMNEiE. 



sides, meeting in the middle so as to conceal it ; but 

 in most genera the mantle is altogether within the 

 shell, its marginal portion exuding the calcareous matter 

 of which the latter consists. In many cases, the foot, 

 or ventral disc, often secretes a calcareous or horny 

 plate, forming an operculum ; this is exemplified in the 

 common periwinkle. 



The limnese move along like the mollusks lately 

 mentioned ; the contraction of the foot takes its hold 

 of a very slight stratum of the water, which they leave 

 above them. Their strength is not great, for the slight- 

 est wind is often sufficient to accumulate them, thus 

 floating, towards the opposite side from which it blows. 

 When danger comes, they withdraw all their parts into 

 the shell, become of a greater specific weight, and fall 

 to the bottom. To return to the surface they are 

 obliged to crawl along the bottom as far as the bank, 

 or to follow the stem of some aquatic plant. The 

 limneee are only to be found in the water, and chiefly 

 fresh water ; and as this fluid cannot serve for respi- 

 ration, they come to the surface from time to time to 

 respire the atmospheric air. If they are found out 

 of the w^ater, it is always close to it, on some aquatic 

 plant. 



Of the glau€us, very little is known. It appears 



