ON COLLECTING SHKLLS. 279 



on the vegetation that abounds in such stations, there are a few 

 exceptions, in which species are found adapted to deep water, in 

 which it is improbable that they can reach the surface and respire 

 the air. The collector will find many species accessible to him 

 along the margins of water. Others will require the aid of a 

 boat, especially such as are found feeding on the weeds in lakes 

 and rivers. To discover some of the minute species found under 

 such circumstances, it may sometimes be advantageous to gather 

 handsful of the weed and gently lift them out of the water. If 

 the operation be rudely performed, the mollusks may be disturbed 

 so as to detach themselves. Many species will be found adhering 

 to the grass-like plants that grow in streams. Others adhere to 

 the stems of flags and bullrushes, and may be discovered very 

 readily by pulling up the plants by the roots, taking care to 

 perform the operation gentl}' and deliberatel3^ Of analogous 

 habits with some of the above are certain small species found 

 concealed under stones just below low-water mark in rivers. 

 The}" are sometimes also found adhering to larger shells. This 

 class embraces only small cup-like shells — " fresh-water limpets " 

 or Ancylus. 



Aside from the air-breathing aquatic mollusca, we have others 

 whose respiration is strict^ aquatic ; the necessities of these 

 restrict them to a narrower range. Hence they are not usually 

 found in stagnant waters, certainly not in waters of limited area, 

 where impurities are generated b^^ decomposing substances. The 

 largest shells of this class are found in the swamps along the 

 rivers of some of the Southern States, and are objects of interest 

 on account of their habits as well as of their value in the cabinet. 

 In their season of active life they are found feeding on aquatic 

 plants. Inhabiting localities subject to drying, they burrow in 

 the mud as the water diminishes. The collector will for conve- 

 nience seek them when they are active. Another class (Yivipar- 

 idffi), smaller than that just mentioned, but affording a greater 

 number of varieties and species, is more widely distributed, 

 being found not only in the waters of the various States, but also 

 in Canada. They inhabit rivers, lakes, ponds and canals, and 

 when circumstances favor their habits they will be found most 

 abundantly burrowing just beneath the surface of the soft mud 

 near the shores : where undoubtedly they are attracted by more 

 abundant supplies of food, and perhaps also by a more agreeable 

 temperature. They will often be found in the muddy banks of 

 rivers in great numbers, congregated at the margin of the water. 

 In canals where conditions of food and temperature are very 

 favorable, they attain a more luxuriant growth than in neighbor- 

 ing rivers. Some localities are remarkable for affording varieties 

 and monstrosities. Xext to this class in size is one that embraces 

 a large number of species included in several genera and sub- 



