nOLLECTING LAND AND FRESH-WATER SHELLS. 391 



to this class in size is one that embraces a large number of species included in 

 several genera and sub-genera. The shells vary in form from a slender turret, to 

 a globular form, variously colored, and sometimes curiously ornamented with 

 tubercles, ridges, and carinations. With a very few exceptions these interest- 

 ing shells are found only in rivers and perennial streams. The different genera 

 of this class seem to be adapted 'to certain raoditications of conditions. Some 

 of these molluscs prefer muddy, sloping river-banks, whei'e they crawl in the 

 comparatively still water on the surface of the mud. Others prefer the rapid 

 current among the rocky portions of streams, where they are found adhering to 

 the surfaces of the rocks. The habits of nearly all molluscs of this class are 

 such as bring them to the shallower portions of the water they inhabit. They 

 can often be reached from the shore with the hand. 



By gradual transitions these genera, with their numerous species, are followed 

 by other and smaller genera, some of which are of comparatively limited range ; 

 others are widely distributed over the whole country. Nearly all of them have 

 habits in some respects similar to the last preceding class, and will be found on 

 the muddy bottoms of shallow portions of rivers, lakes, &c., or feeding on 

 aquatic plants. The small size of many of these sliells renders them some- 

 what difficult to discover, unless the collector has expedients for securing them 

 with ease and certainty. 



Bivalve shells next claim attention, and for convenience they will be consid- 

 ered under two classes, thougli embracing several distinct genera and species. A 

 class of shells (none of which ever attain dimensions much exceeding half an 

 inch) inhabits nearly every perennial stream having a muddy bottom; found 

 also in stagnant waters, lakes, ponds, canals, and, indeed, in every station fitted, 

 for mollusc life. Some species inhabit stations subject to drying during a por- 

 tion of the year, and careless observers have been deceived on finding them 

 alive in their dried Jiahitat, and have inferred they were bivalve laud shells . 

 All the shells of this class barrow just beneath the surface of the mud, and are 

 usually found in greatest abundance near the margin of the water, or where 

 there is but little depth. This class embraces some species remarkable for their 

 fragility, others equally remarkable for their minuteness. They are distributed 

 over all the explored portions of the country. 



Fresh-water muscles. — This class of shells embraces several genera, which on 

 account of the great number of species contained in them will eventually be 

 more minutely classified in sub-genera. No country in the world produces a 

 greater variety of forms in this class of shells than the United States. In the 

 northeastern portion of the United States the number of species found is com- 

 paratively small ; but in the south and west the number of species becomes 

 great, and the variety and beauty discoverable in the almost endless species 

 make this class one of great interest to the collector. These molluscs inhabit 

 lakes, rivers, and canals. Stagnant water is unfavorable to them. They afford 

 abundant food for the muskrat and mink, who collect piles of shells on the shore 

 where they bring the molluscs to feed upon them. The shells left by the musk- 

 rat sometimes serve as a resort for the collector who is not critical to have the 

 best of specimens, while they should serve to point out to him that there are 

 good specimens, alive, not far distant. Except in shallow portions of rivers it 

 is sometimes difficult, howevei', to find muscles ; but where the water is not so 

 deep but that a person may with security wade in it, it is comparatively easy 

 to discover them. They will usually be found partly buried in the mud or 

 gravel, only enough of the shell projecting to enable the mollusc to extend the 

 syphons of his breathing apparatus into the water above him. A little practice 

 will enable the collector to detect the projecting shell. In lakes and ponds, 

 where the water is not too deep, the collector may readily discover the objects 

 of his search from a boat. 



Having thus in general terms given such suggestions as will enable the col- 



