280 ON OOLLKCTING SHELLS. 



genera (Strepomatidffi). The shells vary from a turrited to a 

 globular form, variously colored and sometimes curiously 

 ornamented with tubercles, ridges and carinations. With a few 

 exceptions these interesting shells are found only in rivers or 

 perennial streams. The different genera of this class seem to be 

 adapted to certain modifications of conditions. Some of these 

 mollusks prefer muddy sloping river-banks, where 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 the mollusca of this class are such as bring 

 them to the shallower portions of the water they inhabit. Thej^ 

 can often be reached from the shore by the hand. 



By gradual transitions these genera, with their numerous 

 species, are followed by other and smaller genera (Amnicolidse, 

 etc.), some of which are of comparatiA^ely limited range; others 

 are widel}' distributed over the whole country. Nearly all of 

 them have habits in some respects similar to the preceding class, 

 and will be found on the muddy bottom portions of rivers, lakes, 

 etc., or feeding on aquatic plants. The small size of many of 

 these shells renders them somewhat difficult to discover, unless 

 the collector has expedients for securing them with ease and 

 certainty. 



Bivalve Shells. These next claim our attention, and for 

 convenience they will be considered under two classes, though 

 embi"acing several distinct genera and species. A class of shells, 

 none of which ever attain dimensions much exceeding half an 

 inch (Cyclades), inhabit nearly every perrennial stream having a 

 muddy bottom; found also in stagnant waters, lakes, ponds, 

 canals, and, indeed, in every station fitted for molluscan life. 

 Some species inhabit stations subject to drying during a portion 

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

 them alive in their dried habitat, and have inferred they were 

 bivalve land shells ! All the shells of this class burrow just 

 beneath the surface of the mud, and are usually found in greatCvSt 

 abundance near the margin of the water or where there is but 

 little depth. This class embraces some species remarkable for 

 their fragility, others equall}^ remarkable for their minuteness. 

 They are distributed over all the explored portions of the 

 country. 



Fresh- water mussels (Unionidfe.) This class of shells em- 

 braces several genera, which, on account of the great number of 

 species contained in them will eventuall}^ be more minutely 

 classified in subgenera. No country in the world has produced 

 as great a varietj^ of forms of Unionidae as the United States. 

 In the northeastern portion the number of species is compara- 

 tively small, but in the South and West the number of species 



