CHAPTER XII 



THE FRESH-WATER CLAM AND ITS ALLIES 



THE fresh-water clams belong to the group of lamelli- 

 branch mollusks, characterized by the absence of a distinct 

 head, by the possession of leaf-like gills on the sides of the 

 body, and by the presence of a calcareous shell composed 

 of a right and a left valve. Economically this group is the 

 most important to man of all Mollusca. It includes both 

 marine and fresh-water species. About six thousand 

 species of living lamellibranchs and over ten thousand 

 fossil species are recognized. They all fall into two prin- 

 cipal subdivisions. The first (Siphonata) includes species 

 which possess a siphon and have the mantle edges grown 

 together, while the second (Asiphonata) has no siphon and 

 has the mantle lobes for the most part wholly separated. 1 



Anodonta and Unio are extremely common in the ponds, 

 lakes, and rivers of North America. Anodonta is more 

 apt to be found in still waters and Unio in running 

 waters. They lie partly buried in the mud of the bottom, 

 with the valves of the shell gaping open and the partly 

 united edges of the mantle protruding. They do not lead 

 wholly sedentary lives, but may burrow or plough along 

 the bottom. 



Their food is gained from organic particles borne along 



1 A key to the principal families of bivalve shells will be found in the 

 Appendix, p. 188. 



178 



