3IO THOMAS J. HEADLEE. 



is at least one such difference the size of the shell in propor- 

 tion to its weight. The Anodonta shells are very light, while 

 those of the dark form of U. luteolus are very heavy in propor- 

 tion to surface exposed. Many shells of the former, especially 

 those of A. edcntnla, are easily crushed between the thumb and 

 fingers, while those of the latter will frequently withstand the 

 heavy blow of a hammer. The former is evidently fitted for a 

 life on soft bottom, while the latter finds a most congenial situ- 

 ation on coarse sand or gravel beds. Between these two 

 extremes the other species range, and indeed we find the main 

 facts of their distribution explained by the status of this condition. 



Accordingly A. grandis and A. edentnla are found on the outer 

 edge of the sandy marl banks, the edentnla being better fitted to 

 withstand the bottom conditions, even out in the edge of the dark 

 mud. U. glans and U.fabalis, owing to lightness, are able to 

 occupy about the same region. They are also found inshore in 

 situations not subjected to wave action. U. subrostratus, having 

 medium weight valves, appears on gravel and sand banks, in 

 weed patches, and on chara- covered beds. U. ndrigitiosus, having 

 very heavy valves, is confined to clear sand and gravel banks. 

 The straw-colored form of U. hiteolns, on account of its medium 

 weight valves, is able to live on sand, gravel, in weed patches, 

 and on chara-covered beds. Owing to the fact that so few speci- 

 mens of M. marginata were found, we were unable to draw any 

 conclusions as to its ecology. 



The muskrat is the principal enemy of the mussel ; around his 

 house many mussel shells are found but no live mussels. Shells 

 of all the species in the lake except the smaller ones appear 

 there, those of the Anodontas being in much greater evidence than 

 is proportionate to their total number. They do not appear so 

 on first examination, for they are broken up by the animal and 

 worn by the waves. The conditions on the sand banks beyond 

 reach of wave action are very favorable for Anodonta life, except 

 for the presence of the muskrat. They are absolutely absent 

 from the water some distance from his home where we found 

 Unios rather abundant. This points to the fact that the muskrat 

 confines the Anodonta to the deeper water at the edge of sandy 

 and gravelly banks. 



