306 THOMAS J. HEADLEE. 



parts of the small one, the shallow part of the beach is formed 

 of muck which shades off into marl without the presence of any 

 sand or gravel. 



In general, it may be said that the mussel zone extends from 

 the shore-line to where the bottom changes to very soft mud. 

 This region is covered by from four inches to nine feet of water, 

 although in some places the mud comes to within a few feet of 

 the water's edge, while in others the sandy and gravelly bottom 

 runs out into twenty-two feet of water. 



A. graudis is found just on the outer edge of the sandy and 

 gravelly banks, while A. edentula appears most abundantly a 

 little farther out. A few specimens of both species were taken 

 closer inshore, grandis being sometimes found on sandy bottom, 

 edentnla, however, invariably upon soft bottom. Neither (healthy 

 forms) was taken on hard sand or gravel. U. glans has been 

 taken upon sandy and gravelly bottom in from four feet out. 

 U. fab alls appears in about the same region, except that it goes 

 out on the soft bottom as far as edentula. U. subrostratus ap- 

 pears on the outer edge of the sandy and gravelly banks in about 

 four feet of water, and also further inshore where the bottom is 

 soft. U. Intcolus is the most variable, the most widely distributed, 

 and the most abundant of all the species in the lake. It varies 

 from a moderately thin, light straw-colored shell, marked by 

 radiating greenish lines, to an extremely heavy, almost black 

 form. The gradations of form, color, and size are shown in Figs. 

 9-19 in the plate. The straw-colored variety is found in from 

 four inches to twenty-two feet of water. It is, however, domi- 

 nant inshore, in weed patches (Potamogeton and Ceratopkyllum), 

 and on chara- covered bottoms. The dark variety occupies the 

 same region but is dominant upon sandy and gravelly bottom in 

 from three and one half to twenty-two feet of water. The inter- 

 grading forms cover the same territory as the straw-colored and 

 dark varieties but cannot be said to be dominant anywhere. U. 

 rubiginosus occupies about the same habitat dominated by the 

 dark form of U. hi/coins, except that it was not found in water 

 deeper than ten feet. M. marginata was found so infrequently 

 (only six times) that we could tell little of its distribution. The 



