174 



grandis oeing sometimes found on sandy bottom, edentula, however, 

 invariably upon a soft bottom. Neitlier (liealtliy forms) was ever talien 

 on hard sand or gravel. 



U. glans has been taken upon sandy and gravelly bottoms, in from 

 4 feet out. U. fabalis appeared in about the same region except that 

 it goes out on the soft bottom even farther than edentula. 



U. subrostratus appears on the outer edge of the sand and gi-avel 

 banks in about four feet of water and extends out as far as the light 

 form of U. luteolus. 



U. luteolus is the most variable, the most widely distributed and 

 the most abundant 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 the plate and are very nearly perfect. The 

 straw-colored variety is found in from 4 inches to 22 feet of water; 

 it is, however, dominant inshore, in weed patches (Potomogeton and 

 Ceratophyllum), and on chara-covered bottoms. The dark variety 

 occupies the same region but is dominant upon sand and gravel bottoms 

 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 can not be said to be dominant anywhere. 



U. rubiginosus occupies about the habitat dominated by the dark 

 form of U. luteolus, except that it was not found in deeper water than ten 

 ^eet. 



M. marginata was found so infrequently (only six times) that the 

 writers could tell little of its distribution. The specimens found were 

 taken on sand and gravel, and white marl bottoms in from four to 

 twenty-two feet. 



There are a number of conditions in the environment which sug- 

 gested themselves to us as possible explanations for this distribution 

 —age,, sex, light, heat, food supply and oxygen, pressure, wave action, 

 character of the bottom, and enemies. Sex can not be important, for 

 males and females are found together throughout the habitat; light can 

 have but little to do Avith it, for mussels are absent in places in three feet 

 of water and are abundant in others in fifteen feet, the difference in 

 tight being considerable. Further, the light over some of the immense 

 beds in White River is no greater and perhaps even less than in twelve 

 feet of lake water. That heat has little effect, during the summer at 



