2J[S THE AMERICAN MIDLAND NATURALIST 



generally also that the shells of certain species at least blacken 

 with age, but some effort has been made to ascribe this blackening 

 in part to extraneous influences. Hay, (5), studied U. tuniidus 

 and U. pictorum from the Ouse and Foss, Rivers in England. The 

 Foss river had the more natural conditions, a bottom of mud, 

 abundant aquatic vegetation, a gentler current, and received less 

 polluting material than the Ouse. Its shells were lustrous and with 

 a bright nacre. The Ouse River was wider, had a superabundance 

 of mud in the bottom, and the shells in it were eroded, due either 

 to the rapidity of the current or dissolved CO2 in the water. The 

 shells from this stream were dark or dead brown in color, erosion 

 of the epidermis was considerably advanced, and the pearliness of 

 the nacre was dull, possibly because this stream received the 

 greater abundance and variety of drainage material. Hey believed 

 the differences in coloration observed to be due the amount of such 

 substances received. We may add here in order the observations 

 of two experienced students of the Najades; Messrs. Calvin Good- 

 rich of Toledo, Ohio, and V. S. Frierson, Frierson, La., as kindly 

 given the writer in correspondefice. 



' ' vShells of the same species vary in color of epidermis in different 

 streams, sometimes in the same stream under variations of environ- 

 ment. For example, the shells in the pools of Roche de Boeuf 

 rapids, Maumee River are rough and dull hued. They are most 

 of them covered with limy deposits ^nd blotched wnth some black 

 material which after removal leaves the shells little improved in 

 appearance. The Miami and Erie Canal, upon the bank above 

 these rapids, is fed from the Maumee 7 miles above. The shells 

 of the canal are smooth, polished, often with a sheen like silk and 

 lighter of color. The shells off Catawba Island, Ottav/a Co., Ohio, 

 are a bright, shining lot; those in La Plasaince bay, at the west 

 end of the Lake, much dulled by comparison. The La Plaisance 

 shells seem to be abraded by sand; those of Catawba Island es- 

 caping this treatment. • On the north shore, many of the shells 

 are stained with black and roughened with lime. Algae, lim^, 

 sewage, vegetable growths and mineral matter of one kind and 

 another appear to affect the color of the epidermis. I suppose 

 light has some share in the business. You have no doubt, noticed 

 that the older specimens of the museums show a distinct modi- 

 fication in color." . . . "There i^ near me a lake one quarter of a 

 mile wide, two miles long, five to twenty feet deep. It is really 



