Grier: Morphological Features of Mussel-shells. 153 



as well as the quantity of plankton. A low temperature favors the 

 increase and development of diatoms, while a high temperature 

 favors especially the development of water bloom algae, also such 

 forms as Vorticclla.'' Allen (2) claims that other algae than diatoms 

 form fully as large a part of the food, and lists more species of these 

 from the digestive tract of Naiades than diatoms (63). The colder 

 waters of Lake Erie would therefore tend to inhibit the growth of 

 those forms of algae except diatoms, even in the summer months. 



With the following extract from Kofoid (27) the question of plankton 

 as a source of food for Naiades may be dismissed. "Stream plankton 

 differs from all others in the mingling of plankton from all sources, 

 and in being subject to \ariation in quantity. It appears to be more 

 subject to catastrophic change than that of the lake, possibly on 

 account of the conditions just described. Changes in the volume of 

 the water, the contact of shore and bottom, access of heat and light, 

 and changes in chemical composition are frequently more extensive 

 and more widely effective in streams than in other types of aquatic 

 environments." Kofoid believed Chlorophycecc were somewhat more 

 characteristic of the plankton of rivers than of lakes. Silt (there is 

 more of this in the river) is supposed to hasten the growth of plankton 

 by providing its source of nourishment in an easily obtained form. 

 It would follow in the writer's opinion that fluviatile Naiades, all 

 factors considered, have a more abundant, if less regular, supply of 

 food than those of the lakes. Perhaps the larger amount of silt 

 received in the spring in the lakes accounts for abundance of plankton 

 then, especially when coupled with the rising temperature. 



Characteristics of the Mussel Fauna of Lake Erie. 



The studies of Walker (54, 55) have shown that the Lake Erie 

 fauna did not persist through the Glacial Period. Representative 

 Naiades now found in the Great Lake Region of the Mississippi and 

 Ohio Faunas are the result of post glacial invasion (47). It is Dr. 

 Walker's opinion that to this is to be ascribed the present existence of 

 so large a number of representatives of the Mississippi Fauna in 

 Lake Erie. This migration may be traced at one end of the lake 

 through the Maumee Outlet into the post glacial Lake Maumee; 

 at the other end the ancient headwaters of the Ohio tributaries once 

 emptied (28). Walker ascribes the modifications which have taken 

 place in their size, shape, and appearance to the environmental 



