80 F. B. ISELY. 



an environment gives a constant supply of oxygen, and sufficient 

 food; is frequented by suitable fish; is free from shifting sand 

 and silt accumulation. Those mussels that drop from the fish 

 in these favorable situations develop in large numbers, while 

 the less fortunate, that drop in shifting sand and silt, die early. 



2. In the study of the ecological factors that are inimical to 

 mussel life, more attention should be given to the consideration 

 of the juvenile habitat. Absence of gravel bars and stony 

 situations may sometimes explain the scarcity of the Unionidse 

 in certain streams and lakes where frequently water content has 

 been thought the chief unfavorable factor. 



3. It is a well known fact that in many streams certain stretches 

 of mud-bottom are found loaded with mussels, while other areas, 

 in the same stream, equally favorable from the standpoint of 

 the habitat of the adult mussels, have only scattering specimens. 



This distribution of the adult may be explained by the as- 

 sumption 1 that the average mussel seldom travels far up or down 

 the stream from the place where it begins successful develop- 

 ment. Stretches favorable for juvenile development thus come 

 to be the centers of dispersal in the streams where they occur. 

 As a result, areas of mud-bottom near these favorable habitats, 

 become loaded with mussels by migration. 



4. In the study of the life history of the Unionidae, we may 

 consider the embryonic, the glochidia, the parasitic, the early 

 juvenile, and the adult as distinct periods for separate and special 

 study. 



UNIVERSITY PREPARATORY SCHOOL, 

 TONKAWA, OKLAHOMA. 

 November 25, 1910. 



1 This assumption is fairly well established, by experimental study that I have 

 been carrying on for some time, and will be discussed in a later paper. 



