414 Illinoi>i State Laboratory of Natural History. 



silis are of large parasite capacity; those of Unio (restricted) 

 and of Playiola are of low capacity; while within the genera 

 Quadrida and Alasmodonta we find wide extremes of infesta- 

 tion. Within the limits of the above genera this correspond- 

 ence is more or less evident between members of groups of 

 closely related species, especially when taken in considerable 

 numbers in similar situations. For example, we may note 

 the correspondence between Lamps'dls ligamentinuH and 

 L. liiteolus; Quadrida ehena, Q. trigona, and Q. ruhiginosa; 

 LanipsUis alatus and L. gracilis ; and Quadrula lachrymosa 

 and Q. asperrima. 



Seasonal changes have been found to modify the distribu- 

 tion of the parasites in the case of Atax and Coiirhophtliirus 

 only. As the water grew colder in late October and Novem- 

 ber, the examinations of Unionidce from the Cedar River 

 gave relatively fewer adult Atax and more abundant eggs. 

 The presence of these eggs was regarded as potential infesta- 

 tion, and therefore these data may properly be included in 

 the tabulation. The reliability of the tabulations may be 

 somewhat vitiated by the fact that CoiicJcophthirus is much 

 more plentiful in the warmer months, during which the 

 greater part of my collections were made. 



