400 Illinois State Laboratori/ of Natin-(il Ilistorij. 



a rolling' prairie region. (6) Abbey Creek is a small, slug- 

 gish, and muddy tributary of the Cedar, flowing through a 

 wooded region of rich black soil. (7) The Schuylkill River 

 at Phcenixville, Pa., is rapid and shallow, with a rocky bed. 

 Its waters are largely contaminated with the sewage of the 

 populous region, through which it flows, with manufacturers' 

 wastes, and with the acid pumpings of anthracite coal mines. 

 (8) Pickering Creek, a tributary of the Schuylkill, is an 

 uncontaminated, swift-flowing stream, constant in volume, 

 and draining a restricted rocky area. (9) French Creek, a 

 near-by l)ranch of the Schuylkill, is of similar size, but 

 traverses a country of clay soils, and is suljject to frequent 

 inundations. (10) The West Branch of the Susquehanna 

 Piiver is, upon most of its course, a large but shallow mountain 

 stream with rapid current. It is quiet for some miles at 

 Lewisburg, Pa., where it crosses one of the fertile valleys of 

 the Appalachian system. 



Fifteen huiulred and thirty-seven individual unios from the 

 above localities, belonging to forty-four species, were person- 

 ally examined by me with reference to their parasites ; and 

 with the data thus obtained I have incorporated the results 

 of an examination of seventy- seven individuals, belonging to 

 eighteen species, made by Dr. C. A. Kofoid in 1895 and 

 189G, which were placed at my disposal. It is unfortunate 

 that my material did not furnish equal representation for 

 each host species, and that the examination extended over 

 such a long period of time. For control on both these points 

 an effort was made to examine, if possible, about twenty indi- 

 viduals of each species which were all taken at the same time 

 from the same locality and under the same conditions. 



Whenever it was practicable individuals of or above the 

 adult average in size and age were chosen for examination. 

 Only ninety-seven of the sixteen hundred and fourteen indi- 

 viduals examined could be regarded as immature, and these 

 were well distributed among the several species. The inclu- 

 sion of records for these younger and smaller individuals 

 would presumably reduce somewhat both the probability of 

 and capacity for infestation. However, in the case of 



