Article VIII. — A Statistical Study of the Parasites of the 



Union id<e. By H. M. Kelly. 



The studies on the quantitative occurrence of parasites in 

 different species of the Unionidce which form the hasis of the 

 present paper were made at the Illinois Biological Station, at 

 Havana, early in the summers of 1896 and 1897 and con- 

 tinued both seasons at Mt. Vernon, la., and at Lewisburg and 

 Phcenixville, Pa.«, the material used being secured in the 

 above mentioned localities and examined while fresh. It 

 was the purpose of the investigation to determine, if possible, 

 under what conditions infestation would be found to vary in 

 a series of closely related species of a given locality or of 

 several localities, and to what extent this variation could be 

 regarded as a specific characteristic. 



The Illinois, Iowa, and Pennsylvania waters from which 

 collections were made are indicated in Table I., and for the 

 purposes of this paper may be briefly characterized as fol- 

 lows : (1) The Illinois Paver at Havana is a large sluggish 

 stream with alluvial banks and largely muddy bottom, full 

 of organic matter of various origin. (2) Spoon River, a 

 tributary of the Illinois, is at Bernadotte and Duncan's 

 Mills, where collections were made, small, with rapid current 

 and rocky bottom. It drains a purely agricultural region, 

 and is subject to frequent and sudden Hoods. (3) Quiver 

 Creek, near Havana, is a smaller tributary to the Illinois, 

 draining a sandy plain. (4) Thompson's Lake is a shallow 

 body of water about six miles long and one mile wide, lying 

 in the low alluvial flood plain of the Illinois River, with very 

 muddy bottom and little change of water except as affected 

 by change of level in the river, with which it is connected. 

 (5) The Cedar River, near Mt. Vernon, la., is a rapid shallow 

 stream with shifting sandy bottom for the most part, draining 



