A Stiidf/ of tJie P<(r(i>iifcs of the Vx'ioii'uhc. -415 



Table Y. indicates the range of the different parasites in 

 the several localities supplying the material. In all proba- 

 bility the blanks opposite the more usual parasites are due to 

 the absence of the proper host species or to the examination 

 of an insufficient number of these species rather than to 

 peculiarities in the localities themselves, for the variety of 

 parasites listed for any situation varies with the number of 

 individuals and the variety of species examined from each 

 locality. Thus the absence of .4 fa.r from the Schuylkill and 

 its occurrence in the trilnitary French and Pickering creeks 

 may be accounted for by the fact that with the exception of 

 a single occurrence, this parasite was never found by me, in 

 any locality, in the particular species examined from the 

 Schuylkill. Again, an examination of twenty individuals of 

 Anodonid corpulenta from Abbey Creek, la., made since these 

 tabulations were completed, has increased the list of unionid 

 parasites from that stream to six, adding Aspidogaster, Coty- 

 laspis, free iJlstoiiKifa, Bucephalus, and Conchophthirus, the 

 smaller number of parasites reported in the table being due 

 in a large degree to the particular species of Union'uhe 

 previously examined from the stream. An examination of 

 Table YL shows, however, that there is quite a great varia- 

 tion in the infestation of the same species in different local- 

 ities. This variation is the greatest, as would be expected, in 

 the host species least frequently parasitized, and especially 

 in the case of those parasites that are infrequent or unusual 

 in a given host. Again in the case of the larger streams as 

 compared with the smaller ones, whenever a given host is 

 especially plentiful and Unlon'uUe in general are abundant 

 the infestation is relatively larger and a greater variety of 

 parasites occur. For example, in the Illinois and Cedar 

 rivers, both large streams, a large proportion of the Union\d(C 

 are excessively parasitized, but in the Spoon Eiver, a smaller 

 stream, only such species are extremely infested as are 

 abundant or dominant, as, for example, Qiiadrula tiibercidatd, 

 Villi) (jibhosKS, and Lampsilis gracilis. The fact that Unio 

 romphtnatiis from the Schuylkill Eiver is but slightly parasit- 

 ized in comparison with individuals from its tributaries, 



