.4 Stiidji of flic P(U-(tsit<'s of the riiioiiidw. 417 



French and Pickering creeks, and from the Susquehanna 

 Eiver, may be due to the very peculiar conditions, referred to 

 on page 400, which exist in the SchuylkilL Again, in 

 Al((s)uo(Iont(( mi(rti'niat<( the extensive amount of parasitism 

 in the material from Pickering Creek as compared with that 

 from French Creek, is explained by the fact that this is an 

 abundant and dominant species in Pickering Creek, while it 

 is relatively infrequent in French Creek. 



Some attempt was made to discover whether purely local 

 conditions in the habitat, such as the character of the bottom 

 and association with other species of Un'ionidu', bear any 

 relation to the character and degree of infestation. The 

 examination of representatives of nine species taken at one 

 time from a restricted locality below a bar in the Illinois 

 Eiver, where Uiiionidw were unusually abundant and at least 

 twenty-nine species represented, gave results which did not 

 differ materially or in any one direction — save in the slightly 

 larger infestation to be expected because of the hosts' 

 unrivaled opportunities for infestation — from those obtained 

 from the same species collected in other localities. 



A purely qualitative examination of the food of the various 

 species of Union'uhe showed no differences that could be 

 correlated with their capacities for infestation. The nature 

 of the food would hardly determine to any appreciable degree 

 the parasites of other organs than those closely connected 

 with the alimentary canal, and least of all those whose lodg- 

 ment would be effected by mere entrance to the branchial 

 chamber. 



In conlusion, the results arrived at by the foregoing studies 

 may be summed up as follows : The host species seem to 

 exhibit unlike capacities for infestation, both as to the num- 

 ber of individuals and the kinds of parasites present. It 

 appears that the differences shown are attributable only in 

 a minor degree to the age and size of the host, the size of the 

 stream, and the density of the unionid population. They 

 are not sufficiently accounted for by the seasonal variation, 

 — which is shown to exist to some degree at least in the case 

 of certain parasites, — nor by the very slight difference in 



