A Study of the Parasites of the Unionidce. 411 



the number harbored by each species considered as a unit. 

 In this table and in the succeeding one the data regarding 

 host species in which less than fourteen individuals were 

 examined are not included. While the comparisons between 

 the species included are thus rendered the more reliable, it 

 will be seen that the general conclusions deduced from Tables 

 III. and IV. only confirm the findings of the more general 

 statistics of Table II. While nine kinds of parasites are here 

 listed for the Unionidce, no species of the family was found 

 to harbor more than seven, and the average was but four or 

 five. Moreover, in but four species — Quadrula laehrymosa, 

 Q. ebena, Q. pustulata, and Anodonta suborbiculata — were 

 individuals found with the maximum variety of parasites 

 listed for its species, and in these the maximum variety is 

 four or less. It is perhaps futile to imagine what variety of 

 parasites an individual host might successfully sustain, but 

 it is noticeable that in this table the mean individual infesta- 

 tion lies closer to the species minimum than to its maximum. 

 A close inspection of the data of all examinations further 

 confirms the inference that the individual host is unable to 

 realize the maximum capacity of its species for infestation, 

 since in no case is the presence of an unusual number of one 

 parasite accompanied either by like severe infestation by 

 another or by a considerable variety of parasites. It is true 

 that one individual of Lampsilis gracilis with sixteen speci- 

 mens of Aspidogasier in the pericardium and six in the 

 nephridia, harbored also two of Cotylaspis and one each of 

 Atax and Bucephalus, and that one Lampsilis ventricosiis 

 infested with thirty-one specimens of Aspidogasier contained 

 large numbers of Bucephalus ; but these are exceptional 

 cases, and even in these individuals, when we consider the 

 size of the host and the established maximum capacity of 

 their species, the extreme limit can hardly be said to be 

 reached. The ectoparasites probably require but little from 

 their hosts, but they rarely occur in numbers upon clams 

 exhausted by Bucephalus. 



Table IV. gives the percentage of the hosts which were 

 infested with Aspidogasier, Cotylaspis, and Atax, the most 



