402 Illinois State Laboratdry of Natural History. 



Quadrula pustulosa* where the highest proportion of under- 

 sized individuals is included in the tabulation, — twenty out 

 of eighty-seven, — it appears that the infestation of these 

 younger hosts did not differ materially in kind or degree 

 from that of the larger individuals. 



The sex of fourteen hundred and eighty-three individuals 

 of the sixteen hundred and fourteen examined was deter- 

 mined by microscopical examination, seven hundred and 

 eighty-two being males and seven hundred and one females. 

 In one hundred cases the determination of sex was not 

 attempted. In the thirty-one remaining, it was indetermin- 

 able by microscopic examination, all but five being infested by 

 Bucephalus or other cercaria forms to the utter destruction of 

 the proper reproductive tissue. From the shape of the shells 

 eight of these thirty-one individuals were pronounced males 

 and two females. Others, also belonging to species in which 

 the shells of the two sexes are normally characteristic, had 

 shells of such shape as to render the sex problematical and 

 to suggest that infestation by Bucephalus or other cercaria, 

 when early acquired and long continued, may so alter the 

 form of the shell of the female as to cause it to resemble that 

 of the male or, if acquired later, may produce an intermediate 

 form. Moreover, females infested with Bucephalus or other 

 cercaria rarely (in but three observed cases) carried glochidia, 

 though examined when the marsupia of other females of their 

 species were normally inflated with young. This is especially 

 noteworthy in Lampsilis gracilis, in which determination of 

 the sex of the clam by the form of the shell is usually certain. 

 Of the eighty-nine individuals of this species examined, 

 thirteen were infested with Bucephalus or other cercaria. Of 

 these, seven appeared to be males, one was a female with 

 glochidia in the gill, three were doubtfully regarded as females 

 although no germinal tissue was discernible, while the sex of 

 two was problematical. 



In thirty-eight of the forty-four species examined the niirn- 



* For the convenience of those who have not followed the recent changes in 

 unionid nomenclature a list of the names mentioned in this article is given in the 

 lirst column below, each name being followed in the second column by the one 



