406 Illinois State Laboratory of Natural History. 



apex just below the umbo and its arms reaching even beyond 

 and below the anterior and posterior adductor muscles.* 

 This trematode has not in my experience been found singly, 

 the number associated having varied from four to many 

 hundreds. They are habitually loosely adherent by their 

 suckers to the mantle surface and to each other, but may be 

 slightly insinuated within the loose tissue of the mantle, 

 especially when found between the hinge teeth. The position 

 of this parasite is usually marked by rusty stains in and 

 upon the nacre, by malformation of the shell or of the hinge 

 teeth, and not infrequently by a number of dark, poorly 

 formed pearls. Though these conditions of the superimposed 

 shell do not always accompany infestation by this trematode, 

 and though similar abnormalities are found without its pres- 

 ence being discerned, yet these malformations are very con- 

 stant where the mass of the parasites is considerable, and 

 the size and location of the ferruginous stain or injury corre- 

 spond to those of the infesting colony. When but few are 

 present and there is no injury to the nacre, the irritation is 

 no doubt too slight or too recent for much interference with 

 the normal secretion of the mantle. A malformation char- 

 acteristic of the presence of this parasite but unaccompanied 

 by it, would seem to imply desertion for another host. Such 

 implication is strengthened by the fact that in the case of 

 some of the host species, individuals are frequently found in 

 which none of these salmon-colored masses of trematodes are 

 present, but which nevertheless present malformations of 

 considerable size in which the rusty, altered, and diseased 

 nacre is covered with a normal layer of later deposit. The 

 parasite is, moreover, uniformly immature, no matter at 

 what season it is observed. 



The other three species of Distomum were found encysted 

 in the following situations respectively: (1) in the peri- 

 cardium of a single individual in each of the species Qiiadrula 

 riibiginosa, Plagiola elegans, and Lampsilis anodontoides ; 

 (2) in the ventral muscular margin of the mantle in four 



*H. L. Osborn ("98. Zool. Bull., Vol. I.. Xo. tl) describes in like manner this para- 

 site and its mode of infestation in Anodonta plana (= grandis) and Stropltit»i> 

 edentulus from Chautauqua Lake, X. Y. 



