64 MUSSEL FAUNA OF MAUMEE EIVEB. 



normal fashion. In some of our alcoholic material this contraction 

 and thickening of the mantle is quite markedly shown. The effect 

 of this peculiar form of growth on the shell as a whole is to give it a 

 somewhat more inflated and elongate appearance than is character- 

 istic of the species. Shells of L. ligament'mus thus affected approach 

 the contour of L. luteolus\ and while close approximation of these 

 two species is not rare in normal shells, some of the puzzling cases 

 are probably L. ligamentinu^ which has been modified by this dis- 

 tomid. 



In L. ve7it7'ieosus, which is very susceptible to the attacks of this 

 parasite, we frequently have gaping shells, and others which are 

 markedly thickened. . Some of these thickened shells closely resemble 

 those of L. ligamentinus. The marginal-cyst distomid is of especial 

 interest on account of its connection with pearl formation. 



Suspicion that this distomid had some connection with the forma- 

 tion of pearls was aroused when the objects were first encountered, 

 as their form and position was especially suggestive. This suspicion 

 was intestified by frequently finding small round pearls in the mantle 

 of mussels, closely associated with the cysts. Finally, on decalcifying 

 and clearing in paraffin and xylol, preparatory to embedding, a por- 

 tion of the mantle containing a small round free pearl surrounded by 

 the cysts, a cyst could be clearly distinguished in the center of the 

 pearl. Our material, which was preserved by simply dropping the 

 mussels into strong alcohol, was not in good condition for histological 

 purposes, so that we were unable to demonstrate the nature of the 

 tissues surrounding the pearls. 



Our studies indicate that the marginal-cyst distomid is widely dis- 

 tributed. We have found the cysts in alcoholic material collected in 

 the jNfississippi River, in L. ligamentinus collected by Freeland and 

 Williams in the Illinois RiA^er in 1907, in L. ventricosus collected in 

 the Yellow River, Ind., by ourselves in 1908, and in L. ligamentinus 

 collected in the Ohio by Boepple in 1909. From the appearance of 

 shells examined, Ave judge it to be common in the Minnesota River. 



4. The distomid of Oshorn. — Quite frequently during our mussel 

 investigations we have found Anodonta affected with small white 

 sporocysts, which covered the outside surface of the mantle next the 

 nacre. The nacre of these mussels was often raised into a number of 

 sharp pearly blisters, and in some cases small white areas, conform- 

 ing with the size and shape of the sporocysts, indicate that the sporo- 

 cysts had been actually covered with nacre. Mussels thus affected 

 were found in the Little River ditch near Aboite Station, Ind., and 

 in White Bear Lake and Lake Minnetonka, near St. Paul, Minn., 

 in 1908. Notes on alcoholic examples of Aiiodonfa collected in Wild 

 Cat Creek, Carroll County, Ind., in 1899 refer to " white distomids," 

 which were probably this species. 



