189] FAUNA OF BIG VERMILION RIVER—BAKER 91 
water left on either side of the new drainage ditch are inhabited by 
clean water animals that occasionally get into the ditch during periods 
of high water. Fish, mollusks, and insects have been observed which 
doubtless came from this source. 
PARASITES AND PATHOLOGIC AGENCIES 
As a rule, parasites, either distomids or Unionicola (Atax) were rare 
in the naiads collected in the Big Vermilion and Sangamon rivers. Some 
of the Anodontas had marginal cists of distomids and many shells of this 
genus were discolored from this cause. Clark and Wilson (1912:61), in 
their study of the Maumee River fauna, observed distomids in various 
mussels which were believed to be the distomids described by Osborn and 
Kelly. The affected mussels were Actinonaias ligamentina, Obovaria 
circulus, Elliptio gibbosus, Lampsilis ventricosa, Amblema undulata, Las- 
migona costata (thought to be the distomid of Kelly), and Anodonta 
grandis (thought to be the distomid of Osborn). It is probable that these 
flat-worms also infest many of the mussels of the rivers investigated, but 
they were not observed in the specimens collected. Cotylaspis insignis 
and forms of Unionicola were also found by Clark and Wilson, but these 
parasites were not seen in the mussels examined from the area under con- 
sideration. 
Pearls, usually of small size, were frequently seen attached to the 
shells of mussels, and many pearly growths cause by injuries were also 
observed. A large round pearl was found in a shell of Anodonta grandis 
gigantea from Crystal Lake, which measured almost a quarter of an inch 
in diameter (5 mm.). It has been suggested that many of these pearls and 
pearly growths attached to the mussels may have been caused by parasites, 
such as the distomids before mentioned. The same is also true of the round 
pearls found in the animal tissues of the mussels. 
Crippled shells, those individuals have abnormal valves, were not com- 
mon in either of the rivers examined. Mud was found to cause trouble in 
many cases, getting in between the mantle and the shell below the pallial 
line and causing large blisters. ; 
ECONOMIC CONSIDERATIONS 
The shells known as river mussels or Naiades are used in the manufac- 
ture of pearl buttons. As this industry, the making of pearl buttons, has 
reached large proportions it is imperative that the raw material be con- 
served for the maintenance of the industry. The continued fishing of the 
mussel beds in the larger rivers has greatly depleted the amount of avail- 
able raw material—the mussel beds—, and the whole industry, shell collect- 
ing and button making, is threatened with disaster if means are not found to 
restock the depleted beds (see Coker; 1919:44). The United States Bureau 
( 
