105! FAUNA OF BIG VERMILION RIVER—BAKER 7 
INTRODUCTION 
The present paper embodies the result of investigations of the mollusk 
fauna of one of the smaller rivers of the State of Illinois, the Big Vermilion, 
carried on during the years 1918 to 1920. To this are added notes on 
another river, the Sangamon, for comparison. While all groups of mol- 
lusks were considered, special emphasis has been placed on the Unionidae 
or pearly fresh water mussels (Naiades), on account of their abundance, 
their significance in matters of geographic distribution, and their impor- 
tance from an economic standpoint, as raw material from which pearl 
buttons are made. 
For a number of years, the United States Bureau of Fisheries has been 
engaged in conducting a series of investigations of several rivers of Illinois 
and Indiana, for the purpose of ascertaining the mussel resources of these 
streams. In view of the rapid depletion of the supply in the larger rivers 
(Mississippi, Ohio, Illinois) it becomes necessary to search the smaller 
streams to replenish the older beds. The Maumee and the Kankakee 
(Wilson and Clark, 1912), as well as the Illinois (Danglade, 1914), have 
recently been rather carefully surveyed with interesting and important 
results. 
It was thought that a somewhat similar investigation of the Big Ver- 
milion River would be of value. The scope of the investigation included 
everything that appeared to affect the molluscan life of the river, physical 
characters, pollution, general relation and number of species. It was 
believed, also, that the Big Vermilion, as well as other smaller streams 
in the State, might provide good breeding stock with which to carry on 
artificial glochidial infection of fish, and the results seem to warrant the 
assumption. It will be necessary to make more or less detailed surveys of 
all of the smaller rivers and their tributaries, and the present paper may 
be considered a contribution toward this end, covering fully the Big Ver-* 
milion from its upper waters to the vicinity of Danville, a distance of about 
forty-five miles by stream. ‘A portion of the Sangamon is considered, and 
this river may also prove a valuable source of mussel material. 
Little has been done by field naturalists in the study of the distribution 
of the molluscan fauna of a stream from the headwaters to the larger 
portions of these rivers. Perhaps the most thorough and notable study 
of this character was carried on by Adams (1900, 1915), on the genus Io 
in which the Tennessee River and its tributaries (Powell, Clinch, Holston, 
French Broad, Nolichucky, etc.) were studied from sources to Chattanooga. 
