125] FAUNA OF BIG VERMILION RIVER— BAKER 27 



pression of the shell, for all live in the same section of the stream under 

 identical conditions. The rule cited by Ortmann, however, seems to be 

 applicable in most cases. 



Several species increase in size toward the lower part of the river. This 

 is especially true of Anodonta grandis, Strophitus edentulus, Alasmidonta 

 marginata, Eurynia lienosa, Lampsilis luteola, Amblema undulata, and 

 Lampsilis ventricosa. A few others show some increase at different stations. 

 One species, Uniomerus tetralasmus, is apparentl)'' confined to the upper, 

 smaller tributary streams of Salt Fork. It was common in the ditch north 

 of Urbana; in Crystal Lake, Urbana, in Spoon River; and at Muncie in a 

 small tributary. Two broken valves were found at the station called the 

 natural dam, but these are believed to have been washed into this stream 

 from a nearby tributary which was dry when this part of the Salt Fork 

 was examined (September 25). Tetralasmus probably also occurs in the 



Table VII. Variation of Fusconaia Rubiginosa 



Length Width Per cent Station No. Distance from 



Urbana 



58 23 39 17 15^ mUes 



62 26 42 .. 



61 31 SO .. 



68 27 39 



79 33 42 



86 33 38 24 243^ miles 



89 41 46 



90 31 34 25 27 miles 



82 35 42 .. 



89 47 52 .. 



71 30 42 26 36 miles 



67 30 44 



102 41 40 30 46 miles 



65 30 46 .. 



86 43 50 .. 



upper part of Middle Fork and in North Fork, but the upper parts of these 

 streams have not been examined. The majority of the other species occur 

 at several stations along the Salt Fork and its tributaries and no particular 

 variation in distribution was observed except as already noted. 



Three mussel species and varieties were found in Crystal Lake that 

 are exotic as far as the Big Vermilion River is concerned. These are Ano- 

 donta grandis gigantea, Anodonta corpulenta, and Uniomerus tetralasmus 

 sayii. These species were artificially introduced into this body of water 

 about 1908 by a member of the zoological department of the University of 

 Illinois. They originally came from a stream in western Indiana. That 

 these mussels found a favorable environment and have thrived during these 

 years is evidenced by the number of fine specimens recently collected when 

 the lake was partly drained. Only the Uniomerus was rare, but one speci- 

 men being found. As Crystal Lake is not connected with the Salt Fork 



