123] FAUNA OF BIG VERMIUON RIVER— BAKER 25 



rocky (gravel) habitat while Campeloma prefers a mud or fine sand bot- 

 tom. Physa lives in both mud and on rocks and Ancylus on vegetation or 

 in empty shells of mussels. The ecological preferences of these species, as 

 well as the associated animals, are shown in Table IV. 



ECOLOGICAL VARIATION 



The 35 species and varieties of Unionidae found in the Big Vermilion 

 River west of Danville show an interesting distribution. Table III clearly 

 indicates that there is a more or less gradual increase in the number of 

 species as the stream increases in size. Taking into consideration both dead 

 and living naiads and ignoring for the time the effect of sewage pollution 

 on the distribution, the increase in species correlated with the increase in 

 distance in miles from Urbana may be expressed in the following table: 



Table Number V. Inckease in Species with Distance 



Station No. of Species Distance from Urbana 



Big bend 6 14 . 25 m'les 



One mile north iron bridge 12 16 . 50 " 



Railroad bridge 15 20.00 " 



Two miles above Homer dam 14 24 . 75 " 



Homer Park 28 27.00 " 



South of Muncie 23 36.00 " 



Salt Fork junction 15 44.00 " 



Middle Fork 22 45.00 " 



BigVermUion , . .. 21 46.00 " 



The sudden rise in number of species at Homer Park is noteworthy and 

 is due to the exceptionally favorable environment, good depth of water, 

 favorable bottom, plenty of food, and a fully normal supply of dissolved 

 oxygen provided by the dam just above the Park. The dam appears to be 

 an effective barrier to the migration of mussels, and it would also seem 

 difficult for fish to pass the dam, except at very high water, and thus 

 migration in the glochidial stage is rendered difl&cult or impossible. In 

 the table it may be noted that 17 species occur at five stations, two to 

 eight miles above the dam and 28 species occur below the dam. Seventeen 

 species are common to both areas and 12 species are found below but not 

 above the dam. This distribution is shown in Table VI. 



A striking feature of naiad distribution, noted repeatedly in several 

 species, is the conspicuous change in the shape of the shell as the distance 

 from the headwaters of the stream increases. Species that normally have 

 swollen or globose shells in the larger rivers, occur as flat or compressed 

 forms-in the headwaters of these streams. An increase in length of shell 

 as correlated with decreased obesity is also noted, and also, a decrease in 

 tuberosity. Ortmann (1920) has recently ably discussed this matter and 

 shows that the rule holds good for many species in widely separated areas; 

 the writer cannot fully agree with Ortmann in reducing so many species 



