123] FAUNA OF BIG VERMILION 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 
m empty shells of mussels. The ecological preferences of these species, as 
ell as the associated animals, are shown in Table IV. 
IS 4 ECOLOGICAL VARIATION 
he 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. INCREASE IN SPECIES WITH DISTANCE 
Station No. of Species Distance from Urbana 
Big boeni Gay seats. arte! syaisi steve chapel siwerevs © aie ycronaisycucrete 6 14.25 miles 
One mile north iron bridge................... 12 16550)5 ac 
Railroadibrmd gem amractescrcre clashes nie mrscita ie syne 15 20:00) 
Two miles'above Homer dam.............:.. 14 Zao S 
FomereParkiey are tateieiele sysieiecsl stains) siecosers «tye orseus ie 28 27200) <6 
Southrofi Muncie seyeacia teas Netvsina/sxe esters 6 23 36.00 “ 
Sal tePorksiunchon siya oe cere silat ener 15 44.00 “ 
Mid dles Bork: icrers.cre isos sera eetevistetersie sisiejereiehsess 22 45.00 <“ 
Big Viermilign 51s jssvere/s% viegs use lsps sere s.cie ares 6h 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 difficult 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 
