44 ILLINOIS BIOLOGICAL MONOGRAPHS [142 
The large Anodontas in Crystal Lake are apparently not members of 
the original Salt Fork fauna. Neither grandis gigantea or corpulenia are 
found anywhere in the Big Vermilion drainage, at least above Middle Fork, 
45 miles below Urbana. Since these shells were planted in the lake (see 
p. 27) they have evidently thrived and multiplied. Anodonta grandis foot- 
iana is parasitic in the glochidial stage on the Johnny Darters (Boleosoma 
nigrum, Hankinson, 1908:235) and as this fish also inhabits Crystal Lake 
it may have been the medium for the propagation of the alienfauna. That 
this fauna should have been so easily detected as alien is due to the method 
of examining a stream from its source to its mouth and the distinguishing 
of the foreign population is a striking recommendation of this mode of 
stream study. 
16. Anodonta imbecillis Say. Paper-Shell. 
This beautiful paper-shell occurs abundantly in but one place in the 
Salt Fork—near the cement bridge east of Sidney. Here it is of good size, 
grass-green in color, the rest periods showing as black longitudinal bands. 
The shell is easily known from all others in this State by the very flat 
umbonal region which is flush with the upper or dorsal margin of the shell. 
The largest specimen in the collection measures 75 mm. in length. Imbe- 
cillis was not collected or observed above the cement bridge, 19 miles down 
stream from Urbana. It was, also, not seen below the bed at Homer Park 
and it appears to inhabit only that portion of the stream between these 
points, a distance of about 8 miles. This mussel thrives best on a mud _ bot- 
tom in quiet water and it is not found, normally, on a sand or gravel bottom. 
It did not occur in our Sangamon River collections. 
All of the individuals from Salt Fork bear evidences of distomid infec- 
tion. In nearly all of the valves there are many small pearl-like blisters 
about the size of a pin head which are in all cases confined to the posterior 
two-thirds of the shell. None were noted near the anterior end. 
The species is peculiar and almost unique among naiades in being 
hermaphroditic and in carrying the glochidia within the gills until they are 
ready for independent life, there being no parasitic stage encysted on fish 
as in the case of most Unionidae (Howard, 1914:353). It has an almost 
continuous breeding season, glochidia or embryos having been found in 
the gills during almost every month of the year. The Salt Fork specimens 
were gravid on August 26 and contained well formed glochidia. In this 
mode of reproduction imbecillis is parallelled by Strophitus edentulus, 
which also passes through its metamorphosis without parasitism. 
17. Anodontoides ferussacianus (Lea). Paper-Shell. 
This small naiad was found abundantly in but two places—the Middle 
Fork and Stony Creek near Muncie. _ It occurred infrequenty at all other 
stations. In the upper Salt Fork, north of Urbana, it was common at one 
