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 alien fauna. 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 imbecilMs 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 longittidinal 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 



