1371 FAUNA OF BIG VERMILION RIVER— BAKER 39 



the first two places it is very rare, only a few individuals having been 

 found by Professor Smith in a number of years. In a days search at 

 Homer Park by two experienced collectors, only two living specimens and 

 odd valves of two others were found. The largest specimen from Homer 

 Park measures 72 mm. in length and 63 mm. in height. This species is one 

 of the most abundant of shells in the Big Vermilion below Middle Fork, 

 where specimens measuring 100 mm. in length are common. The distri- 

 bution of this species is a good example of the progressive development of a 

 species in the downward course of a stream, for in the course of about 

 twenty miles the size nearly doubles. Beginning as a rare form at Homer 

 Park it becomes one of the most common forms in the Big Vermilion, 

 twenty miles downstream. 



All of the Big Vermilion tuberculata are of the compressed type, and 

 the shell is covered posteriorly and ventrally with large tear-like pustules. 

 The anterior third of the valve is free from pustulation. The nacre of all 

 shells seen is rich purple, which renders the species valueless for the button 

 makers. Tuberculata does not occur in the portions of the Sangamon River 

 examined. 



11. Elliptic gibbosus (Barnes). Lady-Finger; Spike. 



This mussel does not occur in Salt Fork, nor in any tributaries of the 

 Big Vermilion above Danville that have been examined. It is fairly com- 

 mon in the Sangamon River at Mahomet on a sand and gravel bottom. 

 Young and immature shells are distinctly rayed. The nacre of all speci- 

 mens examined has been purple, no white-nacred individuals being seen. 

 In the Kankakee River white-nacred specimens occur and become the 

 dominant form in the lower part of the stream (Wilson and Clark, 1912 :45). 

 In the Illinois River beds of shells occur which have either a white or a pur- 

 ple interior. (Danglade, 1914:42). This familiar shell will probably have 

 to be known as dilatatus (Rafinesque) if the original description is definite 

 enough to identify it as the gibbosus of Barnes. Dilatatus was described 

 in 1820. It is a pity that these names of Rafinesque could not have been 

 applied earlier to these shells and thus saved the confusion which is now 

 resulting from the changes of the old familiar names which zoologists in 

 our universities have used for years in connection with their classes in 

 systematic zoology. 



12. Unlomerus tetralasmus (Say). 



This species has been found living only in the upper waters of Salt 

 Fork and in Stony Brook near Muncie. It occurs in fair numbers in the 

 stream above Urbana and in Spoon River. Two broken valves were found 

 in Salt Fork at the station called natural dam about 12 miles below Urbana. 

 No living mussels could be found in the stream at this point and it is 

 believed that the odd valves were washed into Salt Fork from a small 



