139] FAUNA OF BIG VERMILION RJVER— BAKER 41 



13a. Strophitus edentulus pavonius (Lea). 



This variety is described by Simpson (1914:348) as "Shell generally long 

 elliptical; epidermis yellowish-green, more or less covered with green or 

 brownish-green rays." In its typical form pavonius is easily separable 

 from edentulus. The variety is almost as common in Salt Fork as is the 

 typical form and there are many intermediate individuals. It is possible, 

 however, to separate all of the edentulus from the different stations, 17 lots, 

 into two groups; one with rhomboid or long-ovate outline and with black 

 or brownish, rayless surface; and the other with long-elliptical outline, 

 brownish surface, and many distinct rays. In pavonius the height is less 

 as compared with the length than in edentulus. These two forms of 

 Strophitus were almost always associated together, indicating their close 

 relationship. On the whole pavonius is much less variable than typical 

 edentulus. Specimens from the Sangamon River at Mahomet are brilliantly 

 rayed with patches of bright green on the ventral margin of the shell. 



Wilson and Clark (1912:48) state that the "question of rays appears 

 to be closely related to clearness of water; in turbid streams mussels are 

 usually dull colored, while in clear streams they are usually brightly 

 rayed." This has been our observation in many cases, but the rayed 

 pavonius in Salt Fork occurs with the rayless edentulus in quiet water 

 on a mud bottom; both also occur in riffles on a sand-gravel bottom. 



Pavonius is credited by Simpson to Ohio and Indiana; it is probably 

 widely distributed in Illinois, but has most likely been listed under edentu- 

 lus in most cases. It is known from^ Cook County, Will County, and the 

 Wabash River (Baker, 1906:72). Its presence in the Sangamon River 

 indicates that it is also an inhabitant of the Mississippi River drainage, 

 as well as the Wabash and Ohio drainages. It is quite probable that the 

 distribution of the variety is coincident with that of edentulus. The nacre 

 of both edentulus and pavonius is usually yellowish or salmon colored al- 

 though white-nacred specimens occur. Pearly growths are not as common 

 among the shells of this species as found in the region under consideration 

 as among the same species from other places. A few individuals had blister 

 and pin-head pearls. A specimen each of the type and the variety had a 

 peculiar pearl formation on the pallial line at or near the posterior end of 

 the shell. These are somewhat dome-shaped, about 5 mm. in diameter 

 and 4 mm. in height and evidently were caused by an effort on the part of 

 the mollusk to cover some irritating object, possibly a nematode worm 

 (Figs. 31, 32). As both shells were without the animal {pavonius had 

 been alive very recently) this point could not be determined. The 

 edentulus was from the railroad bridge east of Sidney (No. Z11098) and 

 the pavonius from below Homer Park dam (No. Z11144 A). 



