139] FAUNA OF BIG VERMILION RIVER—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 riffies 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). 
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