52 ILLINOIS BIOLOGICAL MONOGRAPHS {150 
below the stations examined where the river is larger. Jris has not been 
recorded from the Sangamon River. 
32. Lampsilis luteola (Lamarck). Fat Mucket. 
The mussel known among fishermen as the fat mucket is common or 
abundant almost everywhere in the Big Vermilion and Sangamon rivers. 
It has been killed by the sewage of the Salt Fork from St. Joseph to bench 
mark 655, a distance of five miles down the stream, but the number of dead 
and empty shells found almost everywhere between these points indicates 
that at one time, not very remote, it was common continuously from Spoon 
River, where it now lives in some abundance, to the Wabash River. Below 
the dam at Homer Park it is very common and of darge size, and this abun- 
dance continues down the stream and was also noted in the tributary Mid- 
dle Fork. At Mahomet on the Sangamon River it is also abundant. 
There is great variation both in form and coloration among the shells 
of this species in all of the habitats examined. The male shells are usually 
pointed at the posterior end and are elongated and somewhat compressed. 
From this form they vary by being quadrate in outline with a distinctly 
plow-shaped posterior end, corpulent and almost cylindrical, or flattened 
and oval, in this form greatly resembling Actinonaias ligamentina, from 
which they may be distinguished by the numerous double-looped ridges 
on the umbones. The female shells do not differ so greatly in shape, the 
post-basal swelling for the accommodation of the enlarged branchial marsu- 
pium giving more uniformity to the shell, the variation being principally 
in the width of the shell, which in old specimens is very pronounced. Male 
shells greatly predominate in the collections. In color there is every 
gradation between a bright yellow shell with distinct, narrow dark green 
rays, to a shell that is dark yellowish or brownish without rays or with the 
rays only faintly developed. A few specimens are dark brown or even 
pinkish with narrow, greenish rays. Young shells are very brightly rayed, 
the rays being dark grass-green on a light yellowish background, forming a 
beautiful surface ornamentation. The rays on the adult shells may be 
narrow or broad, or the broad rays may be made up of many fine rays, 
which may also bea trifle wavy. The nacre in all specimens examined from 
the two rivers here considered is pearly white, unmarked by color of any 
kind. The largest specimens seen occur at Homer Park; measurements of 
these are given below: 
Length, 110; height, 59; breadth, 35 mm. Male 
hil) OS «40 mm. Male 
OE Ks 69 «46 mm. Female 
S00) Sea Oo) « 48 mm. Female 
Pearly growths were observed in many of the specimens collected. 
Occasionally a few pin-head pearls occur in a valve but the greatest number 
