52 ILLINOIS BIOLOGICAL MONOGRAPHS [150 



below the stations examined where the river is larger. Iris 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 large 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 o f 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 be a 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 



" 116 " 68 " 40 mm. Male 



" 116 " 69 " 46 mm. Female 



" 100 " 65 " 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 



