36 ILLINOIS BIOLOGICAL MONOGRAPHS ■ [134 



also collected in the Big Vermilion River. One of the largest adult individ- 

 uals found, below Homer Park dam, measured 95 mm. in length; another 

 from the Big Vermilion River measured 103 mm. in length. The species 

 occurs about equally on a mud or sand-gravel bottom. 



A single example from Homer Park is worthy of special note. It is 

 large, inflated, almost twice as wide as the average shell of the same size, 

 and is elongate-quadrate in outline. When viewed from within, the 

 valves are basin-shaped. All of the muscle scars are very heavily impressed 

 and the pseudocardinal teeth are much modified and heavier than in normal 

 rubiginosa. The lateral teeth are very high, wide and massive. The 

 shell was dead when picked up and badly discolored and notes on the ani- 

 mal, which would have been very desirable, could not be made. The 

 measurements of this shell, together with that of a normal shell of the 

 species, from the same habitat, are given below: 



Length, 90; height, 59; width, 50 mm. Z11163 A, Variety. 

 " 82 " 58 " 36 mm. Z11163 B, Normal. 



This species also occurs in the Sangamon River, but does not, appar- 

 ently, attain the dimensions of the Salt Fork specimens, nor does it occur 

 as abunantly. There is but slight variation in form in the Sangamon 

 shells. No pathological specimens were observed in individuals from either 

 river. Rubiginosa is not much esteemed by either pearlers or mussel fisher- 

 men. 



Rubiginosa frequently closely resembles Pleurobema coccineum in the 

 form of the shell and specimens occur which seem difiicult to place satisfac- 

 torily. The animals diflfer in that in rubiginosa all four of the gills are used 

 as marsupia while in coccineum only the outer gills are so used. As far as 

 the Salt Fork and Sangamon River shells are concerned there has been no 

 difficulty in placing any individual. In this material rubiginosa is always 

 quadrate with the umbones large and full, the posterior end of shell is 

 almost sharply truncated and there is a more or less distinct ridge extend- 

 ing from the umbones to the posterior angle of the shell. In coccineum 

 the outline is rather ovate, or rounded, there is no posterior ridge and the 

 position of the umbones gives to the shell an oblique appearance which is 

 very characteristic and is absent in rubiginosa. The ventral margin in 

 coccineum is almost always convex and seldom straight or concave as in 

 rubiginosa. The young shell in coccineum is also usually more distinctly 

 rayed. The surface of the two species is also different, that of coccineum 

 not being 'satiny' as is that of rubiginosa. The interior, and even the 

 exterior, of coccineum is usually pinkish or salmon colored, although 

 individuals occur with white nacre. 



