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 ie oe Ct} “ 36mm. 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 difficult to place satisfac- 
torily. The animals differ in that in rwbiginosa 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 rwbiginosa 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. 
