46 ILLINOIS BIOLOGICAL MONOGRAPHS {144 
the right valve is elongated and thinner than in normal individuals and 
the lateral teeth in both valves are scarcely visible. In the left valve there 
is an abnormally high, long and narrow tooth under the beak. There is 
a pronounced lunule in front of the umbones which is absent in typical 
compressa. This species occurs infrequently in mud at Homer Park. In 
the Middle Fork a single dead and broken shell was found. No represen- 
tatives of this species were found in the Sangamon River. 
20. Lasmigona (Lasmigona) costata Rafinesque. Fluted Shell. 
This characteristic mussel is fairly common at most stations visited 
from bench mark 655 (fifteen miles below Urbana) down the stream to 
Middle Fork. It probably inhabits the lower Big Vermilion to the Wabash 
River. Living specimens, however, were not seen above the station four 
miles above Homer Park dam, nearly 23 miles from Urbana. From Homer 
Park down stream it isa common mussel. The individuals are for the most 
part fine, large, heavy shells with good clean lustre. The shells from Homer 
Park have an olive epidermis beautifully marked with green rays. The 
largest individual collected measured 145 mm. in length and was found at 
the station three and a half miles above Homer Park dam (Z11116A). 
The majority of the specimens of this species are colored light salmon 
on the interior of the shell. Pathological individuals are rare in the collec- 
tions. One specimen from Salt Fork near Middle Fork, found on a sand 
bottom, has a large pearl blister on the posterior margin. A shell from 
Homer Park, taken from a gravel bottom, has an injury in the form of a 
crack in the shell on the outside which had been repaired on the inside by 
the addition of pearly matter forming a long, raised blister, 45 mm. long 
and 2 to 5 mm. wide (Fig. 33). This nodulous blister reaches almost 
to the center of the shell (Z11192 A). Gravid individuals were collected on 
October 8 and 13, 1920. 
The costata from the Sangamon River, where the species is common, 
are somewhat heavier than those from the Big Vermilion River. The 
shell is also less high in comparison with its length. Young individuals of 
this species from either river drainage are very rare, judging by our collec- 
tions. 
21. Lasmigona (Pterosygna) complanata (Barnes). White Heel-Splitter. 
This large, roundish, flat mussel is the most abundant species in the 
Big Vermilion River, occurring commonly or abundantly in all parts of the 
stream, excepting a small stretch of about five miles near St. Joseph, from 
Spoon River to the neighborhood of Danville. It also probably occurs in 
equal abundance below Danville. The abundance of this species in Spoon 
River and below the station bench mark 655, with the break of five miles 
between the beds of living mussels, is strong evidence of the effect of sewage 
pollution. In this barren area only empty shells and odd valves could be 
