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 is a 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. Z2)). 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 di'ainage 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 



