54 



POLYGYRIDAE 



POLYGYRA APPRESSA (Say) 



Usually about one-half to three-fourths inch (15-21 mm.) 

 in diameter, the depressed, horn-colored shell of Polygyra 

 appressa is almost twice as wide as high. The umbilicus is 



covered by a callus. The shell, 

 which has 5 whorls, is marked by 

 a well-developed plait or denticle 

 on the parietal wall and a very 

 small denticle on the peristome. 



In the form of the aperture, 



appressa somewhat resembles 



three other species of Polygyra 



found in Illinois: these are tri- 



dentata, palliata and fosteri. 



Polygyra appressa is rare in the northern part of Illinois and 



has not been seen from the region north of La Salle and Will 



counties. It is most abundant in the southern part of the state 



on the bluffs in the Ohio, Wabash and Mississippi valleys. 



POLYGYRA FOSTERI F. C. Baker 



The horn-colored Polygyra fosteri shell, over one-half inch 

 (15 mm.) in diameter, is characterized by a long, curved, rather 

 heavy plait on the parietal wall and a relatively large labial 



denticle. These features very readily 

 separate fosteri from Polygyra ap- 

 pressa, for in appressa the plait on 

 the parietal wall is small, short and 

 almost straight, and the peristome 

 is marked by only a slight ridge on 

 the lower part of the outer lip. 



There are anatomical differences 

 between the animals of fosteri and 

 appressa. 



Polygyra fosteri is the common appressa-Wkt shell on the 

 limestone bluffs of the Ohio and Mississippi rivers. It is inter- 

 esting to note that the great naturalist Thomas Say distinguished 

 this shell more than a hundred years ago (1821), calling it 

 var. a. The name Polygyra fosteri was published in October, 

 1932, in the conchological monthly periodical, the Nautilus. 



