POLYGYRIDAE 45 



POLYGYRA TRIDENTATA (Say) 



The shell of Polygyra tridentata is approximately one-half 

 inch (13 mm.) in diameter and about twice as wide as high, is 

 horn colored, much depressed, has 5V2 whorls and a distinct 

 umbilicus. In this species the 

 aperture normally has 3 den- 

 ticles or plications, 1 on the pa- 

 rietal wall and 2 on the white 

 peristome, which is widely re- 

 flected. The sculpture of the 

 shell of tridentata consists of 

 distinct riblike lines of growth. 



The animal is dark blue or slate in color, and the eye 

 peduncles are one-fourth inch (6.5 mm.) in length. 



The shell of this species is abundant in parts of Illinois. 

 Most common in the southern part, it has been collected in the 

 Illinois Valley from Cook County southward to Fulton County. 

 Its metropolis is in forests of oak, elm, hickory, dogwood and 

 sassafras on limestone bluffs near the junction of the Ohio and 

 Mississippi rivers. It may be found under forest debris, particu- 

 larly in ravinelike spaces between ledges of limestone. 



Polygyra tridentata tridentata (Say). In the typical form 

 of tridentata the two denticles on the peristome are widely 

 separated, the space between being almost twice as wide as deep, 

 as shown in the figure. Four distinct variations from the typical 

 form are found in Illinois. The distinguishing characteristics 

 of these are noted below. 



Polygyra tridentata frisoni F. C. Baker. In this variation 

 the denticles on the peristome are larger and nearer together 

 than in the typical form, and the width and height of the space 

 between the denticles are about equal. The shell of this variety 

 is pictured on page 18. 



Polygyra tridentata bidentata F. C. Baker. This is a form 

 in which the two denticles on the peristome are present, but 

 the plication on the parietal wall is absent. 



Polygyra tridentata unidentata F. C. Baker. The one- 

 toothed form has a plication on the parietal wall, but the 

 denticles on the peristome are absent. 



Polygyra tridentata edentilabris Pilsbry, In this variation 

 the aperture is entirely without denticulations. 



