POLYGYRIDAE 57 



POLYGYRA CLAUSA (Say)' 



The snail Polygyra clausa is easily confused with small 

 specimens of Polygyra thyroidus. However, clausa generally 

 averages smaller than thyroidus, the shell of the former measur- 

 ing about one-half to five-eighths 

 of an inch (14-17 mm.) in diam- 

 eter. The shell of clausa, which is 

 yellowish, is comparatively higher 

 than that of thyroidus. Further- 

 more, the umbilicus of clausa is 

 much smaller than that of thy- 

 roidus ; sometimes it appears as 

 a mere slit behind the reflected 

 peristome, which is comparatively narrow and somewhat round- 

 ed, not widely expanded and flat as in thyroidus; clausa never 

 has a parietal denticle. These details separate the two species. 



The animal of Polygyra clausa is generally blackish in color. 



Polygyra clausa is widely distributed over Illinois, although 

 usually not many individuals occur in a single colony. For the 

 most part, it "lives in river valleys, in forests of oak, elm and 

 hickory. It has been collected from railway embankments and 

 other parts of the rights-of-way where grass and small shrubs 

 grow abundantly. Such places may be described as prairie 

 habitats. 



Thomas Say described this species as Helix clausa in 1821, 

 in the Journal of the Philadelphia Academy of Natural Sciences, 

 in which were published descriptions of many of the land and 

 freshwater species which he discovered. It is interesting to know 

 that the first specimens came from Illinois, and that Say gave 

 this state as the type locality. 



Polygyra clausa is found in many parts of the United States. 

 Records are available indicating its presence from Minnesota 

 southward to Kansas and Alabama and eastward to western 

 Pennsylvania. Throughout this area it exhibits little variation 

 and in this respect it appears to be one of the most distinct species 

 of American land snails. In Alabama and other parts of the 

 South it might be confused with the small form of thyroidus, 

 known as bucculenta, but in that variation the shell is usually 

 without an umbilical opening and the aperture is not rounded as 

 in clausa. 



