56 POLYGYRIDAE 



POLYGYRA THYROIDUS (Say) 



A snail with a horn-colored shell, often about an inch (20-25 

 mm.) in diameter, Polygyra thyroidus may be recognized by 

 the distinctly open umbilicus and the wide, flaring peristome. 



The majority of specimens have 

 a rather distinct denticle on the 

 parietal wall, but this denticle is 

 absent in many individuals. The 

 characteristics noted will distin- 

 guish thyroidus from any other 

 snail found in Illinois. 



A notable feature of thyroidus 

 is the great variation in size (15-30 mm.). This may occur in 

 the same colony, or one colony may consist of large individuals 

 and another of small specimens. The largest specimen collected 

 in Illinois measures over 1% inches (29 mm.) and the smallest 

 about five-eighths inch (16 mm.) Studies of Illinois thyroidus 

 made by Dr. Thural Dale Foster indicate that the size variation 

 occurs in all material examined, and also that the absence or 

 presence of the parietal tooth is simply an individual variation 

 which cannot be used in classification. 



The animal is grayish or yellowish-white with eye peduncles 

 darker in color. The underside of the foot is a dirty white. 



This snail is found over most parts of Illinois but is appar- 

 ently not common in the extreme northern portion. Its most 

 favorable habitat is on the floodplains of the larger river valleys 

 in woods of oak, elm, beech, hickory and willow. In these places 

 there is usually an abundance of fallen timber rotting on the 

 ground, and a large amount of debris which affords shelter for 

 the snails. On humid days, the snails may often be seen crawling 

 over the ground, over logs or even on standing trees several feet 

 from the ground. 



The name here used, thyroidus, is given the original spelling 

 applied by Thomas Say in 1817 in the Journal of the Phila- 

 delphia Academy of Natural Sciences. Say later used the same 

 spelling in his American Conchology. Subsequent authors have 

 used the spelling thyroides, which is probably etymologically 

 correct, but, since Say has used the original spelling in several 

 places, later authorities have no right to introduce a different 

 spelling. 



