POLYGYRIDAE 61 



POLYGYRA FRATERNA (Say) 



The brown shell of Polygyra fraterna is usually larger than 

 that of Polygyra monodon, and specimens almost one-half inch 

 (11 mm.) in diameter are not uncommon. 



An upland species, Polygyra fraterna inhabits forests of oak, 

 elm, hickory and ironwood, on hillsides or on limestone cliffs. 



Polygyra fraterna fraterna (Say). The typical fraterna, 

 with a shell almost one-half inch (10 

 mm.) in diameter, differs from Pol- 

 ygyra monodon in having the umbili- 

 cal region almost closed and rather 

 deeply indented, and also in having a 

 heavier and longer parietal plait, as 

 shown in the figure. It is abundant on 

 the blufifs of the Ohio and Mississippi 

 rivers, where monodon is absent. In 

 distribution, fraterna fraterna is more 

 common in the southern than in the northern part of Illinois. 



Polygyra fraterna cava Pilsbry & Vanatta. The shell of the 

 variety fraterna cava is larger (11 mm.) than that of Polygyra 

 monodon or fraterna fraterna. It is characterized by an indented 

 umbilical region and by a distinct umbilical perforation which 

 is not so wide nor so nearly round as that of monodon. The 

 variety fraterna cava is rare in Illinois. It has been collected 

 only from Jo Daviess, Carroll, Whiteside, McHenry, Menard 

 and Clark counties. It is, therefore, a form of the northern and 

 central parts of the state. 



The changes of molluscan names and the shifts in their 

 positions in classification are well shown by the history of the 

 monodon-fraterna group of land snails. At one time the small 

 form now known as Polygyra monodon was known as Polygyra 

 leai. Dr. Henry A. Pilsbry indicated that this form was the 

 same as Rackett's monodon, and that the larger form with the 

 partly closed umbilicus was fraterna. For a long time fraterna 

 was placed as a variety or race of monodon, but it is now con- 

 sidered a distinct species. 



Like all other snails of wide distribution, for monodon and 

 fraterna are found from northern United States to Texas, these 

 species have developed several varieties or races, especially in the 

 southern states. 



