MOLLUSCA OF MICHIGAN— WALKER. 471 



Stenotrema monodon, W. G. Biiinev, Man. Am. Land Shells, 280, fig. 298-9, 



(1885). ] 

 Helix jraterna, Sager, Doc. H. Rep. Mich.. 1839, 420, separate, 14. 



, Miles, Rep, Geol. Surv. Mich., 235, (1861). 



monodon, Miles, Rep. Geol. Surv. Mich., 236, (1861). 



Stenotrema , Currier, Shell-bearing Moll. Mich., 6, (1868). 



: , DeCamp, Shell-bearing Moll. Mich., 7, (1881). 



fraterna, DeCamp, Shell-bearing Moll. Mich., 7, (1881). 



Polygyra monodon. Walker, Rev. Moll. Fauna Mich., 17, (1894). 

 /ra^erna, Walker, Rev. Moll. Fauna Mich., 17, (1894). 



Var. ALBIDA Walker. 



Pale greenish-white. 



Polygyra monodon albida, W^alker, Terr. Moll. Mich., 15, (1899). 



The umbilicate form has an abundant and general distribution in all parts 

 of the state. The imperforate variety is apparently restricted to the south- 

 ern part of the state, while the var. albida has thus far been reported only 

 from Charlevoix and Grand Rapids. This species has been commonly 

 known as P. monodon Rackett, the imperforate form being designated as 

 var. fraterna Say. But it seems that Rackett 's species is that described by 

 ^Vard as P. leaii. It follows that this form must be given Say's name. 

 The umbilicate and imperforate forms inosculate too closely to justify vari- 

 etal distinction. 



POLYGYRA MONODON (Rackett). 



Shell umbilicate, giobosely depressed, light horn-color, whorls 

 ^ 6, obsoletely striate, narrow, slowly and regularly increasing, con- 

 C^^ vex, the last somewhat flattened below the periphery and con- 

 stricted behind the peristome; suture distinctly depressed; base 

 convex, rather widely excavated around the umbilicus; a promi- 



®nent oblique white tooth on the parietal wall; peristome narrowly 

 reflected. 



Alt. 5, greater diam. 9, lesser 8 mm. ♦ 



Alt. 4, greater diam. 7, lesser 6^ mm. 

 Helix monodon, Rackett, Linn. Trans. XIIL, 42, pi. V.. fig. 2, 



(1822). 

 Stenotrema monodon leaii, W. G. Binnev, Man. Am. Land Shells, 281, fie;. 



297, (1885)^. ' 



leai, Currier, Shell-bearing Moll. Mich., 6, (1868). 



leaii, DeCamp, Shell-bearing Moll. Mich., 7, (1881). 



Polygyra , Walker, Rev. Moll. Fauna Mich., 17, (1894). 



Generally distributed south of the Saginaw-Grand valley. Alpena, the 

 type locahty, is the only reported locality further north. 



This abundant little species is easily distinguished from P. fraterna by its 

 smaller size, with one more whorl, larger umbilicus and wider basal exca- 

 vation and lack of epidermal hairs. Its habits also are quite different as it 

 is usually found in low damp places, such as river margins, where it is asso- 

 ciated with the Succineas, Zonitoides nitidus, Pomatio'psis lapidaria and 

 other small moisture-loving species; while fraterna is usually found in the 

 dryer localities frequented by the larger helices. It has until quite recently 

 been known as P. leaii (Ward), but Dr. Pilsbry has shown that this form 

 w^as the type of Rackett's species. 



Fig. 24. 

 P. monodon. 



