POST-WOODFORDIAN MOLLUSKS 17 



that of H. oampanulatum is flatly coiled. Living H. antrosa is widely distributed 

 in North America, but populations are especially numerous east of the Missis- 

 sippi River. 



Helisoma trivolvis (Say) 1817 



Planorbis trivolvis Say 1817, Nicholson's Encyclopedia, Amer. Ed., 

 v. 2, pi. 2, fig. 2 (no pagination) . 



Helisoma trivolvis (Say), F. C. Baker 1928, Freshwater Moll. Wisconsin, 

 pt. 1, p. 330, pi. 20, figs. 1-13, 22, 23. 



The shells of this large pulmonate aquatic gastropod may exceed 3 mm 

 in diameter. It is widespread in North America, except in the western mountains, 

 where it occurs less frequently. In the Pacific area, populations previously 

 referred to trivolvis are often given other names. H. trivolvis is not common 

 in the collections from the two bogs under study, and those populations that do 

 occur are obviously late colonizers (fig. 3) since shells occur only in the upper- 

 most 4 to 6 feet at localities 135-1 and 135-2. No shells of H. trivolvis were 

 recovered at the Batavia bog (locality 136). 



Helisoma oampanulatum (Say) 1821 



Planorbis oampanulatus Say 1821, Acad. Nat. Sci. Phila . Jour. , v. 2, p. 166. 

 Helisoma oampanulatum (Say), F. C. Baker 1928, Freshwater Moll. 

 Wisconsin, pt. 1, p. 345, pi. 21, figs. 1, 2, 4, 5, 8, 9, 13, 14. 



H. oampanulatum is about the same size asH. antrosa , that is, the shell 

 is 10 to 15 mm in diameter; the aperture is expanded in a bell-like fashion, 

 hence the name. Superficially it resembles H. antrosa; its distinguishing 

 characters have been noted. 



H. oampanulatum occurs infrequently in the assemblages from localities 

 135-1 and 135-2, but is present in every one of the seven faunal collections 

 from the Batavia bog, locality 136 (fig. 3). The living species is distributed 

 largely in northeastern United States, but records exist for several widely 

 scattered localities in Canada. It does not seem to occur south of Illinois and 

 Ohio. 



Genus Planorbula Haldeman 1842 



The shells of the pulmonate aquatic gastropods of this genus are typically 

 planorbid, the whorls are tightly wound, and the lip is more or less thickened 

 within. The truly characteristic feature of these shells, however, is the persistent 

 set of six dentiform lamellae a short distance back from the aperture. The inter- 

 nal anatomy is similar to that of Gyraulus. The genus occurs in North America 

 and in Africa . 



Planorbula armigera (Say) 1818 



Planorbis armigerus Say 1818, Acad. Nat. Sci. Phila. Jour., v. 2, p. 164 

 Planorbula armigera (Say), F. C. Baker 1928, Freshwater Moll. Wisconsin, 

 pt. 1, p. 355, pi. 8, figs. 27-30. 



Planorbula armigera displays the characters of the genus; the planorbid 

 shell is about 7 mm in diameter. According to Baker (1928, p. 359), it is dis- 

 stributed from New England and Great Slave Lake to Nebraska and south to 

 Georgia and Louisiana. It is widespread as a Pleistocene fossil. 



