POST-WOODFORDIAN MOLLUSKS 19 



Ferrissia parallela (Haldeman) , La Rocque 1968, Ohio Div. Geol. Surv. 

 Bull. 62, pt. 3, p. 521, fig. 375. 



The shells of F. parallela are so named because the sides of the patel- 

 liform shell are straight and nearly parallel, although they widen somewhat 

 anteriorly; the apex is turned slightly to the right and is situated slightly anter- 

 ior of center. The shells are thin and delicate and easily broken, but many were 

 recovered from auger cuttings in the study of the two bogs. Although there was 

 some variation in the precise form of the shells recovered, it seems best to 

 refer all of them to F. parallela. 



This little ancylid, rarely more than 5 mm in length, is distributed 

 somewhat erratically through the faunal assemblages studied; how much the 

 apparent occurrence (or lack of it) depends upon the hazards of collecting such 

 delicate shells is not known. They thrive in a pond environment, especially 

 if large vascular plants such as Typha are present. 



F. C. Baker (1928, p. 397) quotes Walker as saying that F. parallela 

 is a northern species distributed from southern Canada to northern Ohio and 

 Indiana. A few records exist for southern states, however; and it is widely 

 distributed as a Pleistocene fossil in the Great Plains. 



Family Physidae 



Genus Physa Draparnaud 1801 



Species of the genus Physa are aquatic pulmonates with simple, sinis- 

 trally spiral shells. They are quite sensitive to local environmental factors, 

 and thus recognition of biological species is extremely difficult. The result has 

 been proliferation of names ad nauseam. For many years F. C. Baker (1928, 

 p. 408) preferred to recognize the genus Physella of Haldeman (1842, Monogr. 

 Limnaides N. America, pt. 8, p. 14, 38) on the basis of the digitations of the 

 mantle, but in the last years of his life Baker abandoned this stand. 



Physa gyrina hildrethiana Lea 1841 



Physa hildrethiana Lea 1841, Amer. Philos. Soc. Proc . , v. 2, p. 32. 



Physella gyrina hildrethiana (Lea), F. C. Baker 1928, Freshwater Moll. 

 Wisconsin, pt. 1, p. 453, pi. 27, fig. 36, pi. 28, figs. 2-4, 7-14. 



The physid shells that occur throughout the faunal assemblages at the 

 Strawn and Batavia localities (135-1, 135-2, 136; fig. 3) are referred to this 

 name combination with some misgivings, but the shells in question closely 

 resemble those previously referred to as P. g. hildrethiana. One of the problems 

 in such an assignation is that hildrethiana is described by Baker (1928, p. 454) 

 and others as living in temporary ponds, where few animals ever reach maturity 

 before the pond dries up. The shells recovered in these studies are small as 

 described, but there is no evidence that the ponds were in any sense ephemeral, 

 since they supported throughout their history abundant populations of branchiate 

 mollusks that do not survive drying conditions for very long. Whatever the fact may 

 have been, physid shells occur in moderate numbers in every faunal assemblage 

 studied, and with almost no variation of shell characters. The animals must 

 have formed a significant and persistent element of the total molluscan fauna. 



Genus Aplexa Fleming 1820 



Although the shells of this genus are sinistral like those of Physa , 

 animals have been separated from that genus on the basis of mantle and other 



