POST-WOODFORDIAN MOLLUSKS 11 



P. compression is distributed widely in North America, from northern 

 Canada to California and Mexico. 



Pisidiwn nitidwn Jenyns 1832 



Pisidiwn nitidwn Jenyns 1832, Cambridge Philos. Soc . , v. 4, p. 304, 

 pi. 20, figs. 7-8. 



Pisidiwn nitidwn Jenyns, Herrington 1962, Revision Sphaeriidae 

 N. America, p. 45, pi. 5, fig. 6, pi. 7, fig. 17. 



Pisidiwn nitidwn is a small pelecypod, usually less than 3 mm in length; 

 it has a generally rounded form, inconspicuous beaks, and very fine striations 

 on the shell. It is distributed widely in Europe as well as in North America. 



It occurs in 24 of the 2 7 faunal assemblages studied (fig. 3), but at 

 none of them were the valves numerous. In many of the assemblages, pairs of 

 valves occurred intact, indicating that the species is indigenous to the deposits 

 even though not abundant in them. Some of the shells possess the characters 

 referred to as the "form pauperoulwn, " which does not seem to be a valid 

 taxonomic entity. 



Class Gastropoda 



Subclass Streptoneura 

 Order Ctenobranchiata 

 Family Valvatidae 



Genus Valvata Miiller 17 74 



The genus Valvata encompasses a few species of small, spiral, turbinate 

 gastropods that carry on gaseous exchange by means of gills; they therefore re- 

 quire permanent water, reasonably free of silt. There are probably no more than 

 three or four species of this genus, which is unique to North America, but many 

 "forms" of little or no taxonomic value have been described (Baker, 1928, p. 7- 

 32; La Rocque, 1968, p. 358-368). 



Valvata lewisi Currier 1868 



Valvata lewisi Currier 1868, Kent Sci. Inst. Misc. Pub. No. 1, p. 9. 

 Valvata lewisi Currier, La Rocque 1968, Ohio Div. Geol. Surv. Bull. 62, 

 pt. 3, p. 360, fig. 211. 



Valvata lewisi is a small gastropod with a spiral, turbinate shell bearing 

 regular striae that have been compared to the winding of a thread on a spool. 

 The shells are about 4.5 mm in diameter, and are 3 or more mm high. In this 

 study, V. lewisi occurred in only 8 of the 27 faunal assemblages, and in 

 these the species was represented by no more than 1 to 4 shells. Occurrences 

 were limited to the deeper levels of localities 135-1 and 135-2; no shells were 

 found in the abundant faunal assemblages at locality 136 (fig. 3). 



This species is distributed across Canada and the northern part of the 

 United States; it has been reported living in La Salle County, Illinois (Baker, 

 1928, p. 28). 



Valvata tricarinata (Say) 1817 



Cyclostoma tricarinata Say 1817, Acad. Nat. Sci. Philadelphia Jour., 

 v. 1, p. 13. 



Valvata tricarinata (Say), F. C. Baker 1928, Freshwater Moll. 

 Wisconsin, pt. 1, p. 11, pi. 1, figs. 1-3. 



