Spire slightly elevated, convex. 



Whorls rapidly enlarging- as seen from 

 above, umbilicus narrow. 



Genus VITREA 36 

 Whorls enlarging more gradually, su- 

 ture more impressed, whorls more 

 rounded above. Genus ZONITOIDES 37 

 **-Peristome more or less reflexed. 



i — Shell not more than 3 mm. in diameter. 



Conical, compact, whorls narrow, body whorl with 



revolving lamellae within. Genus STROBILOPS 38 

 Depressed, with rapidly enlarging whorls; aper- 

 ture without teeth or folds. Genus VALLONIA 39 

 ii — Shell more than 5 mm. in diameter. 



Genus POLYGYEA 40 

 4 — Shell very oblique, body whorl enlarged and elongated. 



Genus SUCCINEA 41 



Ill-With bivalve shell. 



A — Small, less than 25 mm. long. 



a — Very oblique, mostly very small and inflated. 



Genus PISIDIUM 42 



b — Less oblique, larger. 



Valves very thin, fragile. Genus MUSCULIUM 43 



Valves thicker ; opaque and usually brown. 



Genus SPH^EIUM 44 



B — Larger, more than 25 mm. long. 



a — Pseudocardinal and lateral teeth both present. (The old 

 genus JJnio.) 

 1 — Without tubercles or plications. 



* — Elongated, lower margin usually nearly parallel with 

 hinge line. 



Shining, usually rayed; beaks sculptured with fine 

 ridges; sometimes winged above the hinge. 



Genus LAMPSILIS 45 

 Dull, not rayed; beaks with coarse ridges; not 

 winged. Genus UNIO 46 



(Toothed forms of Margaritana monodonta 

 might also be sought here.) 

 **-More nearly iso diametric. 



i — Posterior ridge well developed, 

 o — Epidermis dull brown. 



Genus QUADRULA (in part) 47 

 oo-Epidermis greenish or yellow; usually with 

 rays. 



Posterior truncate area with longitudinal 

 wrinkles. Genus TBUNCILLA 48 



78 



