12. Posterior ridge ending below the mid line; nacre thickened at the mar' 



gin and anteriorly, darker in the cavities; not common 

 Anodonta implicata Say 

 Posterior ridge ending at about the mid line; shell thin throughout; 

 nacre bluish; common 



Anodonta cataracta Say Eastern Floater 



13. Usually with faint wavy rays (color markings); southeastern states 



Anodonta couperiana Lea Wavy'rayed Floater 

 Usually rayless or with a few faint rays; western species 14. 



14. Posterior ridge curved slightly upwards and ending in a slight turned' 



up projection 



Anodonta wahlametensis Lea 

 Posterior ridge straight 15. 



15. Shell very thin; beak sculpture doublc'looped 



Anodonta impura Say Western Floater 

 Shell more solid; beak with almost straight, parallel markings 

 Anodoyita oregonensis Lea Oregon Floater 



16. Shell fluted on posterior slope 17. 

 Posterior slope not fluted 19. 



17. Length about two times the height; central states 



Lasmigona costata (Raf.) Fluted Shell 

 Length not over one and onc'third times the height 18. 



18. With a double row of pustules down the center; Mississippi and Ohio 



drainages and west to Texas 



Arcjdeyis coy^fragosa (Say) Rock Pocketbook 

 Posterior half of shell with oblique wrinkles or folds; Arkansas to 

 Oklahoma 



Ar\ansia wheeleri Walker and Ortmann 



19. Shell not much longer than high, flattened, and with a low or high pos' 



terior wing; central states 



Lasmigona complanata (Barnes) White Heel Splitter, Hatchet- 

 back 

 Not so 20. 



20. With two distinct posterior ridges; Cumberland and Tennessee Rivers 



Pegias fahida (Lea) 

 Only one or no pronounced posterior ridge 21. 



21. Little or no posterior ridge; shell lengthened 22. 

 With a posterior ridge; shell oval or triangular 28. 



22. Cardinal teeth heavy; nacre tinged with red or purple; most of U. S., 



in mountain streams 



Margaritana margaritifera (Linn.) River Pearl Mussel 

 Cardinal teeth very small or blurred; nacre white, salmon or purple' 

 tinted 23. 



23. Shell gaping before and behind; shell slightly twisted when viewed from 



above or below; rare 24. 



Shell not normally gaping; not twisted 25. 



149 



