Shell lined with nacre; cardinal teeth (strictly, pseudocardinals) under 

 or before beaks, with the lateral teeth posteriorly; usually becoming 

 larger True Clams or Mussels 4. 



2. Anterior end longer than the posterior; apparently only one siphon; 



shell triangular 



P^sidium Pfeiffer 

 Anterior end shorter than the posterior; with two distinct siphons; shell 

 ovoid or rectangular 3. 



3. Embryonic part of shell distinct from the adult growth; shell rectangular 



Musculium Link 

 Not so, shell ovoid 



Sphaerium Scopoli 



4. Lateral teeth blurred or absent 5. 

 Lateral teeth complete 34. 



5. No indication of cardinal teeth 6. 

 Cardinal teeth present or represented by minute tubercles or swellings 



of the hinge line 16. 



6. Shell almost cylindrical; with fine, concentric umbonal markings; Missis' 



sippi and St. Lawrence drainages 



Anodonta ferussaciana Lea Cylindrical Paper Shell 

 Anodontoides ferussaaanus (Lea) 

 Shell thin below; umbonal markings usually double looped, nodulous or 

 parallel bars 7. 



7. Shell almost flat, not much longer than high, winged both anteriorly and 



posteriorly and gaping at both ends; central states 

 Ayiodoyita suhorhicuhta Say Flat Paper Shell 

 Utterbac\iana suhorhiculata (Say) 

 Not so 8. 



8. Umbones very flat, looking as if they had been sliced off; Mississippi 



drainage 



Anodonta ohiensis Raf . Paper Shell 

 Utterhac\ia imhecillis Say 

 Umbones rounded 9. 



9. Umbones swollen and extending above the hinge line; umbonal mark' 



ings usually nodulous (with minute tubercles) ; most of U. S. except 

 the extreme east and the Rocky Mt. area and westward 

 Anodonta grandis Say Floater 

 Umbones not so much elevated; umbonal markings not usually nodulous 



10. 



10. Dorsal margin not winged posteriorly; eastern states 11. 

 Dorsal margin usually straight and sloping slightly upwards to form a 



low wing posteriorly; central, western or southeastern statcG 13. 



11. No marked posterior ridge; posterior slope rounded; nacre dull silver; 



not common 



Anodonta marginata Say Silver Paper Shell 

 Posterior ridge somewhat double; nacre bluish to salmon 12. 



148 



