Genu's Obovaria Rafinesque. 

 Subgenus O'BVAria Rafinesque. 



1. Group of Obovaria retusa. 

 Subgenus Pseudoon Simpson. 



2. Group of Obovaria ellipsis. 

 Genus Plagiola Rafinesque. 

 Subgenus Plagiola Rafinesque. 



1. Group of Plagiola securis. 



Subgenus Amygdalonaias Crosse and 

 Fischer. 



2. Group oi 'Plagiola elegaus. 



3. " " " donaciformis. 

 Subgenus Ptychoderma Simpson. 



4. Groui> of Plagiola cyrcnoides. 



5. " " " nicaragiiensis. 



DIGEN^. 

 Genus Tritogonia Agassiz. 



MESOGEN.E. 

 Genus C^'PROGiiNiA Agassiz. 

 Genus Obliquaria Rafinesque. 



PTYCHOGEN^. 

 Genus Ptychobranchus Simpson. 



ESCHATIGEN^. 

 Genus Dromus Simpson. 



DIAGEN^. 

 Genus Strophitus Rafinesque. 



HOMOGEN^. 

 Genus Anodonta Bruguiere. 



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