Common genera • — Lampsilis Ohovaria 



Proptera Glehula 



Carunculina Medionidus 



Truncilla Lemiox 



Plagiola Dysnomia 



Family DREISSENSIIDAE 



Small; no nacre; beaks terminal; valves narrow and unequal; 

 with conspicuous siphons; foot with byssus (thread or threads 

 used for attachment to rocks, etc.) ; Atlantic coast (Not included 

 in the key) 



Common genus — Congeria 

 Family CYRENIDAE 



Small; shell porcelain-like; with hinge plate; with cardinal teeth 

 and anterior and posterior lateral teeth; with two distinct si' 

 phons; southern Atlantic and Gulf coasts (Not included in the 

 key) 



Common genus — Polymesoda (Cyrena) 

 Family SPHAERIIDAE 



Small; no nacre; no hinge plate; with cardinal teeth and an' 

 terior and posterior lateral teeth; generally distributed (Some 

 times grouped with the preceding family) 

 Common genera — Sphaerium Musculium 



Pisidium 



Enpera (Tropical species, not included 

 in the key) 

 Family CYRENELLIDAE 



Small; no nacre; with cardinal teeth but no lateral teeth; with 

 two long siphons, contractile, completely united; Florida (Not 

 included in the key) 

 Common genus — Cyrenella 

 Family RANGIIDAE 



Large; no nacre; with cardinal teeth and anterior and posterior 

 lateral teeth; with two short siphons, united below; in brackish 

 waters of the Gulf States (Not in key) 

 Common genus — Rangia 



112 



