Posterior truncate area without distinct 

 longitudinal wrinkles. Genus PLAGIOLA 49 

 ii — Posterior ridge absent or indistinct. 



Very oblique, with beaks well at anterior end. 



Genus OBOVAEIA 50 

 Less oblique. Genus QUADRULA (in j^art) 47 

 2 — With tubercles or plications. 

 * — Elongated. 



Tubercles numerous, well defined. 



Genus TRITOGONIA 51 

 Indistinct tubercles or short folds extending across 

 shell in oblique row from beak. 



Genus PLEUROBEMA 52 

 Shell somewhat wrinkled posteriorly, often winged. 



Genus SYMPHYNOTA (in part) 56 

 **-More nearly isodiametric. 



Tubercles, or knobs very large, in one row. 



Genus OBLIQUARIA 53 

 Tubercles less prominent, few or many, not re- 

 stricted to one row. 

 Beak sculpture very faint. 



Genus CYPROGENIA 54 



Beak sculpture coarse. 



Genus QUADRULA (in part) 47 

 b — Pseudocardinal teeth present, laterals absent or indistinct. 

 (The old genus Margaritana.) 



1 — Elongated, ends rounded or merely angular. 



* — Less than 5 cm. long, elliptical, ends rounded. 



Genus HEMILASTENA 55 



**-Distinctly larger. 



Posterior end angular, wrinkled. 



Genus SYMPHYNOTA (in part) 56 

 Ends rounded, epidermis dark. 



Genus MARGARITANA 57 

 (Some forms of Lampsilis leptodon, lighter 

 colored and usually rayed, might be sought 

 here. ) 



2 — Isodiametric, or abruptly truncated. 



Surface with plications and wrinkles. 



Genus ARCIDENS 58 

 Plications none, or obscure and only on truncate end. 



Genus ALASMIDONTA 59 

 c — Hinge without well developed teeth. (The old genus Aiio- 

 donta.) 



1 — Rather heavy for the group, dark, opaque. 



Genus STROPHITUS 60 



79 



