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Third Genus METAPTERA. 



Shell oval, triangular, dilated into a wing pos- 

 teriorly; ligament inclined over the wing; cardi- 

 nal tooth bilobed, crenulated ; lamellar tooth 

 curved, detached from the edge of the wing; 

 axis extramedial ; contour slightly thickened; 

 three muscular impressions; animal resembles 

 that of the Unio. 



The name signifies posterior wing ; I at first 

 adopted that of Proptera, erroneously, because it 

 signifies anterior wing. 



SPECIES XVII. Metaptera megaptera (Unio 

 megaptera.) 



Shell thin, compressed ; epidermis brown, flexu- 

 ously rugose ; nacre purple ; wing very large, 

 smooth inside ; lamellar tooth double in the right 

 valve, and with an oblong protuberance at the 

 extremity. Length f diam. f axis of the 

 breadth. 



A fine species, common in the Ohio ; beautiful 

 nacre, purple, iridescent ; frequently with pearli- 

 form tubercles ; cardinal tooth, lobes nearly 

 equal in size, smooth exteriorly; crenulated, 

 compressed, furrowed interiorly ; anterior muscu- 

 lar impressions strongly marked, striated; the 



