64 



Bryant Walker 



Subgenus BULLELLA Simpson, 1900. 



Shell thin, greatly inflated, somewhat triangular, with a high, sharp pos- 

 terior ridge; beaks very full, having exceedingly strong, concentric sculp- 

 ture extending w^ell on to the disk; pseudocardinals reflexed, compressed. 



Fig. 185 

 Type: Margaritana arcula Lea, fig. 185. 



Genus SIMPSONIOONCHA Frierson, 1914. 



Fig. I 



Shell small, elongate elliptical, rounded 

 in front and behind, often slightly incurved 

 at the central base ; beak rather sharp, but 

 not full ; sculpture consisting of fine par- 

 allel ridges which are looped up in the mid- 

 dle and open behind; epidermis brownish, 

 rayless ; teeth imperfect, a single, irregular, 



compressed tooth in each valve; laterals nearly or quite wanting; anterior 

 end of the shell much thickened; nacre dull whitish. ^Nlarsupium occupying 

 the whole of the outer gills. 



Type: Alasmodonta ainbigiia Say, fig. 186. 



Subfamily LAAIPSILIX.l^ Ortmann, 1912. 



"Inner lamina of inner gills rarely more or less free from the abdominal 

 sac, generally connected with it throughout ; supra-anal opening separated 

 from the anal, rarely entirely closed ; branchial openings w^ell defined : edge 

 of the mantle in front of the branchial opening smooth to crenulated or 

 with peculiar papilla? or a flap; marsupium rarely formed by the whole 

 outer gill, generally only by or within the ])Osterior part of the outer gill; 

 edge of marsupium, when charged, distending and bulging out beyond the 

 original edge of the gill, generally assuming a beaded appearance ; water- 

 tubes simple in the gravid female ; glochidium semicircular or semi-elliptic, 



