GENERA OF SHELLS. 19 



V. perforans, nucleus, irus, exotica, distans, crenata, car- 

 ditoides. 



NYMPHACEA. 



Two or more cardinal teeth on the same valve ; shell often 

 gaping slightly at lateral extremities ; ligament exterior ; 

 nymphce generally projecting outside. 



NYMPHACEA SOLENARIA. 



SANGUINOLARIA. Shell transverse, subellip- 

 tic, slightly gaping at lateral extremities, supe- 

 rior edge arcuated, not parallel to the inferior. 

 Hinge with two approximate teeth on each 

 valve. 



The sanguinolaria is distinguished from solen, by 

 the superior margin not being parallel to the inferior ; 

 they also gape but little at the lateral extremities. 



S.X)ccidens, rosea, livida, rugosa. 



PSAMMOBIA. Shell transverse, elliptic or oval 

 oblong, nearly flat, slightly gaping at each side, 

 beaks prominent. Hinge with two teeth on the 

 left valve, and one fitting between them on the 

 opposite valve. 



The psammobiae approach in form nearer to tellina 

 than to solen ; but besides gaping at the sides, they have 

 not the irregular fold of the former, though they fre- 

 quently have an angle or fold of the same form, on the 

 anterior side of both valves. 



P. virgata, feroensis, vespertina, florida, maculosa, caeru- 

 lescens, elongata, flavicans, squamosa, alba, Cayennensis, 

 laevigata, tellinella, pulchella, aurantia, fragilis, livida, gala- 

 thsea, lusoria, fusca. 



PSAMMOTEA. Shell transverse, oval or ob- 



