GENERA OF SHELLS. 51 



valve small, flat, like an operculum. Hinge 

 toothless ; an oblong cardinal pit. 



The curved beak of these shells usually projects 

 considerably, either above the upper valve or laterally. 

 They are almost always free shells, and are all fossil, 

 but one species. 



G. angulata vel tricarinata. 



OSTREA. Shell adherent, inequivalve ; irreg- 

 ular ; beaks distant, becoming very unequal by 

 age ; the superior valve becoming displaced 

 during life. Hinge toothless ; ligament semi- 

 interior, inserted into the cardinal fossa ; fossa 

 of the inferior valve increasing with age to a 

 great length. 



The shell of the ostrea is rude, rugged, often scaly, 

 sometimes singularly plaited at the margins, and fre- 

 quently very thick. It does not curve upwards like 

 that of the gryphsea. The texture of the valves is 

 loose, foliated ; the lower one, which is the largest, 

 and by which it adheres to marine bodies, is more 

 convex than the upper. 



(1.) Margins simple or waved, not plaited. 



O. edulis, hippopus, borealis, Adriatica, cochlearis, cristata, 

 gallina,mumisma, lingua, tulipa, Brasiliana, scabra, rostralis, 

 parasitica, denticulata, spathulata, ruscuriana, Virginica, Can- 

 adensis, excavata, mytiloides, sinuata, trapezina, tuberculata, 

 rufa, margaritacea, gibbosa, australis, elliptica, haliotidea, 

 deformis, fucorum semicylindrica. 



(2.) Margins plaited. 



0. cornucopia, cucullata, doridella, rubella, limacella, 

 erucella, folium, labrella, plicatula, glaucina, fusca, turbi- 

 nata, crista-galli, imbricata, hyotis, radiata. 



