GENERA OF SHELLS. 7 



truncated at both extremities, open at summit, 

 closed at base by a membrane. 



T. balaenarum. 



CORONULA. Shell sessile, apparently uni- 

 valve, suborbicular, conoid or obtusely conic, 

 truncated at extremities ; parietes very thick, 

 interiorly excavated into longitudinal cells. 

 Operculum with four obtuse valves. 



C. diadema, balgenaris, testudinaria, denticulata. 



BALANUS. Shell sessile, attached, univalve, 

 conic, truncated at summit, closed at base by 

 the adhering testaceous lamina ; aperture sub- 

 triangular or elliptical. Operculum interior, 

 quadrivalve ; valves movable. 



B. angulosus, sulcatus, tintinnabulum, nigrescens, cylin- 

 draceus, calycularis, roseus, ovularis, miser, amphimorphus, 

 perforates, laevis, spinosus, radiatus, palmatus, stalactiferus, 

 plicatus, duplocomus, patellaris, semiplicatus, galeatus, sub- 

 imbricatus, rugosus, plancianus, pustularis, crispatus, punc- 

 tatus, fistulosus, latus geniculatus. 



ACASTA. Shell sessile, ovate, subconic, com- 

 posed of separable pieces ; cone formed of six 

 equal, adnate, lateral valves, having an orbi- 

 cular plate at base, internally concave, resem- 

 bling a patella or basin. Operculum four 

 valved. 



A. montagui, glans, sulcata. 



CREUSIA. Shell sessile, fixed, orbicular, con- 

 vexo-conic, quadrivalve ; valves unequal, ad- 



