DESHAYES. 601 



upon a perpendicular tooth. Mya, Corbula, 

 Pandora. 



11. OSTEODESMES. Shell inequivalve ; ligament in- 



ternal ; a free ossicle at the hinge. Osteo- 

 desma, Periploma, Anatina, Thracia. 



12. MACTRACEA. Shell equivalve, closed or slightly 



gaping ; ligament internal with or without an 

 external ligament. Lutraria, Mactra, Meso- 

 desma, Crassatella, Erycina, Amphidesma. 



13. PETRICOLID^E. Shell free or boring, not en- 



closed in a tube, gaping, scarcely regular, 

 ligament external. Saxicava, Petricola, Vene- 

 rupis, Hyatella, Byssomia. 



14. TELLINIDID^E. Shell free, regular : two cardi- 



nal teeth at most in each valve, sometimes 

 lateral teeth ; ligament external, on the lesser 

 side of the shell. Sanguinolaria, Psammobia, 

 Tellina, Donax. 



15. LUCINID.E. Shell free, regular, closed ; ligament 



external or subinternal ; hinge variable ; mus- 

 cular impressions very large. Corbis, Lucina, 

 Ungulina. 



16. CYCLAD^E. Shell fluviatile ; hinge simple or 



with lateral teeth, or with lateral and hinge 

 teeth ; ligament external. Iridina, Cyclas, 

 Cyrena, Galathea. 



17. CONCHA. Shell free, regular, closed ; cardinal 



teeth from two to four, sometimes a single 

 lateral tooth ; ligament short, external. Cy- 

 prina, Venus, Astarte. 



18. CARDIACEA. Shell regular ; cardinal teeth irre- 



gular, one or two lateral teeth ; ligament 

 external. Cypricardium, Isocardium, Cardium. 



19. TRIDACNAD^E. Shell regular, truncate, some- 



times gaping on one side ; ligament external. 

 Tridacna, Hippopus. 



** Shell irregular. 



20. CAMACEA. Shell irregular, adherent, inequi- 



valve ; a large hinge tooth. Chama, Diceras. 



Order II. 

 * Shell regular. 



21. NAYADES. Shell regular, fluviatile, nacred ; 



hinge without teeth or articulation ; ligament 

 external. Uiiio. 



22. CARDITES. Shell regular : one or two oblique 



hinge teeth. Cardita. 



