DESHATES. 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 wiih 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. LuciNiDiE. Shell free, regular, closed ; ligament 



external or subinternal ; hinge variable ; mus- 

 cular impressions very large. Corbis, Lucina, 

 Ungulina. 



16. Cyclad^. Shell fluviatile ; hinge simple or 



with lateral teeth, or A^dth lateral and hinge 

 teeth ; ligament external. Iridina, Cyclas, 

 Cyrena, Galathea. 



17. CoNCH.E. 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^. 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. Unio. 



22. Cardites. Shell regular : one or two oblique 



hinge teeth. Cardita. 



