22 INTRODUCTION. 



7. Cardium : Hinge with remote penetrating lateral teeth. 



8. Mactra: Hinge with a comphcated triangular mid- 



dle tooth, and an adjoining hollow. 



9. DoNAx: Hinge with a generally remote lateral tooth, 



not let into the opposite valve. 



10. Venus: Hinge with generally three approximate di- 



varicate teeth. 



11. Spondylus: Hinge with two teeth, separated by a 



small hollow. 



12. Cham a: Hinge in one shell, with two oblique obtuse 



teeth. 



13. Arc a: Hinge with numerous penetrating teeth. 



14. Ostrea: Hinge without teeth, but an ovate hollow. 



15. Anomia: Hinge without teeth, but generally a linear 



depression on the rim, the beak of one valve curved 

 over the hinge. 



16. Mytilus: Hinge without teeth, with a subulate de- 



pression, and generally fixed by a silky beard. 



17. Pinna: Hinge without teeth, valves united at one 



end, and open at the other. 



III. UNIVALVES. 



1. WITH A REGULAR SPIRE. 



18. Argonaut a: Shell with one cell, spiral, involute. 



19. Nautilus: Shell with many cells, with a siphon of 



communication. 



20. Conus: Aperture effuse, longitudinal, without teeth. 



21. Cypr^ea: Aperture effuse, lineal, longitudinal, toothed 



on each side. 



22. Bulla: Aperture a little contracted, and placed ob- 



liquely; toothed on one side only. 



