u 



INTRODUCTION. 



6. Tellina: Hinge with the lateral teeth of one valve 



not let into the other. 



7. Cardium : Hinge with remote penetrating lateral 

 teeth. 



8. Mactra: Hinge with a complicated triangular mid- 

 dle tooth, and an adjoining hollow. 



9. Don ax ; 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. Chama: Hinge in one shell, with two oblique ob- 



tuse teeth. 



13. Arca: Hinge with numerous penetrating teeth. 



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



15. Anomia: Hinge without teeth, but generally alinear 



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. Argonauta: Shell with one cell, spiral, involute. 



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



communication. 

 '20. CoNus: AperturcefTuse, longitudinal, without teeth. 



