ESSENTIAL GENERIC CHARACTERS. 



11. VENUS. Hinge with three teeth, all near together ; one 

 of them placed longitudinally, and inclining outwards. 



12. CHAMA. Hinge rugged and gibbous, with the teeth 

 locking obliquely into the opposite valves. 



13. ARCA. Hinge with numerous teeth, alternately locking 

 within each other. 



14. OSTREA. Shell with the valves generally unequal : hinge 

 without teeth, but furnished with a cavity in the middle, and 

 generally lateral grooves. 



15. ANOMIA. Shell with unequal valves; one of them 

 usually flat and perforated near the hinge, through which it 

 is fixed to other substances : hinge without teeth ; an oblong 

 transverse projection on the flat valve connecting them by a 

 strong cartilage under the beak. 



16. MYTILUS. Shell generally fixed by a mass of silky fila- 

 ments : hinge without teeth, marked by a longitudinal groove 

 which is sometimes finely notched. 



17. PINNA. Shell brittle, gaping at the larger end, and fixed 

 at the other by a mass of silky filaments : hinge without teeth ; 

 the valves connected by the cartilage nearly the whole breadth. 



C. With a single valve, 



1 . More or less spiral. 



18. NAUTILUS. Shell "with several compartments or cells, 

 communicating with each other by means of a small tube or 

 perforation. 



19 CYPRJEA. Shell involute, oval, obtuse at both ends : 

 aperture very narrow, reaching the whole length of the shell, 

 and toothed on both its edges. 



20. BULLA. Shell convolute, without teeth : aperture con- 

 tracted, oblong, reaching nearly or entirely the whole length 

 of the shell, without beak, entire at the base: pillar oblique, 

 smooth, 



21. VOLUTA. Shell more or less spiral: aperture narrow, 

 extending much down the shell, without beak: pillar plaited 

 or toothed. 



22. BUCCJNUM. Shell spiral, gibbous : aperture oval, ending' 

 in a canal leaning to the right hand. 



23. STROMBUS. Shell spiral : aperture ending in a canal 

 leaning to the left ; the outer-lip more or less expanded. 



24. MUREX. Shell spiral, often rough with folds or protu- 

 berances : aperture oval, ending in a straight canal, which is 

 sometimes a little reflected. 



25. TROCIIUS. Shell spiral, more or le^s conic: aperture 



a little 



