XXvi 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. 
id. Arca. Hinge with numerous teeth, alternately locking 
within each other. 
14. Ostkea. 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. JVith a single valve. 
1 . More or less spiral. 
IS. Nautilus. Shell with several compartments or cells, 
communicating with each other by means of a small tube or 
perforation. 
19 Cyprtea, 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, reachiug 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. Buccinum. 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 Ie>s conic: aperture 
a little 
