ESSENTIAL GENERIC CHARACTERS. 
A. With more than two valves. 
1. Chiton. Shell boat-shaped, with several valves disposed 
in a transverse manner along the back, and incumbent on each 
other at their front margin. 
2. Leras. Shell attached to other substances, with several 
erect unequal valves. 
3. Piiolas. Shell with two larger primary valves, gaping 
at both extremities, and one or more lesser valves about the 
hinge; teeth long, curving inwards, one in each valve on the 
inside. 
4. Teredo. Shell with two primary hemispherical valves, 
attached at the posterior end to a cylindrical testaceous tube : 
teeth long, curving inwards, one in each valve on the inside. 
B. With two valves . 
5 My a. Shell generally gaping at one end : hinge with 
mostly a single strong broad tooth, not inserted into the op¬ 
posite valve. 
6. Solen. Shell oblong, open at both ends: hinge with a 
reflected subulate tooth or two, not inserted into a groove of 
the opposite valve. 
7. Tellina. Shell with the cartilage side generally sloping, 
or curved to one side: hinge with usually three teeth; the la¬ 
teral tooth in one of the valves flat, or nearly obsolete. 
8. Cardium. Shell with the valves equal, mostly ribbed 
longitudinally and toothed round the margin : hinge with two 
teeth near the beaks, and a lesser remote one on each side, 
locking into the opposite valve. 
9. Mactra. Shell with equal valves, generally unequal at 
the sides: middle tooth of the hinge complicated, with a small 
hollow on each side; the lateral teeth remote, and locking 
into the opposite valve. 
10. Donax. Shell with the cartilage side very obtuse and 
abrupt: hinge with two teeth in the middle, and a single re¬ 
mote lateral one. 
11. Venus. 
