ESSENTIAL GENERIC CHARACTERS. 



A. Wiih 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. LEPAS. 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 valyes, 

 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. MYA. Shell generally gaping at one end : hinge with 

 mostly a single strong broad tooth, not inserted into the op- 

 posite valve. 



6. SOL EN. Shell oblong, open at both ends : hinge with a 

 reflected subulate tooth or two, not inserted into a groove of 

 the ot:pOt'-te 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 xvith the valves equal, mostly ribbed 

 longitudinally and toothed round the margin : hinge with two 

 teeth near the beaks, ami 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 teeJi in the middle^ and a single re- 

 mote lateral one. 



11. VENUS. 



