189 



receiving each other crosswise ; two lateral ones, remote 

 and inserted. Recent and fossil. PL viii. fig. 12. 



4. Cypricardia. An equivalved, inequilateral bivalve, 

 extended transversely and obliquely, with three teeth under 

 the beaks, and a lateral one under the corselet. 



Differing from cardita in having, instead of one tooth 

 under the beaks, three, like venus. 



5. Isocai^dia. An equivalved, cordiform, ventricose 

 bivalve; the beaks separate, diverging, and turned in a 

 spiral; two flat, inserted cardinal teeth, one of which is 

 recurved under the beak ; a lateral tooth extended beneath 

 the corselet ; the ligament external. Recent and fossil. 

 PI. viii. fig. 8. 



Arcacece. With numerous small penetrating teeth dis- 

 posed on both valves in a straight or bent line. 



1. Cucullea. An equivalved, inequilateral, trapeziform, 

 ventricose shell, with detached beaks, separated by a liga- 

 mental area ; marked with oblique lines meeting in angles 

 at the union of the valves ; the hinge linear, straight, set 

 with small transverse teeth, with from tw T o to five at the 

 ends disposed obliquely. Recent and fossil. PI. viii. fig. 20. 



2. Area. A transverse, subequivalved, inequilateral 

 shell, with distant beaks, separated by a ligamental area; 

 the hinge straight and linear, formed by numerous teeth 

 closely set in a row : the teeth of one valve inserted between 

 those of the other ; the ligament external. Recent and 

 fossil PI. viii. fig. 17. 



3. Pectunculus. An orbicular, almost lenticular, equi- 

 valved, close, and subequilateral, bivalve ; the hinge with 

 numerous teeth, obliquely inserted in bent rows, obsolete 

 and scarcely any in the middle ; the ligament external. 

 Recent and fossil. PI. viii. fig. 19. 



4. Nucula. A transverse, ovato-trigonal or oblong, equi- 

 valved and inequilateral bivalve ; no intermediate area ; the 



