EXPLANATION OF THE PARTS OF SHELLS. 19 



tinct, often elevated ; and in general are inserted in a cavity 

 in the opposite valve. They however differ very much in some 

 shells, but may easily be distinguished. Plate IV. fig. 1, 3, 

 and 7. a a a a a. These are also termed the Cardinal Teeth. 



Lateral Teeth, Plate IV. fig. 10. are teeth which diverge from 

 the umbo, and are in general long and flat, often double and di- 

 vided by a groove or hollow. Plate IV. fig. 2, 4, 6. and 7. 

 bhhh. 



Double Teeth, Plate IV. fig. I. c c. 



Incurved Teeth are those which are bent round, as in the sin- 

 gle tooth of the Solen Siliqua. Plate IV. fig. 8.// 



Recurved Teeth, are those which are bent backwards, as in the 

 hinge of the Panopea. Plate XVIII. fig. 9j and Spondylus, 

 Plate II. fig. 8.// 



Middle Teeth, Plate IV. fig. 4-. d d. These are also termed 

 Cardinal Teeth. 



Numerous Teeth, are those small upright teeth, set in rows, 

 of which the hinges of all the species of the genus Area are 

 formed. Plate IV. fig. 10. k k. 



Cavity of the Hinge. The hollow depression in which the li- 

 gament of the Ostrea is situated, generally of a triangular form. 

 Plate IV. fig. II. g. 



Ligament of the Hinge, or cartilage, is that flexible fibrous 

 substance by which the valves are united, and the hinges kept 

 in their proper places ; always situated under the beaks of the 

 Shell. Plate II. fig. 1, 3. and 4. Z / / ; Plate IV. fig. 3. n. 



Beak is the extreme point of the summit of bivalves, which in 

 many species turns spirally downwards, or to the one side, as in 

 most species of Venus, &c. From this circumstance it is sel- 

 dom the highest part of the shell. Plate II. fig. 5. r r. 



Seam. When the valves are closed, the line of separation be- 

 tween them is so called. Plate 11. fig. 6. 1 1. 



Umbo. That part situated immediately under the beak. Plate 

 I. fig. 6. w ; and Plate II. fig. 4. w. 



Ears. The processes on each side of the beak, in most spe- 

 cies of that division of Ostrea, called scallops ; some have one 

 ear very large, and the other small ; and some are scarcely 

 observable on one side. Plate II. fig. 2. h h. 



Superior Ear. Plate IV. fig. 11. h. 



Inferior Ear. Do. do. i. 



