EXPLANATION OF THE PARTS OF SHELLS. 21 



DESCRIPTION OF PLATE IV. 



HINGES OF THE LINNEAN GENERA OF BIVALVES. 



Fig. 1. Hinge of the Mya pictorum. (Unio pictorum, La- 

 nmrck.) a. Primary tooth, c c double teeth. 



Fig. 2. Hinge of the Solen siliqua, b. Lateral teeth. 



Fig. 3. Hinge of the Tellina radula. (Lucina radula, La- 

 marck.) a a. Primary teeth, n. cartilage. 



Fig, 4. Hinge of the Cardium aculeatum. b. Lateral tooth, 

 d d. middle teeth, I L spines. 



Fig. 5. Hinge of Mactra hians. (Lutraria hians, Lamarck.) 



Fig. 6. Inside of both valves of the Donax trunculus. A is 

 the left valve, and B is the right valve, b. lateral teeth, b. primary 

 complicated tooth, or cleft in the middle, m m. crenulated margin. 



Fig. 7. Hinge of Venus gaUina. (Ortygia gallina. Leach.) 

 a a. Primary teeth, b lateral tooth. 



Fig. 8. Hinge of Spondylus gcederopus, ff. incurved teeth. 



Fig. 9. Hinge of Chama Cor. (Isocardia Cor, Lamarck.) 



Fig. 10. Hinge of Area pilosa. (Pectunculus pilosus, La- 

 marck.) k k. numerous small teeth ; the distinguishing charac- 

 teristic of the genus Area, 



Fig. 11. Hinge of Ostrea varia. (Pecten vari us, Lamarck.) 

 g. cavity of the hinge, h. superior ear, i. inferior ear. 



Fig. 12. Hinge of Anomia Ephippium. 



The shells composing this order are far more numerous than 

 the two preceding, both in genera and species ; and it requires a 

 considerable degree of attention, to discriminate many of the spe- 

 cies, as they run into each other so m.uch ; and they are divested 

 of the strong and distinct character afforded by the teeth of bi- 

 valves ; besides many of the species there are several varieties. 



In the examination of shells of this order, the general contour 

 or outline of the whole shell, is the first particular to be attended 

 to, as this leads to those distinctions necessary in the definition 

 of simple, spiral, or turbinated shells, or, more strictly according 

 to the Linnean method of discrimination, Univalves with a re- 

 gular spire, and those without a regular spire. The genera of this 



