EXPLANATION OF THE PARTS OF SHELLS. 25 



Fig. 7- — Hinge of Venus Ga///n«.(Ortygia gallina, Leacli.) 

 a a. Primary teeth, b. lateral tooth. 



Fig. 8 — Hinge of Spondylus gcederopus, f f. incurved 

 teeth. 



Fig. 9 Hinge oiChama Cor. (Isocardia Cor, Lamarck,) 



Fig. 10. — Hinge of Area pilosa. (Pectunculus pilosus, 

 Lamarck.) k k. numerous small teeth; the distinguishing 

 characteristic of the genus Area. 



Fig. 11 Hinge of Ostrea varla. (Pecten varius, La- 

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



Fig. 12. — Hinge of Anomia Ephippium. 



UNIVALVE SHELLS. 



The shells composing this order are far more numerous 

 than those of the two preceding, both in genera and species. 

 It requires a considerable degree of attention to discrimi- 

 nate many of the species, as they often closely resemble 

 each other; and they are divested of the strong and distinct 

 character afforded by the teeth of bivalves; besides many 

 of the species present 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 neces- 

 sary in the definition of simple, spiral, or turbinated shells ; 

 or, more strictly according to the Linntean method of dis- 

 crimination. Univalves with a regular spire, and those with- 

 out a regular spire. The genera of this order are formed 

 principally from the shape of the aperture, taken in con- 

 junction with the general shape of the shell; from the spire 

 being lengthened or depressed, being with or without a 

 canal; the length of the beak and its direction, together 

 with the particular form of the outer lip. The colour of 

 shells only serves as a specific distinction, and cannot in 

 this respect, in all cases, be depended upon, although in 

 others it is an unvarying test. The particular manner in 

 which the spots are .disposed, frequently characterizes spe- 

 cies. 



