18 INTRODtJCTORY. 



the valve, suppose it with its inside lying undermost. Plate I. 

 fig. 4. 0. 



Inside, the concave part of a valve. Plate IV. fig. 6. n. 



Cicatrix or Tongue, is the impression left on the inside of the 

 valves, by the adhering muscles of the animal. It differs in 

 most shells^ according to the shape of the muscle, as semi-ovate, 

 round, lunate, elongated, &c. As a specific distinction, it is 

 often of great use ; being, with a very few exceptions, alike in 

 shells of the same species. Some shells have only one cicatrix, 

 as the Edible Oyster, and Muscle ; others have two, and some 

 few more ; the Tellina Fausta for example. Plate I. fig. 6. 

 and 8. e e e e. 



Lunule. The lunated depressions, situated in the anterior 

 and posterior slopes. In different species of Venus they are 

 prominent, characteristic marks, often of much service in ascer- 

 taining a species. Plate I. fig. 4. h h. 



Ligament perforation. The circular aperture, or perforation, 

 through which the ligament passes ; by which the animal of the 

 Anomia attaches itself to stones, and other marine extraneous 

 bodies ; it is in general situated in the fiat valve, though there are 

 a few exceptions to the contrary. Plate I. fig. 7. m. 



Hinge, is the point at which bivalve shells are united, it is 

 formed by the teeth of one valve inserting themselves between 

 those of the other, or by the teeth of one valve fitting into the 

 cavities or sockets of the opposite one. 



It is on the peculiar construction of the hinge that the generic 

 character of bivalve shells is principally founded, together with 

 the general contour of the shell. Plate II. fig. 1. and 3. q q q. 



Teeth of the Hinge. Upon the number and relative situation 

 of the teeth, principally depend the specific distinctions ; they 

 are of various forms, and very differently placed ; some are single 

 and large, others numerous and small, orbicular, spatuliform, la- 

 minated, &c. Some hinges have no visible teeth, and are termed 

 inarticulate. — When a primary tooth has a groove or hollow in 

 its centre, it is called complicated. Plate IV. Fig. 6. e; those 

 with few teeth are termed articulate. Plate IV. fig. ]. and 12. 

 and those with many teeth multiarticulate. Plate IV. fig. 2. 3. 

 4<. &c. Plate I. fig. 6. and 8. g g g g. 



Primary Teeth, are those teeth in general situated in the cen- 

 tre of the hinge, and are for the most part broad, large, and dis- 



