EXPLANATION OF THE PARTS OF SHELLS. 21 



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. 9. and II, 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 ascertaining a species. Plate I. fig. 7, h h. 



Ligament perforation. — The circular aperture, or per- 

 foration, 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 

 flat valve, though there are a few exceptions to the con- 

 trary. Plate I. fig. 10, 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 S, q q q. 



Teeth of the Hinge. — Upon the number and relative 

 situation of the teeth, principally depend the generic distinc- 

 tions; they are of various forms, and very differently placed ; 

 I some are single and large, others numerous and small, orbi- 

 I cular, spatuliform, laminated, &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 com- 

 plicated. Plate IV. fig. 6, e. Those with few teeth are 

 termed articulate. Plate IV. fig. 1. and 12, and those with 

 many teeth mvltiarticulate. Plate IV. fig. 2, 3, 4, &c. 

 Plate I. tig. 6. and 8, g g g g. 

 Primary Teeth are those teeth in general situated in the 



* Two distinct tilings are confounded in this delinition. The cica- 

 trix, or sinus, is a mark left by the siphon-tube, as in Fig. 11, and the 

 e to the riglit. Muscular impressions are the marks left by the adduc- 

 tor muscles, as in Fig. 9, e, e. 



