12 INTRODUCTORY 



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.l and 12 ; and 

 those with many teeth multiarticulate. Plate IV. fig. 2, 3, 4, 

 &c. Plate I. fig. 6, and Q, g g g g- , 



Primary, teeth are those teeth in general situated in the centre 

 of the hinge, and are for the most part broad, large, and distinct, 

 often elevated ; and in general are inserted in a cavity in the op- 

 posite 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 1. b b 

 b b. 



Double Teeth, Plate IV. fig. 1, c c. 



Incurved Teeth are those which are bent round, as in the single 

 tooth of the Solen. Plate IV. fig. 8,//. 



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

 hinge of the Panopea and Spondylus. 



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

 Cardinal Teeth. 



JWimerous 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. \Q,kk. 



Cavity of the Hinge. The hollow depression in which the 

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

 Plate IV. fig. 11,^. 



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

 stance by which the valves are united, and the hinges kept in 

 their proper places ; generally situated under the beaks of the shell. 

 Plate II. fig. 1, 3, and A, 1 1 I; 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 

 some species of Venus, «fec. From this circumstance it is seldom 

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



