22 INTRODUCTION. 



centre of the hinge, and for the most part broad, large, and 

 distinct, often elevated, and usually inserted in a cavity in 

 the opposite valve. 'I'hey however differ very much in 

 some shells, but may easily be distinguished. Plate IV. 

 fig. 1, 3, and 7, aa a a a. These are also termed the Car- 

 dinal Teeth, or the Central Teeth. 



Lateral Teeth, Plate IV. fig. 10, are teeth which diverge 

 from the umbo, or are situated at a distance from it, and 

 are in general long and fiat, often double and divided 

 by a groove or hollow. Plate IV. fig. 2, 4, 6, and 7, 

 bbbb. 



Double Teeth, or teeth formed of two laminae. Plate IV. 

 fig. 1, cc. 



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

 single tooth of the Solen Siliqua. Plate IV. fig. 2. 



Recurved Teith are those which are bent backwards, as 

 in the hinge of the Panopea. Plate XVIII. fig. 9; and 

 Spondylus, Plate IV. fig. 8,//. 



Middle Teeth, Plate IV. fig. 4, dd. Another term for 

 the Primary, Central, or Cardinal teeth. 



Numerous 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. 10, kk. 



Cavity cf the Hinge. — The hollow depression in which 

 the ligament of the Ostrea is situated, generally of a tri- 

 angular form. Plate IV. fig- II, g. 



Ligament of the Hinge, or the Cartilage, is that elastic 

 fibrous substance by which the valves are united, and the 

 hinges kept in their proper places; always situated under 

 the beaks of the Shell. Plate II. fig. 1, 3, and 4, III; 

 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 most species of Venus, &c. From this cir- 

 cumstance it is seldom the highest part of the shell. Plate 

 II. fig. 5, r r. 



Seam. When the valves are closed, the line of separation 



between them is so called. Plate II. fig. 6, 1 1. 



