ji INTRODUCTORY 
minated, &c. Some hinges have no visible teeth, and are termed 
tnarticulate.—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.1 and 12; and 
those with many teeth multiarticulate. Plate IV. fig. 2, 3, 4, 
&c. Plate I. fig.6, and 8, ee 9 2. 
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, aa 
aaa. 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 7. 56 
bb. 
Double Teeth, Plate IV. fig. 1, c ¢. 
Incurved Teeth are those which are bent round, as in the single 
tooth of the Solen. Plate IV. fig. 8, ff. 
Recurved Teeth, are those which are bent backwards, as in the 
hinge of the Panopea and Spondylus. 
Middle Teeth, Plate IV. fig. 4, dd. These are also termed 
Cardinal Teeth. 
Numerous Teeth, are those small upright teeth, set in rows, of 
which the hinges of all the species of the genus Arca are formed. 
Plate IV. fig. 10, k k. 
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, 2. 
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 Il. fig. 1, 3, and 4, 222; 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, &c. From this circumstance it is seldom 
the highest part of the shell. Plate IL. fig. 5,r7. 
