PARTS OF BIVALVES EXPLAINED. r 
lock into a cavity in the opposite valve. Those. shells 
which have no distinct teeth, are called inarticulate. 
Primary Teeth are those situate in the centre of the 
hinge, and which are usually broad and large, or elevated 
and acute, Fig. 22. c. 
Cardinal or Primary Teeth. See above. 
Erect Teeth are exemplified in the teeth of Mya and 
Panopea, fig. 25. a. 
Lateral Teeth are those which diverge from the umbo; 
they are generally long and flat, seldom erect. In some 
species they are double. Fig. 22. d. 
Numerous Teeth are exemplified in the genera Pectun- 
culus, Cucullea, &c. Fig. 28. a. 
Ears. Those processes on each or one side of the beak. 
Fig. 20. c. and d, 
Superior Ear. Fig, 20—d. 
Inferior Ear. Fig. 20—ce. 
Valves are used to denote the two pieces composing a 
bivalve shell. These move on a hinge, consisting of a liga- 
ment, or cartilage, or teeth, or with all of them. 
Right Valve is that valve which, when viewed from the 
inside with the umbo uppermost, has the anterior slope 
pointing to the right hand. 
Left Valve. The reverse of the above, or with the ante- 
rior slope pointing to the left hand, when viewed from the 
inside. Fig. 22. 
Margin. The extreme edge of the whole shell, or cir- 
cumference of the valve all round. 
Crenulated Margin. Those fine numerous notches which 
invest the edges of many bivalves, particularly in the genus 
Cardium, and Cypricardia. Fig. 22. e e. 
Length is taken from the hinge or umbo, fig. 20. 5. to 
the opposite extremity, aa. Shells are termed longitudinal 
when their length is greater than their breadth. For ex- 
ample, the common Mussle is a longitudinal shell; as are 
also. the shells of the genus Pecten. Fig. 20; and plate 
VII. Figs. 29 and 30. 
Breadth, from the extreme of one side to that of the other. 
Fig. 22. ff. It is not uncommon for the breadth of a shell 
to be greater than its length, as in the genns Anadonta or 
fresh water Mussles, and strongly exemplified in plate VII. 
fic. 13; plate VIII. fig. 19; and plate IX. fig. 12. In this 
case they are called lateral or transverse shells. 
Strie are fine or coarse thread-like lines which ornament 
