10 ‘INTRODUCTORY 
Feelers are those crenated arms, evolved from the side of the 
Anatifera. While the animal is in the water it continually moves 
its feelers, evidently for the purpose of entangling minute marine 
insects, as food. Plate I. fig. 2, £ x. 
Accessory valves are small plates which cover the apex at the 
hinge of the Pholades, or are situated below the hinge. Plate I. 
fig. 3, a. 
Margin. A fleshy border in which the valves are attached in 
the genus Chiton. Plate I. fig. 11, dd. 
BIVALVE, 
Bivalve shells consist of two parts or valves, connected by a 
cartilage, and a hinge which is generally composed of teeth ; those 
of the one valve locking into a cavity in the other. 
The valves of some bivalve shells are formed exactly alike, and 
others are very different ; the one being smooth, the other rugose ; 
one flat and another convex; and often one is shorter than the 
other. 
The shells of the Mya, Solen, Tellina, Venus, and others, have 
in general both valves alike, while those of the Spondylus, Ostrea, 
and Anomia, have in general dissimilar valves. The first of these 
kinds are called equivalve, and the latter inequivalve. | 
Equilateral shells, are those whose sides are alike, as in the 
shells of the genus Pecten. Plate II. fig. 2, and Plate VII. fig. 
14, This is also exemplified in the Pectunclus. 
Inequilateral valves are shells whose sides are unequal; and 
of different shapes, as in the Mactra, Donax, &c. 
Summit is the most elevated point of that part of the shell in 
which the hinge is placed. Plate I. fig. 4, 7 k. 
In naming this the summit we do not follow the axiom of 
Linneus, but because we consider it more properly the summit 
of the shell than the opposite extremity. 
Base is the reverse of the above, or that part of the shell im- 
mediately opposite the summit. Plate I. fig. 6 and 7, d d. 
Sides, the right and left parts of the valves. Plate I. fig. 6, c. 
Posterior slope is that part of the shell in which the ligament 
