_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, E E. _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, _b b_. 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, _i k_. In naming this the summit we do not follow the axiom of Linnæus, 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 immediately 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 is situated. In viewing the posterior slope in front, the beaks of the shell retire from view. Plate I. fig. 4, _i_. _Anterior slope_, that part of the shell opposite the posterior slope; in viewing it in front, the beaks point to the observer. Plate I. fig. 4, _k_. _Disk_, the convex centre of a valve, or most prominent part of the valve, suppose it with its inside lying undermost. Plate I. fig. 4, _o_. _Inside_, the concave part of a valve. Plate IV. fig. 6, _m_. _Muscular impression_ is the impression left on the inside of the valves, by the adhering muscles of the animal. It differs in most shells, according to the shape of the animal, as semi-ovate, round, lunate, elongated, &c. As a specific distinction, it is often of great use; being, with a very few exceptions, alike in shells of the same species. Some shells have only one cicatrix, as the Edible Oyster and Mytilus; others have two, and some few more; the Tellina for example. Plate I. fig. 6 and 8, _e e e e_.