MALACOLOGY. C67 



wards and to the right side. Some shells have the mouth on the 

 left side and are called sinistral ; those of the ordinary form have 

 the mouth on the right side, and are called dextral. 



BIVALVE shells are composed of two pieces, kept together by 

 a sort of hinge. When the two valves are equal, the shell is said 

 to be equivalve ; when unequal, unequivalve. They may be 

 round, elliptical, ovate, linear, or of various forms. The more or 

 less prominent part of the valve at the joint, is the umbo. When 

 the umbo is nearly in the middle, the shell is said to be isomeral 

 or equilateral; when not, anisomeral or inequilateral. The 

 hinge may be plain, but it generally presents various promi- 

 nences, called teeth, with depressions, the teeth of one valve filling 

 the depressions of another. The valves are farther kept together 

 by an elastic fibrous ligament, which tends to throw them open. 

 They are brought near to each other by a pair of strong muscles 

 extended internally from one valve to the other, and leaving 

 strong impressions on the inner surface. The teeth are distin- 

 guished into cardinal or central, and lateral. The surface may 

 be convex in various degrees; concentrically striate, laminate, or 

 rugose, or radiated from the umbones (or bosses) with striae, 

 ridges, grooves, ribs or spine. In the natural position, the hinge 

 is uppermost on the back ; that end of the shell to which the lig- 

 ament is nearest, is above, and is called the posterior end ; the 

 other or lower, toward which is the head of the animal, is the 

 anterior end ; the thin edges of the valves are their ventral mar- 

 gins. On the inner surface of the valves are seen the impres- 

 sions made by the muscles, and that left by the mantle. (See 

 figures of Plate XVII. together with the explanations of the 

 same.) 



Some shells, as the Pearl Oyster, Avicula margaritifera, the 

 Pinna and the Modiola, (see Chart,) fix themselves by silky fila- 

 ments called a byssus; some by a sort of cement, as the Oyster, 

 (Ostrea ;) others by forming a vacuum, as the Tatella or 

 Limpet, and still others attach themselves to rocks by the 

 same substance as that of which the shells are made, as the 

 Vermetus. The shells, which by any of these means are ren- 

 dered stationary, are called fixed shells, subsisting upon the 

 little animals which are brought near by the motion of the 

 water; the other shells are called free. Mollusks are also (1) 

 terrestrial. These feed on vegetables, have always four ten- 

 tacula, and their eyes placed at the tip of these organs ; (2) 

 fluviatile or fresh-water shells, that have only two tentacula, 

 which are flat, and have eyes at the base ; (3) Marine, which are 

 most numerous, most beautiful, and most highly prized. We 



