EXPLANATION OF THE PARTS OF SHELLS. 19 



merits vary considerably in their texture, being scaly, prickly, 

 smooth, or punctated. 



Ridges are certain convexities in many of the Lepas 

 tribe, sometimes longitudinal and sometimes transverse. 

 Plate I. fig. 1, FF. 



Peduncle. — A sort of stem by which the shells of the 

 second division of Lepas are attached to wood, &c. It is 

 composed of a membranaceous tube, filled with a liquid 

 which perhaps affords nourishment to the animal. Plate I. 

 fig. 5, c c The peduncle is usually affixed to a piece of 

 wood as represented at g, or to some other solid body. 



Feelers are those ciliated arms, evolved from the upper 

 or hind part of the Lepas anatifera, and other shells of the 

 genus Lepas. While the animal is in the water it con- 

 tinually moves its feelers, evidently for the purpose of 

 entangling minute marine insects, as food. Plate I. fig. 5, ee. 



Accessory Valvis are small plates which cover the apex 

 at the hinge of the Pholades, or are situated below the 

 hinge. Plate L fig. 4, A. 



Margin. — A fleshy or cartilaginous border in which the 

 valves are encased in the genus Chiton. Plate L fig. 6, 6, b. 



BIVALVE SHELLS. 



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 corresponding 

 cavities in the other. 



The valves of some bivalve shells are formed exactly 

 alike, but those of others are very diflferent ; the one being 

 smooth, the other rugose ; one flat, and another convex ; 

 and often one is shorter than the other. 



The shells of the genera Mya, Solen, Tellina, Venus, and 

 others, have in general both valves alike, while those of the 

 genera Spondylus, Ostrea, Anomia, and Pinna, have in gen- 

 eral dissimilar valves. The first of these kinds are called 

 equivalve, and the latter inequivalve. 



Equilateral shells are those of which the sides are alike. 



