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, 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. 



JMargin. A fleshy border in which the valves are attached in 

 the genus Chiton. Plate I. fig. \\, b b. 



BIVALTE, 



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 cavit}' in the other. 



The valves of some bivalve shells are formed exactly alike, and 

 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 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 

 Hhells of the genus Pecten. Plate II. fig. 2, and Plate VII. fig. 

 14. This is also exemplified in the Pectunclus. 



Ineqjiilateral Tya^Tyes are shells whose side^ are unequal ; and 

 of diflerent 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 

 Linnaeus, 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 



