10 INTRODUCTION. 



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 1. 

 %. 3. .1. 



Margin. — A fleshy border in which the valves are attached in 

 the genus Chiton. Plate I. fig. 11. J5 B. 



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. 



Eqjiilateral 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 arc unequal ; and 

 of different shapes, as in the Mactra, Donax, &c. 



Suminit, is the most elevated point of that part of the shell in 

 which the hinge is placed. Plate L fig. 4. and 8. a a. 



In naming this the summit, we do not follow the axiom of 

 Linnseus, 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 



