16 INTRODUCTORY. 



animal, and inserted at their sides into a marginal tough liga- 

 ment. Plate I. fig. I, AAA. 



Operculum consists of four small valves on the summit of the 

 lepas, which shut up the superior orifice ; it is in a certain degree 

 stationary, and different from the operculum of univalve shells, 

 which will hereafter be described. Plate I. fig. 1. a. Fig. 9. re- 

 presents a profile view of the operculum removed from its place, 

 A the front valves, B the back valves. Fig. 10. a front view of 

 the operculum. 



Base, is that part of the shell by which it is fixed to rocks and 

 other bodies. Plate I. fig. 1. and 2. b b b. — h is a piece of stone 

 to which the base is fixed, and g a piece of wood to which the 

 shells of this section are generally attached. 



Ligament, is the membranous or tendinous substance by which 

 the valves or parts of the shell are attached. Some raultivalve 

 shells are connected, by the parts of one valve locking into an- 

 other. Plate I. fig. 2. d d d.. The ligaments 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 a mem- 

 branaceous substance, similar to a bladder but materially 

 thinner, and filled with a liquid which evidently affords nourish- 

 ment to the animal. Plate I. fig. 2. c c. the peduncle is usually 

 affixed to a piece of wood as represented at a. 



Feelers, are those crenated arms, evolved from the side of the 

 Lepas anatifera, and other shells of the second division of Lepas, 

 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 L fig. 11. B B. 



