EXPLANATION OF THE PARTS OF SHELLS. 



MrLTITALVE. 



A MULTiTAivE shcU is composed of move parts than two. 

 Every part of a shell which is connected by a cartilage, ligament, 

 hinge, or tooth, is called a valve of such shell; thus, the Chitons 

 have eight transverse, broad, but very short valves, placed on the 

 back of the animal, and inserted at their sides into a marginal 

 tough ligament. Plate L fig. 11, a a a. 



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 L 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 multivalve 

 shells are connected by the parts of one valve locking into 

 another. Plate L 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, 

 r F. 



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

 are attached to wood, &c. It is a membranaceous substance, 

 similar to a bladder but materially thinner, and filled with a liquid 

 which evidently affords nourishment to the animal. Plate L fig. 

 2, c c ; the peduncle is usually aflftxed to a piece of wood as re- 

 presented at G. 



