GENERAL CHARACTERISTICS 3289 



cockle, and most other bivalves. In one group, however (Chitonidce) , the shell 

 takes the form of a series of eight adjacent plates, and in one group of bivalves 

 {Pholadidce) there are one or more accessory pieces in addition to the two principal 

 valves. In the bivalves, with one or two exceptions, the shell is always external. 

 Not so with the univalves, in some of which it is quite concealed beneath the skin, 

 in others it is partially so. Shells are mainly composed of carbonate of lime, with 

 a small admixture of animal matter. Their microscopic examination has revealed 

 a great diversity of structure. Some are termed porcellaneous, others horny, 

 glassy, nacreous, and fibrous. The shell is essential to the life of the inhabitant, 

 it forms part of its organization, and if it be removed, death ensues sooner or later. 

 Although Mollusks have the power of repairing injuries to their shells, no case is 

 known of a species removed from its shell having secreted a fresh one. Many shells 

 exhibit an outer coat of animal matter termed the periostracum. It is generally of 

 an olivaceous tint, but varies considerably in thickness and appearance. It is 

 quite smooth, or of a velvety or silky aspect, or it may be produced into hair-like 

 extensions. Its special function is the preservation of the shell from atmospheric 

 and chemical action. Were it not for the periostracum, the shells of fresh-water 

 mollusks would in time be dissolved by the carbonic acid gas with which water 

 is more or less saturated. Owing to the thinness of the periostracum, or to its 

 having been worn away, the apical portion of many fresh-water spiral shells and 

 the tips or beaks (um bones) of the bivalves are frequently more or less eroded 

 through this chemical action. 



The shells of gastropods are enlarged with the growth of the 

 inhabitant by the addition of fresh layers to the margin of the aper- 

 ture, so that a shell which at its birth had only two volutions or whorls, may 

 eventually consist of about a dozen. The growth of bivalves is more readily 

 understood, being effected by additional concentric deposits along the outer margin 

 of the valves. The adult condition of many shells cannot be detected by a super- 

 ficial inspection. This is particularly the case with the bivalves. On the contrary, 

 a little experience soon teaches us to recognize in the majority of cases if a gastro- 

 pod shell is immature; but even among this class there are many exceptions where 

 the most practiced eye would fail to determine the period of growth. 



Many shells exhibit conspicuous prominences on the surface in the form of 

 spines and ridges, and it is a great puzzle to the uninitiated how this ornamentation 

 is produced. It is, however, a very simple matter. Wherever a spine occurs on a 

 shell we may be certain that it resulted from a corresponding filament or process 

 upon the edge of the mantle, and these processes may be brought into use only 

 periodically. A striking example of this periodic formation of spines occurs among 

 the typical Muricidce. On the contrary, spines or extensions may occur only upon 

 the lip or edge of the aperture when the shell has arrived at maturity, and conse- 

 quently we may assume that the necessary prolongations upon the edge of the 

 mantle for the secretion of such spines are only developed at this period of the 

 mollusk'slife. 



The diversity in shells with respect to form, size, and solidity is simply enor- 

 mous. What resemblance, for example, is there between a Chiton and a Dcntalium^ 



