GENERAL CHARACTERS. 321 



either of a single piece, as in the snail, whelk, etc., or of two portions (valves) as 

 in the oyster, cockle, and most other bivalves. In one group, however (Chitonidai), 

 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 con- 

 cealed 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 porcel- 

 laneous, others horny, glassy, nacreous, and fibrous. The shell is essential to the life 

 of the inhabitant, it forms part of its organisation, and if it be removed, death 

 ensues sooner or later. Although Molluscs 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 preserva- 

 tion of the shell from atmospheric and chemical action. Were it not for the 

 periostracum, the shells of fresh- water molluscs would in time be dissolved by the 

 carbonic acid gas with which water is more or less saturated. Owing to the thin- 

 ness of the periostracum, or to its having been worn away, the apical portion of 

 many fresh- water spiral shells and the tips or beaks (umbones) 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 

 aperture, 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 recognise in the majority of cases if a 

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

 where the most practised 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 ornamenta- 

 tion 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 

 consequently 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 mollusc's life. 



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

 enormous. What resemblance, for example, is there between a Chiton and a 



vol. vi. — 21 



