GENERAL CHARACTERS, 
3 21 
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 ( 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 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-lilce 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 
Growth. . . ® 
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 w T e 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 wdien 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 
