THE CONCHOLOGIST. 



The shell thus commonly consists of three 

 distinct layers, distinguished as nacreous or 

 pearly, prismatic, and epidermic. 



The nacreous or pearly layer lies next to, 

 and is secreted by the surface of the animal 

 mantle. 'I'he outer prismatic layer is said 

 to be fornied only by the free edge of the 

 mantle. The external horny cuticle, "epider- 

 mis" or periostracum, being secreted by the 

 extreme edge of the mantle, where also the 

 pigments or colouring matters are principally 

 formed or secreted. 



The periostracum appears to be the first 

 formed substance, and next the ])rismatic 

 layer, by which the shell increases in super- 

 ficial extent, the shell increasing in thickness 

 by the continued deposition of the nacreous 

 layer. The growth of the shell is thus 

 affected in two ways : — the increase in size 

 by additions to the epidermic and prismatic 

 layers, and in thickness by continued deposi- 

 tion of the nacreous layer. The proposition 

 has been brought forward that the prismatic 

 layer has been formed by the re-arrangement 

 of the particles of carbonate of lime of which 

 it is composed, into a crystalline condition. 

 This has been objected to on the grounds 

 tliat the prismatic layer is formed where the 

 growth of the shell goes on most rapidly, 

 its apparent existence there independent of 

 the nacreous layer, and that the calcareous 

 particles in its substance do not possess 

 a crystalline character. Another theory has 

 been brought forward to the effect that the 

 shell grows by intussusception. This may be 

 better expressed by saying that the shell 

 itself has life, and formulates from the car- 

 bonate of lime in its surrounding medium, 

 and in connexion with the animal, an external 

 and protective shell. 



According to its chemical composition the 

 shell consists of carbonate of lime— (CaCO;,), 

 and an organic matrix, called conchyolin, 

 usually presenting a laminated structure. In 



any investigations into the formation and 

 character of the moUuscan shell, the minera- 

 logical and chemical characters of the calcic 

 carbonate should be borne in mind. Both 

 these factors, it is probable, play an impor- 

 tant part in the formation of the shell. 



The hexagonal-form of the carbonate of 

 lime (or more correctly aragonite, as minera- 

 logists term this form), is one that is common 

 to that mineral in a state of nature. It would 

 appear as though the prismatic layer of the shell 

 has changed its character since its secretion 

 by the animal — there being as it were two 



i forces at work, the animal secreting the 

 amorphous particles, while another force is 

 converting these amorphous [jarticles into a 

 crystalline condition. As we shall see, how- 

 ever, I do not think this is probable. 



All moUuscan shells, at some period of 



j their growth, possess an epidermal covering, 

 formed at the margin of tlic animal mantle, 

 termed the periostracum. It is formed in 

 advance of the shell, and varies greatly in 

 thickness and the form it may assume. It is 

 mainly composed of an organic substance 

 termed " chitin," having a complex chemical 

 composition (CioH2,;N20,o)- This chitinous 

 cuticle is generally supposed to protect the 

 shell from decomposition by the carbonic acid 

 gas usually present in the waters where the 



j mollusca exist. 



I The developmental history of the shell 



is of some importance, as the bearing of 

 embryology upon morphological and physio- 



I logical questions is universally recognised. 



' Unfortunately, the information at hand upon 

 this point is somewhat meagre, more atten- 

 tion having naturally been paid to the 

 development of the animal, the history of the 

 shell being of secondary importance. The 

 embryological history of mollusca show they 

 all undergo a metamorphosis, which transfor- 

 mation may be passed through either within 

 or outside of the egg-capsule. Very often a 



