Fie-. 21. Pinna. 



Fi 



22. Terebratulu. Fig. 23. Pearl* 



are formed by alternate layers of very 



The nacreous shells 



thin membrane and carbonate of lime, but this alone does not give ' 



the pearly lustre which appears to depend on minute undulations j 



of the layers, represented in fig. 23. This lustre has been sue- I 



cessfully imitated on engTaved steel buttons. Nacreous shells, \ 

 when polished, form "mother of pearl:" when digested in weak 



acid, they leave a membraneous residue which retains the original ^ 



form of the shell. This is the most easily destructible of shell- | 



textures, and in some geological formations we find only casts of \ 

 the nacreous shells, whilst those of fibrous texture are completely 



preserved. \ 



Pearls are produced by many bivalves, especially by the ! 



Oriental peaii-mussel {avicula margaritifera), and one of the j 



British river-mussels {miio margaritijerus). They are caused by i 

 particles of sand, or other foreign substances, getting between 



the animal and its shell ; the irritation causes a deposit of nacre, ] 

 forming a projection on the interior, and generally more brilliant 



than the rest of the shell. Completely spherical pearls can only \ 



be formed loose in the muscles, or other soft parts of the animal. ' 



The Chinese obtain them artificially, by introducing into the \ 

 living mussel foreign substances, such as pieces of mother-of-pearl 

 fixed to wires, which thus become coated with a more brilliant 



material. ; 



* Figs. 21, 22, 23. Magnified sections of shells, from Dr. Carpenter. 



Fragments of shell gi'ound very thin, and cemented to glass slides with Canada , 



balsam, are easily prepared, and form cm-ions microscopic objects. A great \ 



variety of them may be procured of Mr. C. M. Topping, of PentonviUe. \ 



