38 



MANUAL OF THE MOLLUSC A. 



Fio-. 23. 



The nacreous shells are formed by alternate layers of very, 

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

 the pearly lustre which appears to depend on minute undulations 

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

 cessfully imitated on engraved 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 pearl-mussel {avicula mai'garitiferd), and one of the 

 British river-mussels {unio margaritiferus). They are caused by 

 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-peai'l 

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

 material. 



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

 Fragments of shell ground very thin, and cemented to glass slides with Canada 

 balsam, are easily prepared, and form curious microscopic objects. A great 

 variety of them may be |5rocured of Mr. G.lSl. Topping, of Pcntonville. 



