Lamellihrmichia 



39 



measures over a yard in diameter. A large specimen may be seen 

 in the Geological Department . 



The Pteriidae include the ' Pearl ' and ' Hammer " Oysters and 

 the Wing-shells. The Hammer Oysters (Malleus. Case I4(i) are 

 remarkable for the grotesque shape of the shell, an exaggeration 

 of a tendency seen in Avicula (Case 145), Pteria, and certain species 

 of Margaritifera. Avicula zebra lives on a Hydroid, Halicornaria 

 insignis, and imitates its colour-markings with remarkable close- 

 ness. 



The Pearl Oysters (Margaritifera) are exhibited in Case 147 

 and Wall-ease 1). Species of this genus occur on the coasts of 

 India, Ceylon, and North- West Australia. In the Ceylon pearl 

 fishery 80,000,000 shells were dredged in 1905. The interior of 

 the shell of these molluscs is lined with dense nacre or mother- 



Fig. 29. 



Trigonia margariiacea. Case 141. 

 From The Cambridge Natural History (by permission of Messrs. Macmillan& Co. 



of -pearl (v. p. 9), and the isolated pearls found in the tissues of 

 the animal are formed of the same substance. The most impor- 

 tant types of pearl are the ' orient ' and ' muscle ' pearls. Orient 

 pearls are by far the most valuable ; they are often formed 

 round a parasitic worm that has found its way into the tissues of 

 the mantle. Round this centre of irritation are built up successive 

 layers of nacre until an irregular or spherical body is formed. 

 Sometimes the pearl contains another sort of nucleus, e. g. a sand 

 grain. Muscle pearls, which are usually found in the neighbour- 

 hood of muscles, do not contain a nucleus of intrusive substance, 

 but are formed round particles of nacre. Such is the origin of 

 most of the small seed-pearls. The well-known Japanese ' culture 

 pearls ' are produced by grafting into the mantle of a Pearl Oyster 

 small pieces of the mantle epithelium of another individual, each 

 wrapped round a small piece of nacre, which serves as a nucleus. 

 ' Blister pearls ', which are of no commercial value, are found on 

 the inner surface of the shell. These result from the intrusion of 



