GLOSSARY 143 



Oriental pearls. Pearls from true pearl oysters of the genus 



Margaritifera ; applied first to pearls from the Indian seas. 

 Paar. The banks or shallows in the Gulf of Manaar on which 



the pearl oysters live. 

 Pallial cavity. The space inside a bivalve shell, which is part 



of the outer world, and usually filled with sea water or fresh 



water according to the home of the mollusc. 

 Pearl sac. The little sac or bag of epithelium which seci-etes 



the layers that build up a pearl. 

 Periostracum. The outermost horny layer of mollusc shells. 

 Petal pearls. Flattened, leaf-like pearls. 

 Pinna, A bivalve, common in the Mediterranean, from which 



pearls are obtained. These pearls have little lustre. 

 Placuna. The window-pane oyster of Ceylon. 

 Prismatic substance. The substance of which the prismatic 



layer of mollusc shells is composed. It lies just under the 



outermost layer of the shell, and on its inner surface comes 



into contact with the nacreous or mother-of-pearl laver (see 



Fig. 2). 

 Seed pearls. Small pearls, round or irregular, but only } grain 



or less in weight. 

 Skinning or peeling. The removal of the outer layer of a pearl. 

 Slugs. Nacreous excrescences ; irregularly shaped pearly masses 



from fresh water shells. 

 Tapes. A bivalve, common off" the coasts of England and France. 

 Trematode. A flat worm which is parasitic eithei- externally or 



internally. The most familiar example is the "hver fluke" 



of the sheep. Another species causes pearl formation in the 



edible marine mussel. 

 True pearls. Pearls which are formed in a pearl sac, and which 



are found free in the shellfish, without any connection with 



the shell. They may not be composed of any nacre. Any, 



or all, layers of the shell may be found. 

 Twinned pearls. Pearls which have come together and then 



been coated with layers of nacre (see Fig. 7). 



