130 MADRAS FISHERIES BULLETIN [VOL XVI, 



utilized such masses in the production of grotesque figures when 

 from time to time jewellery of this design is fashionable. 



True gem-pearls are those composed of lustrous nacre and of 

 symmetric shape, round or pear-shaped preferably. These are 

 produced normally in the mantle in the region between the pallial 

 line (the curved line marked by a row of muscle scars) and the limit 

 to which the deposit of nacre extends marginally — from half to three 

 quarters of an inch inwards from the free edge of the shell. Such 

 pearls seldom occur in the visceral mass area of the mantle or 

 within the muscles. As will be seen later, these gem-pearls have 

 frequently some foreign intrusive body as the nucleus, whereas 

 the less valuable pearls found in and around the muscle insertions 

 have some particle produced by the oyster itself, as the centre of 

 deposition. 



In all cases an envelope of secreting tissue — the pearl-sac — 

 surrounds the developing pearl. In the case of gem-pearls this 

 arises usually as an invagination of the external epithelial layer, 

 for the intrusive foreign body is generally found in the first 

 instance between the inner surface of the shell and the secretory 

 surface of the mantle. The latter being delicate yields readily to 

 the pressure of the intrusive body which then comes to lie in a pit 

 within the mantle substance. At first this pit is wide-mouthed, 

 but as the foreign object sinks deeper in, the mouth of the pit 

 narrows to a neck, and eventually may close; the next stage is for 

 the cyst containing the intrusive body to separate from its connex- 

 ion with the ectoderm and to assume a saccate shape conformable 

 with that of the enclosed body. For this reason, Prof. Herdinan 

 and I named pearls of this origin ' cyst pearls ' in contradistinction 

 to the small and usually irregularly shaped 'muscle pearls 'formed 

 within the muscles. This classification has the merit of simplicity 

 and I see no reason to amend it. 



Cyst-pearls in number are relatively very scarce as compared 

 with muscle pearls, and large cyst-pearls, the true gem-pearls, are 

 again relatively much scarcer than small sizes. The former con- 

 stitute the so-called Orient pearls, pre-eminent above all for their 

 lustre and purity of colour and for a peculiar suggestion of 

 translucency not seen in other pearls. 



The origin of these pearls has been a battlefield of theory in the 

 past ; the resultant confusion appears to me to be due in large part 

 to the lack of recognition that there are these two main categories 



