270 HABITATS OF THE PEARL-OYSTER. 



the Indian Ocean ; and his statement still holds good. 

 The avicula abounds, for example, off the shores of 

 Ceylon, accumulating in banks of several miles in extent. 

 It is very plentiful, also, in the Persian Gulf. Of late 

 years a large number have been drawn from the Arabian 

 Gulf, being popularly known as Red Sea jewels. Amerigo 

 Vespuccio, who unwittingly deprived Columbus of the 

 honour of giving his name to the New World, records 

 that they were abundant in the regions he visited in the 

 course of his second voyage. The pearl-oysters are found, 

 in fact, at Panama in South America, St. Margarita in 

 the West Indies, the Coromandel Coast, the Sooloo 

 Islands, the Bahrein Islands, and the Mediterranean. 



The avicula is of an oblique, oval form, with a shell 

 longitudinally ribbed, and marked by concentric folia- 

 tions which gradually disappear as the animal grows 

 older. There are about twenty species. 



Most of the molluscs, as our readers will remember, are 

 provided with a fluid secretion which they use to line the 

 interior of their shells, and to coat the otherwise rugged 

 material with a smooth, lustrous surface. This is not 

 only a matter of beauty but of usefulness, since it saves 

 the soft, tender body of the animal from injurious friction. 

 The secretion is laid in semi-transparent films of great 

 tenuity, which, as a whole, are remarkable for their 

 iridescence, and in some species acquire a sufficient thick- 

 ness to be wrought and moulded by human skill. The 

 material itself, in its hardened state, is called nacre, or 

 " mother-of-pearl." 



Now the pearl which is so highly prized as an orna- 

 ment of rank and beauty is nothing more than a spherical 

 or rounded fragment of nacre. But whereas the nacre is 



