FAMILY 3. AVICULACEA. 151 



innovation : in classifying the whole system of organic nature, both animal 

 and vegetable, this discreet naturalist had an immense scheme to contend 

 with, and was therefore especially cautious in the introduction of new 

 genera; still, whilst we esteem his operations for their simplicity and 

 distinctness, we must admit that the efforts of speculators, such as Schu- 

 macher, De Montford and Gray, are useful and important when fully and 

 fairly illustrated. The genus Avicula was then revived by Bruguiere, 

 including those species which Lamarck afterwards distinguished by the 

 new title of Malleus. 



The Avicula rnargaritifera is that mollusk so celebrated for the produc- 

 tion of the true Oriental pearl ; the nacre with which it lines the interior 

 of its shell exceeds that of all other species in lustre and brilliancy, and 

 it is on this account that the pearls of this animal are so highly esteemed 

 for their purity and beauty. Pearls are small nacreous balls, that become 

 formed and hardened within the body of the animal ; they are found 

 deposited in the most fleshy parts, particularly within and around the 

 adductor muscle, and are said to be occasioned by the overcharge of those 

 glands, whose function it is to secrete the nacreous fluid destined for the 

 internal lining of the shell. When the animal is thus diseased, this 

 beautifully iridescent fluid is very irregularly discharged, being also de- 

 posited upon the inner surface of the shell in little heaps of hemispherical 

 excrescences ; these are often detached, and form articles of commerce as 

 pearls of inferior value, the former being considered more precious both 

 on account of their rotundity of form and the clearness and beauty of 

 their complexion. The disease to which we are indebted for these valu- 

 able gems is very much increased by the introduction of a piece of stick 

 or any sharp instrument within the shell, particularly if thrust into the 

 animal in the fleshy parts of the mantle, or between the lobes. In 

 fact, many of the proprietors of the Indian Pearl Fisheries preserve the 

 Avicula mar garitif eras alive ; they are said to keep large vivaria of them 

 for the sole purpose of producing an inward accumulation of pearls by 

 this cruel mode of irritation. 



Pearls have been found in many other species of Mollusca ; in all the 

 Aviculacea, in the Tridacnas, the Naiades, and even in the Haliotides 



