118 MADRAS FISHERIES BULLETIN [VOL. XVI, 



season. This comparatively short life involves a corresponding 

 small size in the pearls produced ; the larger oysters of Australia — 

 the Gold lip (M maxima) particularly — have a far longer life and in 

 consequence the maximum and average sizes of pearls produced by 

 them are much greater, but the lustre and skin are not so perfect as 

 in the Indian and Ceylon pearls. 



The shell substance consists of three principal superimposed 

 layers, with a fourth at those places where the muscles of the body 

 are attached to the inner surface of the valves. The outermost 

 layer, the periostracum, is tough, thin and horny, and yellowish in 

 colour; the second, the prismatic layer, is thicker and consists of 

 tiny columnar prisms of carbonate of lime, arranged at right angles 

 to the surface of the shell; in this layer are the red and yellow 

 pigments that give bright colouring to very young shells and a 

 reddish mottling to the older ones. Both these layers are secreted 

 by the cells on or towards the edge of the mantle as will be 

 described later. The nacreous or third layer is the one that yields 

 mother-of-pearl. Its lustrous, white, iridescent appearance is 

 characteristic, and is due to the way in which its substance is built 

 up of tiny and slightly irregular lamellae deposited horizontally 

 and overlapping one another. As a result the surface shows a fine 

 lineation that causes a deflection of light rays, resulting in the 

 peculiar lustrous iridescence associated with this substance. True 

 pearls consist of the same material. Both the periostracum and 

 the prismatic layer are limited in their growth as they are produced 

 normally only by the cells near the mantle edge ; nacre on the 

 contrary being secreted by the greater part of the exterior surface 

 of the mollusc's body, increases continuously throughout life, at 

 first both superficially and in thickness, and later, when full size 

 has been attained, in thickness only. Thus in old oysters the 

 nacreous layer may attain quite a considerable thickness, parti- 

 cularly in the inner region near the hinge. The fourth layer, or 

 hypostracum, is closely akin to the prismatic layer in structure, 

 being built up of distinct columnar prisms ; it covers only those 

 portions of the shell to which muscles are attached. The largest 

 area of hyprostracum is that beneath the insertion of the great 

 adductor muscle, towards the hinder central region of each valve. 

 It has much importance in the origination of muscle pearls, which 

 comprise the greater number of seed and baroque pearls as we 

 sha 11 see below. 



