222 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



diameter of the shell, must be a continuous function ; but after full 

 growth it may be doubted whether the layers follow one another 

 so rapidly. If a pearl oyster perishes the animal quickly disappears, 

 being eaten by fish, crabs and small molluscs, and within twenty- 

 four hours in warm tropical waters the shell will lose its brilliancy 

 and become quite dull or " dead " owing to the chemical action of 

 the salt water. When the mother-of-pearl oyster is alive and open 

 the two mantle lobes cling closely to the surfaces of the upper 

 and lower shells, excluding the sea-water from direct contact with 

 the shell, but leaving a considerable space for it to fill between the 

 two mantle lobes. It is obvious, therefore, that the larval pearl- 

 inducing parasite or any other intruding object must first enter 

 between the mantle lobes and not between the mantle and the shell. 

 The mantle lobes' adhesion to the shell surfaces is so strong that 

 considerable leverage with the blade of the knife has to be exerted 

 to force them apart, and it is not improbable that a certain degree 

 of suction action prevails, in addition to the presence of a slime 

 of a mucilagenous cba icter which promotes adhesion. It is quite 

 obscure how a stray shell or dead "crab or other object which is 

 found in a blister attained access to the shell surface beneath the 

 mantle lobe. Nothing is known of the activities of the oyster 

 when it is closed, or what convolutions of the animal occur, but it 

 is quite possible that occasionally each mantle lobe is turned inwards, 

 bringing the slimy external epidermis into contact with the inner 

 clean ciliated epidermis of the cavity in order to clear it of intruders, 

 large or small. In this manner cestode larvae piercing the cavity 

 lining would enter under different conditions to those piercing the 

 outer lining. 



The magnified section in Fig. 5 (PI. VIII) of the artificially 

 produced Japanese blisters represents a special adaptation of a 

 custom amongst Chinese from time immemorial to produce figures 

 of Buddha, and other objects, as blisters in shells : the same thing 

 was done by Saville Kent in Australia. The configuration of the 

 layers of nacre in this figure exemplifies the process of reduction in 

 height. 



Fig. 6 (PI. VIII) is a highly magnified section of what there is 

 reason to believe is actually what it is represented to be — a mother- 

 of-pearl bead covered with layers of nacre within the tissues of a 

 living pearl oyster and converted into a pearl. The remarkable 

 difference between these layers both in thickness and in regularity 

 and those in the two following Australian pearls is of importance. 

 These new Japanese culture pearls vary of course in quality, but 

 the one from which this section was cut was of about 5 grains much 

 above the ordinary quality. Mr. Mikimoto, the originator of the 

 enterprise; does not profess to produce anything much larger than 

 7 grains. According to the specification of his American patent, 

 taken out in 1919, a portion of the shell-secreting epidermis is 



