THE CEYLON PEARL-OYSTER. 317 



central cavity, seconrlary cavities in its svibstance, in which 

 organic particles are lodged (fig. 20). 



The variations of coarsely alveolar structure which amorphous 

 substance shows (e. g. PI. XL. fig. 29 a and PL XLII. figs. 40 & 41) 

 recall those sti'uctures which Biitschli (6, Taf. iii. figg. 20-33) 

 describes in the sphasrocrystals of (?) Trydimite formed when the 

 siliceous concretionary substance of the Bamboo (known as Tabaxir 

 or Tabasheer) is heated ; this structure is probably in great measure 

 the expression of the physical conditions (surface tension, etc.) 

 which prevail when two substances in solution or in a colloidal 

 state separate from one another to form a sphserocrystalline 

 mass. 



PI. XLII. figs. 37 & 39 are of intei'est as showing another 

 variation of the nacre, in the direction of columnar substance. 

 In this variety of nacre, the conchyolin-layei's are connected 

 by a number of thickened junctions, which tend to occur 

 in groups and which are arranged in radial rows. In surface 

 view these junctions appear as groups of dark spots on the 

 conchyolin-layers ; in radial section they are as shown in fig. 37, 

 and can also be seen in PL XXXVI. fig. 13. These junctions 

 seem to be thickenings of the walls which normally connect 

 the several conchyolin-layers of the nacre to one another; they 

 may, in fact, be regarded as local exaggerations of the condition 

 described as " granular rejiair-nacre." 



It is interesting to note the peculiar manner in which these 

 repair-substances occur in Japanese " Culture Pearls." This 

 name was given by the late Professor Mitsukuri (27 c, pp. 283-4, 

 pi. xi. fig. 1) to pearl-like bodies — "blisters," as they would be 

 called on the Australian fisheries — which are artificially produced 

 in the Japanese Pearl-Oyster, Margaritifera martensii * Dunker. 

 The production of these " Culture Pearls " is an extensive 

 industry supporting about 100 persons, and is carried on by 

 Mr. Mikimoto on leased areas of sea-bottom in the Bay of Agu, 

 Shima Province, on lines originally suggested by Prof. Mitsukuri 

 in 1890. It has been going as a commercial success since 1898, 

 when the first crop of " Culture Pearls " was marketed. In 1905 

 the number of oysters operated on per year was from 250,000 to 

 300,000. 



The process, which is protected by patents, is analogous to that 

 adopted by the Chinese in the production of " Buddha Pearls " 

 in the fresh-water mussel, Dipsas plicatus, and to the method 

 discovered by Linnteus in the 18th century (see Herdman, 16 a), 

 and consists in the introduction between the shell and the mantle t 

 of a bead of nacre, which in due course (the time allowed in 

 Japan is four years) becomes thickly coated over with nacre, 



* This mollusc is regarded by some naturalists as a local race of M. vulgaris, to 

 which it is undoubtedly very closely related. Whether it be called M. martensii or 

 31. vulgaris var. martensii is largely a matter of individual taste. 



t Tliis is apparently done via the edge of the shell and not by drilling as in the 

 Linnffius process. 



