346 DR. H. LYSTER JAMESON ON 



granular matter. The remaining two, which were decalcified but 

 not sectioned, had no obvious nuclei, the centres being, apparently, 

 like those of CXLYl and OXLVII. Of the sixteen minute 

 pearls four were sectioned. Preparation CXXXY had in its 

 centre a simple cavity, like the peai-ls in Dr. Kelaart's material 

 of M. vulgaris. Preparation CXXXVI showed a central cavity, 

 with around it some columnar substance, or perhaps hypostracum. 

 Preparation OXXXVII had a large dark pseudo-nucleus, over 

 half a millimetre long, which might easily have been taken for a 

 foreign body, when examined entire, biit which on being sectioned 

 proved to be composed of concentrically laminated amorphous 

 substance, passing over in places into columnar repair-substance, 

 and containing a minute central cavity. Preparation CXXXVIII 

 had in its centre a little brown body, probably composed of 

 amorphous substance, but -this specimen did not prove satisfactory 

 on being sectioned. The twelve remaining pearls were decalcified 

 and examined entire in oil of cloves, but were not sectioned. Of 

 these four had either a small central cavity, surrounded by 

 ordinary naci-eous conchyolin, or no obvious nucleus or j^seudo- 

 nucleus. Six had such cavities, partly or entirely surrounded by 

 repair substances or hypostracum, one had a few yellowislx 

 granules in its cavity, and one had a dark body, about '3 mm. in 

 diameter, of doubtful nature, but showing no characters that 

 would warrant its identification as a Cestode. 



The absence of evidence in support of Mr. Hornell's Cestode 

 theory of the origin of Placuna Pearls is an additional argument 

 in favour of the opinion that the supposed observation of these 

 worms by Mr. Hornell and Professor Herdman in the pearls of 

 M. vulgaris was a mistake. 



M. Pearls from Margaritifera margaritifera var. cumingii Reeve 

 {the Black- edged M other- of- F earl Oyster), from Rikitea, 

 Gamhier Archipelago. 



In order to enable me to compare the pearls of M. vulgaris with 

 those of this species, in which Seurat claims that Cestodes occur as 

 nuclei, M, iSeurat most generously placed at my disposal some 

 material. This consisted of three slides on which sections of 

 pearls from this species had been mounted, but which had not 

 been stained or completed. I stained and finished these three 

 slides. In two of them the centre of the pearl could not be found, 

 in the third there was not a section through the exact centre, but 

 there was one that cannot have been far from it. This showed no 

 Cestode remains, but a pseudo-nucleus rather less than half a 

 millimetre in diameter, consisting in its more central part of 

 concentricall}' deposited columnar repair-substance, passing over 

 periphei-ally into alternate layers of amorphous and columnar 

 substance, which in their turn passed over into nacre, through 

 catenulated and granular transitional substance, such as I have 

 desciibed in M. vulgay'is. 



