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 CXLVI and CXLVII. Of the sixteen minute 
pearls four were sectioned. Preparation CXXXV_ had in its 
centre a simple cavity, like the pearls in Dr. Kelaart’s material 
of MW. vulgaris. Preparation CX XX VI showed a central cavity, 
with around it some columnar substance, or perhaps hypostracum. 
Preparation CXXXVII had a large dark pseudo-nucleus, over 
half a millimetre long, which might easily have been taken for < 
foreign body, when examined entire, but 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 CX XXVIII 
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 nacreous conchyolin, or no obvious nucleus or pseudo- 
nucleus. Six had such cavities, partly or entirely surrounded by 
repair substances or hypostracum, one had a few yellowish 
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. cumingil Reeve 
(the Black-edged Mother-of-Pearl Oyster), from Likitea, 
Gambier 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. Seurat 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 
concentrically deposited columnar repair-substance, passing over 
peripherally 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 
described in J. vulgaris. 
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