THE CEYLON PEARL-OYSTER. 345 
and the fishery is carried on by the natives from whom the pearls 
are bought by licensed traders. The annual yield is between £3000 
and £4500 worth of pearls, as valued locally. The first of the two 
pearls examined was decalcified by me, but was not sectioned. 
The centre was a double pseudo-nucleus, slightly over a millimetre 
in length, and a little under a millimetre in breadth. It was 
obviously composed of stratified columnar substance. External 
to this the pearl, which was quite a good one, was composed of 
normal nacre. 
The second example was decalcified and examined entire, and 
then sectioned. The centre was a spherical pseudo-nucleus, 
rather less than half a millimetre in diameter, also composed of 
stratified columnar substance, with a minute central cavity, about 
0-02 mm. in diameter. 
These were certainly not “ Muscle” pearls, but small “ fine 
pearls. The preparations have been returned to the Imperial 
Institute. 
” 
L. Pearls from Placuna placenta, from Lake 
Tampalakamam, Ceylon. 
In view of Mr. Hornell’s statement, referred to in the account of 
the work in Ceylon, to the effect that he had determined that a 
Cestode larva, similar to or identical with that found in the pearl- 
oyster, caused the pearls produced in such quantities by Placuna 
placenta, the Window-pane Oyster, I thought that a study of this 
form would throw light on the problem in J/. vulgaris. I made 
several fruitless efforts to obtain material from Ceylonand elsewhere. 
I examined one small pearl, decalcified and mounted whole, in 
Professor Herdman’s collection (No. 78), labelled “ Pearl Shelled 
from Mantle of Placuna.” This pearl measures about half a 
millimetre in diameter, and the “nucleus” is a little yellowish 
body, about ‘05 mm. in diameter, and thus too small to be Hornell’s 
larva (which is said to be from *2 to ‘4 mm. in diameter), even if 
it is of foreign origin. However, it was necessary to have 
material to section before the matter could be investigated. As 
I could not get other material Professor Dunstan very kindly 
allowed me to take a few Placuna pearls from the collection on 
exhibit in the Ceylon Court at the Imperial Institute. Two very 
well marked size-groups occur in the pearls in this collection, 
there being a majority of minute irregular pearls, 1 mm. in 
diameter and under, and a minority of larger ones, averaging from 
2 to 3 mm. in diameter, Prof. Dunstan allowed me to take 
four of the latter and sixteen of the former. All these were 
decalcified, and two of the larger and four of the smaller were 
sectioned. 
Of the larger pearls, Preparation CXLVI (sectioned) had for 
its centre a cavity, ‘(05 mm. in diameter, containing a little 
irregular columnar repair-substance. Preparation CXLVII had 
a central cavity containing a small amount of yellow refractive 
[87] 
