Tllli: CEYLON rEAIlL-OYSTEll. 34^ 



and the fishery is carried on by the natives from whom the pearls 

 are boiioht by licensed tiuders. The annual yield is between £3000 

 and £4500 wortli 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 disimeter, also composed of 

 stratified columnar substance, with a minute central cavity, about 

 0'02 nun. in diameter. 



These were certainly not " Muscle '' pearls, but small " fine " 

 l)ear]s. The preparations have been returned to the lm[)erial 

 Institute. 



L. Pearls from Placiinsi ])]acenta, from Lake 

 Tampalakamam, Ceylon. 



In view of Mr. Hornell's statement, referred to in the account of 

 the work in Ceylon, to the efiect 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 Flacuna 

 placeitta, the Window-pane Oyster, I thought that a study of this 

 form would throw light on the problem in If. vulgaris. I made 

 several fruitless eflforts to obtain material from Ceylon and elsewhere. 

 I examined one small pearl, decalcified and mounted whole, in 

 Professor Herdman's collection (No. 78), labelled '' Pearl Shelled 

 from Mantle of Placuna." Tliis pearl measures about half a 

 millimetre in diameter, and the " nucleus " is a little yellowish 

 l)ody, 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 



1 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 

 lour 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 



