THE CEYLON PEARL-OYSTER. 299 



(xv.) Twenty pearls from Placuna placenta, from Lake Tam- 

 palakamam, Ceylon, from the Imperial Institute. 



(xvi.) About a dozen pearls from Margaritifera margaritifera 

 var. cumingii, from the Gambler Archipelago. 



In the course of these obsei'vations, apart from studies on the 

 structure and formation of pearls in other forms, 356 pearls 

 derived, with perhaps a few exceptions, from Margaritifera 

 vulgaris, chiefly from Ceylon, have been decalcified and examined, 

 175 of these having been studied in sections. 



(8) Methods. 



For decalcification, whether the pearls were free or in situ, 

 preserved in alcohol or dry, I found alcohol of about 50 per cent. 

 strength, to which a few drops of nitric acid had been added, was 

 the best. Of course, a preserved pearl nevei- decalcifies as well as 

 a fresh one. In the case of dry pearls, where only the nucleus 

 a,nd central parts are required for examination, it is often 

 an advantage, di;ring decalcification, to strip off the outer layers 

 of conchyolin, thus facilitating the penetration of the reagent. 

 Bubbles of carbon dioxide, generated in the process of decalci- 

 fication between the conchyolin-layers, cause a great deal of 

 trouble, especially in old dry pearls, where the conchyolin seems to 

 be particularly leathery and impei-meable. Such bubbles often 

 greatly distort the normal structure of the pearl, as seen in section, 

 by tearing the conchyolin-layers apart and causing great spaces 

 between them. The most suita,ble reagent for expelling the bubbles 

 is absolute alcohol, in which the decalcified pearl is placed for 

 a. few days. In some cases, however, it was necessary to extract 

 the gas under an air-pump. 



For staining decalcified pearls ha?matoxylin was used, also 

 borax carmine. Sections of the pearls in the tissiies were 

 stained sometimes with borax carmine and picro-indigo-carmine*, 

 sometimes with hfematoxylin and eosin or orange, occasionally 

 with other reagents. 



(9) Structure of the Shell-Substances. 

 The shell of Margaritifera consists of the following parts : — 

 (i.) the outermost layer or so-called Periostracum ; 



(ii.) the prismatic layer, forming with (1) the " back" of the 

 shell, the fragile, dark-coloured " lip,"' and the lappet- 

 like processes of the margin ; 



* Borax carmine (Grenadier's) in bulk : — 



Picro-iudigo-carmine as under, on the slide. 



A. Saturated solution of picric acid in 90 per cent, alcohol. 



B. Saturated solution of indigo-carmine (Grubler) in 70 per cent. 



alcohol. 

 1 part of A, 2 parts of B, 6 parts of 70 per cent, alcohol. 



