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 Gambier Archipelago. 
In the course of these observations, 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) Mrrnops. 
For decalcification, whether the pearls were free or in siti, 
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 never decalcifies as well as 
a fresh one. In the case of dry pearls, where only the nucleus 
and central parts are required for examination, it is often 
an advantage, during 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 impermeable. 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 
betweenthem. The most suitablereagent 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 hematoxylin was used, also 
borax carmine. Sections of the pearls in the tissues were 
stained sometimes with borax carmine and picro-indigo-carmine ™*, 
sometimes with hematoxylin 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 (Grenacher’s) in bulk :— 
Picro-indigo-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. 
[41] 
