Bd4 
Fig. 
Fig. 
Fig. 
Fig. 
Fig. 
Fig, 
DR. H. LYSTER JAMESON ON 
18. The centre of the same pearl, consisting of a cavity, about ‘08 mm. in 
diameter, surrounded by granular repair-nacre; the cavity contains a few 
irregular strands or lamine of conchyolin-like substance, together with 
certain granular bodies, possibly of cellular origm. Preparation LXX1 3. 
x 450. 
19. Group of muscle-pearls and hypostracum-pearls, from an old unlabelled 
example of Margaritifera vulgaris in the British Museum. The centres 
of the pearls have been brought into the same plane in the drawing. 
The pseudo-nuclei range from ‘08 mm. to “15 mm. in diameter. At the 
bottom and on the left-hand sides of the sketeh are two pearls with 
eccentric pseudo-nuclei, in these eases composed of hypostracum, 
showing that the nacre-secreting epithelium arose on one side of 
the sac only. ‘The preservation does not allow of any histological 
details of the tissues being shown. Four hypostracum-pearls are also 
shown. mac., nacre; »v., coluramar pseudo-nucleus; nw.’, hypostracum 
pseudo-nucleus, only partly surrounded by nacre; hy.p., hypostracum- 
pearls ; mesc., muscle-bundles. Preparation XXWII, 4. X 380. 
PuaTE XXXIEX. 
g. 20. Centre of the pearl shown on the upper side of fig. 19. zac., nacre; col., 
colmmuar substance ; @m., amorphous substance; g7., granules in central 
cavity. Preparation XX VII, 4. xX 350. 
.21. A hypostracum-pearl, decalcified. gr., granules in central cavity; am., 
amorphous substance; hy. orgamic residue of the hypostracum. 
Preparation XXVII,1. x 550. 
21a.Part of a section ground through the middle of a hypostracum-pearl, 
showing the columnar and stratified nature of its substance, and the small 
central cavity. From an unlabelled specimen in the British Museum. 
Preparation LXXVITI c. xX 70. 
22. Portion of the mantle of Margaritifera vulgaris, near the margin, from a 
second unlabelled specimen in the British Museum. Cleared in oil of 
cloves, and examined entire before decalcification. ‘The specimen shows a 
number of hypostraeum-pearls 7x situ. At rac. is seen a large hypostracum- 
pearl in course of transformation into a nacreous pearl by the deposition 
of nacreous layers around it. Preparation XXX. X 20. 
x, 23. The same decalcified and examined in oil of cloves. mac., as above. X 20. 
1G Geo) OU 
24. A double nucleus, with abundant contents, from a pearl in the same 
series of sections as figs. 19-21. gi., granular substance, apparently 
derived from dead organic matter: a@m., amorphous cuticle-like layers; 
am.’, « more deeply staining portion of the same substance, immediately 
surrounding the granular contents of the nucleus ; col., columnar substance ; 
nac., nacre. Preparation XXVIII, 7. > 250. 
g, 25. A small hypostracum-pearl. Central cavity without contents. Preparation 
XXVIL 11. x 100. 
ig. 26. A double hypostracum-pearl, forming the nucleus of a naereous pearl. 
am., (2) amorphous substance; fy., hypostracum ; hy.’, do. at periphery of 
amorphous substance; nae., nacre; ac.’, columnar repair-substance. 
Preparation XXVIII, 11. x 75. 
27. Small pearl, with nacreous substance overlain by hypostracum. _ ¢., central 
cavity, containing a few granules ; am., amorphous substance which passes 
over into col., columnar substance; mac., nacre, which is succeeded by 
hy., a layer of hypostracum, forming the outermost layer of the pearl. 
This specimen shows well the difference between the reaction to stain 
of hypostracum and the other substances, e. g. colamnar substance. 
Preparation XXVII, 7. xX 350. 
28, Pseudo-nucleus of the left-hand constituent of the double pearl shown in 
fiz. 49 (Plate XLIV.). From one of the unlabelled specimens of 
Margaritifera vulgaris in the British Museum. c., central mass composed 
of (2) amorphous substance and granules; col., columnar substance ; 
col.’, outer layers of the same, which pass over imperceptibly at one point 
into nae.’, Nacreous substance; aec., complete layers of nacre around 
nucleus; a., horny layer, probably composed of amorphous substance ; 
96] 
