354 DR. II. LYSTER JAMESOK OX' 



Fig. 18. Tlie centre of the sinme pearl, consisting of a cavity, about "08 mm. in 

 diameter, smTounded by granular repair-nacre; the cavity contains a few 

 ii regular strands or laminie of conchyolin-like substance, together with 

 certain granular bodies, possiblj' of cellular origin. Preparation LXXI J. 

 X 450. 



Fig. 19. Group of muscle-pearls and hyiwstracura-pearls, from an old unlabelled 

 exi\m\Ac oi 3Iargaritifera vulffaris 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 sketch are two pearls with 

 eccentric pseudo-nuclei, in these cases 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, nac, nacre; nu., columnar pseudo-nucleus; ■»?«.', hypostracum 

 pseudo-nucleus, only partly surrounded by nacre ; Jri/.p-, hj'postracum- 

 pearls : ))?«.>tc., muscle-bundles. Preparation XXVII, 4. X 30. 



PXATE XXXIX. 



Fig. 20. Centre of the pearl shown on the upper side of fig. 19. 7iac., naci'e ; roL, 

 columnar substance ; am., amorphous substance ; //»'., granules in central 

 cavity. Preparation XXVII, 4. X 350. 



Fig. 21. A hypostracum -pearl, decalcified, ^r., granules in central cavity ; am., 

 amorphous substance; Jii/., organic residue of the hypostracum. 

 Preparation XXVII, 1. x'550. 



Fig. 21 a. Part of a section ground tlirough the middle of a hypostracum-pearl, 

 showing the columnar and stratified nature of its substance, and the small 

 central cavitv. From an unlabelled specimen in the British Museum. 

 Preparation LXX VI II c. X 70. 



Fig. 22. Portion of the mantle of Marc/aniifera vulgaris, near the margin, from a 

 second unlabelled specimen in the British Museum. Cleared in oil of 

 cloves, and examined entire before decalciiication. The specimen shows a 

 number of hypostracum-pearls in situ. At nac. is seen a large hj'postracum- 

 pearl in course of transformation into a nacreous pearl by the deposition 

 of nacreous layers aroiind it. Preparation XXIX. X 20. 



Fig. 23. The same decalcified and examined in oil of cloves, nac, as above. X 20. 



Plate XL. 



Fig. 24. A double nucleus, with abundant contents, from a pearl in the same 

 series of sections as tigs. 19-21. gr.. granular suljstance, apparently 

 derived from dead oi'gauic matter; am., amorphous cuticle-like layers; 

 am.', a more deepl}' staining portion of the same substance, immediately 

 surrounding the granular contents of the nucleus ; col., cohmiiuir substance ; 

 nac, nacre. Preparation XXVII, 7. X 250. 



Fig. 25. A small hypostracum-pearl. Central cavity without contents. Preparation 

 XXVII, 11. X 100. 



Fig. 26. A double hypostracum-pearl, forming the nucleus of a nacreous pearl. 

 am., (?) amorphous substance; /y/., hypostracum ; hii.', do. at periphery of 

 amorphous substance; nac, nacre; nac.', columnar repair-substance. 

 Preparation XXVII, 11. X 75. 



Fig. 27. Small pearl, with nacreous substance overlain by hypostracum. c, central 

 cavity, containing a fev>' granules ; am., amorphous substance which passes 

 over into col., columnar substance ; nac, nacre, which is succeeded by 

 /),(/., a layer of hypostracum, forming the' outermost layer of the pearl. 

 This specimen shows well the dift'erence between the reaction to stain 

 of hypostracum and the other substances, e. q. columnar substance. 

 Preparation XXVII, 7. X 350. 



Fig. 28. Pseudo-nucleus of the left-hand constituent of the double pearl shown in 

 fig. 49 (Plate XLIV.). From one of the unlabelled specimens of 

 Margaritifera vulgaris in the British Museum, c, central mass composed 

 of (?) amorphou-S substance and graimles ; col., columnar substance : 

 col.', outer layers of the same, which pass over imperceptibly at one point 

 into nac.', nacreous substance ; nac, complete layers of nacre around 

 nucleus; am., horny layer, probably composed of nmorphous substance : 



