356 DR. H. LYSTER JAMESON ON 



tlie imievmost layer of the columnar pseuJo-micleus, col.; nac, nacre; 

 nac.', radiallj' marked, eaten uiated variety of nacre, characterised by 

 radial rows of junctions which occur in small groups between conchyolin- 

 layer^ (jimc.) ; nac", the normal nacre, passing over into this specialised 

 variety of nacre. Preparation LIV g. X 300. 



Fig. 38. Tangential section through the alveolar-columnar substance shown at 

 col. in lig. 37. X 300. 



Fig. 39. Tangential section through the radially catenulated repair-nacre, shown at 

 nac' in fig. 37. June, the groups of junctions in radial rows, seen in 

 surface view. The section is near the inner surface of this layer. X 300. 



Fig. 40. Centre of the imperfectly decalcified brown pearl, shown in PI. XLVI.. 

 fig. 57. WM., nucleus with rhombohedra of calcite ; am.', lining of 

 amorphous substance ; col., columnar layer ; col.tr., transition-layer from 

 alveolar-columnar substance to amorphous substance; am., amorphous 

 substance, with scattered alveoli, containing calcium carbonate. Pre- 

 paration XL. X 250. 



Fig. 41. Part of the same pearl, external to that shown on fig. 40, showing the 

 transition from amorphous substance, am., with scattered alveoli, through 

 an intermediate substance, tr., to needle-like prismatic substance, col. 

 X 250. 



Fig-. 42. Transition from the needle-like prismatic substance, col., shown in fig. 41, 

 to the ordinary pi-ismatic shell-substance, ^r., similar to that of the shell. 

 al., minute alveoli in the thickenings of the walls of the prisms, similar 

 to those observed by Romer (32). X 250. 



Plate XHII. 



Fig. 43. A portion of the repair-substance in the body of a pearl, shown at col. 

 ill fig. 36, showing transitions from amorphous substance to nacre. 

 nu., outer part of the pseudo-nucleus; am., amorphous substance, with 

 alveoli, which passes over into finely columnar substance, col. This 

 ill turn goes over through granular repair-nacre, gr., into normal nacre, 

 nac', and is followed by layers of normal nacre {nac) e.xternally. Pre- 

 paration LIU. X 400. 



Fig. 44. Portion of the contents of the central cavity of one of the pearls purchased 

 in Cej'lon ; showing also a portion of the columnar substance which 

 surrounded the centre. The contents drawn are selected from a number 

 of the sectioas in the series cut from this specimen, nac, nacreous 

 substance of the pearl ; col., columnar repair-substance surrounding the 

 nucleus; «t»(., -aw?.', amorphous substance; c^ia., diatoms ; sjyie., fragments 

 of sponge-spicules ; veg., vegetable debris. Preparation LIV j. X 500. 



Fig. 45. Fragment of a Radiolarian shell, from the centre of another of the same lot 

 of pearls. Preparation LIV e. X 600. 



Plate XLIV. 



Figs. 46, 46 a. Pearl from a cluster of pearls in one of Dr. Kelaart's specimens. 

 Decalcified and examined, 46, entire in oil of cloves, 46 a, in section. The 

 diameter of the pearl was just 2 mm., that of the central denser part was 

 1'3 mm. There is a simple central cavity. Preparation XVI. X 27. 



Figs. 47, 47 a. Another pearl from Dr. Kelaart's material, decalcified and examined, 

 47, entire, and, 47 a, in section. The diameter of the pearl was 2 mm. ; that 

 of the dark, opaque, yellowish centre rather over 1 mm. Examined entire 

 the nucleus appears amorphous, and might well be mistaken for the dead 

 remains of a parasite ; but this peculiarity was found, on sections being 

 made, to be due to a break in the continuity of the nacreous layers 

 and the interpolation of a dark layer, composed of irregularly calcified 

 conchyolin, and granules which are probably of extraneous origin. There 

 is a small central cavity, as in the rest of Dr. Kelaart's pearls, with a plug 

 of conchj^olin-like substance at one side, gr., granular dead matter ; col., 

 columnar substance ; n, nacre. Preparation XV. X 27. 



Fig. 48. A "double" pearl from Dr. Kelaart's material, decalcified, cleared, and 

 examined entire. Sections through this pearl are shown on Plates 

 XXXVII. & XXXVIII. figs. 16-18. The pearl was 2-25 mm. in diameter. 

 The pseudo-nucleus was a dark opaque body, about 0'5 mm. in diameter, 

 similar to that shown in fig. 47, and might easily be taken, on examination 

 of the entire pearl, for the shrivelled remains of a dead parasite. Sections 



