﻿THE 
  STOBY 
  OF 
  A 
  PEARL. 
  47 
  

  

  by 
  Holdsworth, 
  at 
  Ceylon 
  ; 
  the 
  two 
  fishes 
  with 
  strong 
  jaws 
  

   alluded 
  to 
  by 
  Bouchon-Brandely 
  at 
  Tuamotu 
  ; 
  or 
  Batistes, 
  

   which 
  also 
  eats 
  the 
  Pearl-Oysters 
  ; 
  or 
  in 
  a 
  combination 
  of 
  them. 
  

  

  In 
  shape 
  pearls 
  vary 
  greatly. 
  They 
  are 
  round, 
  oblong, 
  pear- 
  

   shaped, 
  kidne} 
  r 
  -sliaped, 
  drop-like, 
  biconvex 
  (Mobius's* 
  pearl, 
  which 
  

   has 
  a 
  darker 
  brownish-green 
  middle 
  mass), 
  slightly 
  dumb-bell- 
  

   shaped, 
  and 
  of 
  various 
  irregular 
  outlines. 
  In 
  colour 
  they 
  are 
  

   white, 
  grey, 
  black, 
  lilac, 
  roseate, 
  blue, 
  brown, 
  or 
  reddish 
  brown, 
  

   and 
  some 
  are 
  fancifully 
  termed 
  golden 
  (S. 
  Grand). 
  

  

  Whilst 
  it 
  may 
  be 
  stated 
  generally 
  that 
  they 
  are 
  formed 
  of 
  

   layer 
  upon 
  layer 
  of 
  the 
  iridescent 
  nacreous 
  secretion, 
  yet 
  they 
  

   diverge 
  much 
  in 
  appearance 
  and 
  structure. 
  Thus, 
  in 
  a 
  section 
  

   of 
  a 
  decalcified 
  pearl 
  from 
  the 
  Common 
  Mussel 
  (Plate 
  I., 
  fig. 
  4), 
  

   the 
  field 
  is 
  only 
  marked 
  by 
  the 
  concentric 
  rings 
  of 
  conchyolin, 
  

   which 
  are 
  more 
  distinct 
  in 
  stained 
  preparations, 
  and 
  which 
  

   encircle 
  the 
  nucleus, 
  or 
  it 
  may 
  be 
  nuclei. 
  The 
  rings 
  vary 
  in 
  

   breadth, 
  sometimes 
  being 
  proportionally 
  broad 
  round 
  the 
  nucleus, 
  

   sometimes 
  narrow. 
  Only 
  in 
  certain 
  examples 
  is 
  a 
  very 
  minute 
  

   reticulation 
  apparent, 
  and 
  it 
  seems 
  to 
  be 
  different 
  in 
  character 
  

   from 
  that 
  of 
  the 
  succeeding 
  form. 
  On 
  the 
  other 
  hand, 
  a 
  section 
  

   of 
  a 
  pearl 
  from 
  the 
  Mussel 
  (Margaritana) 
  of 
  the 
  Tay 
  (Plate 
  I., 
  

   fig. 
  2) 
  presents 
  over 
  the 
  entire 
  surface 
  a 
  series 
  of 
  bold 
  reticula- 
  

   tions 
  with 
  striated 
  walls, 
  from 
  the 
  prismatic 
  condition 
  of 
  the 
  

   structure, 
  whilst 
  the 
  concentric 
  lines 
  are 
  less 
  conspicuous. 
  

   Moreover, 
  in 
  certain 
  decalcified 
  sections 
  (Plate 
  I., 
  fig. 
  3) 
  the 
  

   radii 
  of 
  conchyolin 
  are 
  in 
  great 
  strength, 
  and 
  form 
  a 
  complex 
  

   and 
  beautiful 
  series, 
  bound 
  at 
  short 
  intervals 
  by 
  the 
  concentric 
  

   bands. 
  The 
  same 
  pearl, 
  indeed, 
  may 
  present 
  both 
  appearances 
  

   in 
  different 
  parts, 
  according 
  to 
  the 
  line 
  of 
  section. 
  The 
  same 
  

   structure 
  is 
  observed 
  in 
  a 
  section 
  of 
  either 
  kind 
  of 
  pearl 
  without 
  

   decalcification 
  (Plate 
  L, 
  fig. 
  1). 
  

  

  As 
  in 
  the 
  Common 
  Mussel, 
  the 
  pearl 
  of 
  the 
  Mussel 
  from 
  the 
  

   Tay 
  is 
  surrounded 
  by 
  a 
  sheath 
  of 
  epithelium 
  of 
  considerable 
  

   thickness 
  (Plate 
  I., 
  fig. 
  5), 
  and 
  it 
  is 
  supported 
  by 
  connective- 
  

   tissue 
  and 
  muscular 
  fibres. 
  At 
  some 
  period 
  the 
  sheath 
  of 
  the 
  

   pearl 
  becomes 
  continuous 
  with 
  the 
  outer 
  cellular 
  layer 
  of 
  the 
  

   mantle, 
  though 
  the 
  origin 
  of 
  the 
  sheath 
  may 
  be 
  independent. 
  

  

  * 
  ' 
  Sitzungs-bericht 
  d. 
  Gesell. 
  Nat. 
  Freunde 
  zu 
  Berlin,' 
  10 
  Jan. 
  1902, 
  

   No. 
  1, 
  p. 
  1. 
  

  

  