﻿MUSSEL 
  RESOUECES 
  OF 
  THE 
  ILLINOIS 
  RIVER. 
  35 
  

  

  Those 
  shells 
  which 
  have 
  received 
  an 
  external 
  injury, 
  however, 
  and 
  

   are 
  distorted 
  in 
  consequence, 
  seldom 
  contain 
  anything 
  of 
  value; 
  

   but 
  it 
  does 
  appear 
  unquestionable 
  that 
  other 
  distorted 
  shells 
  owe 
  their 
  

   peculiar 
  condition 
  to 
  the 
  presence 
  of 
  a 
  pearl; 
  the 
  abnormal 
  shape 
  is 
  

   the 
  direct 
  result 
  of 
  an 
  accommodation 
  to 
  the 
  disturbing 
  nacreous 
  

   growtli 
  within, 
  which 
  as 
  it 
  slowly 
  develops 
  maintains 
  its 
  position 
  

   relative 
  to 
  the 
  margin 
  of 
  the 
  mantle 
  and 
  valves. 
  Certain 
  shells 
  

   wliicli 
  have 
  come 
  to 
  the 
  writer's 
  attention 
  contained 
  valuable 
  pearls 
  

   as 
  well 
  as 
  showing 
  clearly 
  the 
  abnormal 
  condition. 
  As 
  viewed 
  from 
  

   the 
  outside, 
  such 
  shells 
  liave 
  a 
  slight 
  and 
  gradually 
  broadening 
  ridge 
  

   running 
  diagonally 
  down 
  from 
  near 
  the 
  beak 
  or 
  umbone 
  to 
  the 
  tip 
  

   or 
  the 
  posterior 
  end 
  on 
  one 
  valve 
  and 
  a 
  corresponding 
  valley 
  or 
  

   depression 
  down 
  the 
  other. 
  Within 
  the 
  shell 
  the 
  opposite 
  condition 
  

   of 
  the 
  ridge 
  and 
  valley 
  is 
  true. 
  From 
  this 
  circumstance 
  it 
  is 
  possible 
  

   to 
  estimate 
  with 
  some 
  degree 
  of 
  accuracy 
  the 
  age 
  of 
  the 
  pearl 
  by 
  the 
  

   lines 
  of 
  growth 
  on 
  the 
  external 
  surface 
  of 
  the 
  shell, 
  noting 
  where 
  

   the 
  disturbance 
  begins 
  and 
  ends. 
  In 
  studying 
  these 
  distorted 
  shells 
  

   the 
  peculiar 
  fact 
  is 
  brought 
  out 
  that 
  the 
  pearl 
  is 
  always 
  on 
  that 
  side 
  

   of 
  the 
  shell 
  whose 
  valve 
  shows, 
  viewed 
  externally, 
  the 
  valley 
  or 
  

   depression. 
  In 
  a 
  large 
  percentage 
  of 
  these 
  distorted 
  shells 
  the 
  

   pearls 
  have 
  been 
  found 
  on 
  the 
  left 
  valve. 
  The 
  pearl, 
  near 
  the 
  margin 
  

   of 
  the 
  mantle, 
  causes 
  tlie 
  abnormal 
  action 
  of 
  the 
  latter, 
  which 
  pro- 
  

   duces 
  the 
  slight 
  inward 
  curve 
  of 
  the 
  nacreous 
  deposit, 
  so 
  that 
  it 
  might 
  

   be 
  said 
  that 
  the 
  pearl 
  lies 
  on 
  or 
  near 
  a 
  ridge. 
  For 
  the 
  opposite 
  

   valve 
  the 
  mantle 
  condition 
  is 
  reversed, 
  resulting 
  in 
  the 
  outward 
  

   deflection 
  of 
  the 
  nacre, 
  or 
  the 
  valley. 
  

  

  Some 
  of 
  these 
  apparently 
  valuable 
  crippled 
  mussels 
  contain 
  no 
  

   pearl 
  whatever, 
  although 
  the 
  evidence 
  is 
  plainly 
  \vritten, 
  both 
  within 
  

   and 
  without, 
  in 
  the 
  texture 
  of 
  the 
  shell. 
  When 
  this 
  condition 
  is 
  

   observed, 
  the 
  pearls 
  are 
  said 
  to 
  have 
  been 
  lost 
  or 
  ''shed." 
  If 
  the 
  

   pearl 
  were 
  lost 
  some 
  years 
  before 
  the 
  mussel 
  was 
  taken, 
  the 
  shell 
  often 
  

   recovers 
  in 
  part 
  its 
  natural 
  shape 
  from 
  that 
  time 
  on, 
  as 
  shown 
  by 
  the 
  

   lines 
  of 
  growth 
  and 
  the 
  posterior 
  edge 
  or 
  lip, 
  and 
  it 
  is 
  hardly 
  worth 
  

   the 
  time 
  and 
  trouble 
  to 
  try 
  to 
  find 
  a 
  pearl 
  within. 
  

  

  Accompanying 
  plates 
  show 
  some 
  examples 
  of 
  distorted 
  or 
  other- 
  

   wise 
  injured 
  mussels 
  and 
  will 
  assist 
  in 
  understanding 
  this 
  interesting 
  

   subject. 
  

  

  QUALITY 
  AND 
  VALUE 
  OF 
  ILLINOIS 
  PEAELS. 
  

  

  The 
  quality 
  of 
  the 
  Illinois 
  River 
  pearls 
  and 
  slugs, 
  though 
  very 
  good, 
  

   does 
  not 
  quite 
  equal 
  that 
  of 
  the 
  pearls 
  and 
  slugs 
  of 
  the 
  Wabash 
  and 
  

   some 
  other 
  tributaries 
  of 
  the 
  Mississippi. 
  Nearly 
  all 
  of 
  the 
  different 
  

   species 
  of 
  mussels 
  have 
  produced 
  pearls 
  and 
  slugs, 
  but 
  the 
  washboard 
  

   (Quadrula 
  Tieros) 
  is 
  the 
  principal 
  pearl-bearing 
  shell 
  of 
  the 
  Illinois, 
  and 
  

   the 
  blue-point 
  and 
  three-ridge 
  {Q. 
  UTidulata 
  and 
  plicata) 
  are 
  among 
  the 
  

  

  22889°— 
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  33 
  

  

  