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  UNUSUAL 
  FORMS 
  OF 
  FOSSIL 
  CRINOIDS 
  SPRINGER 
  43 
  

  

  The 
  introduction 
  of 
  a 
  narrow 
  belt 
  of 
  supplementary 
  pieces 
  between 
  the 
  

   basals 
  and 
  radials 
  would 
  be 
  sufficient 
  to 
  transform 
  any 
  Dichocrinus 
  into 
  an 
  

   Acrocrinus. 
  

  

  When 
  along 
  with 
  this 
  is 
  correlated 
  the 
  further 
  fact 
  that 
  the 
  

   radials 
  in 
  this 
  form, 
  by 
  reason 
  of 
  their 
  extreme 
  relative 
  shortness, 
  

   are 
  not 
  the 
  radials 
  of 
  Dichocrinus 
  but 
  of 
  Acrocrinus, 
  the 
  conclusion 
  

   logically 
  follows 
  that 
  it 
  is 
  best 
  placed 
  under 
  the 
  latter 
  genus. 
  

  

  This 
  means 
  that 
  the 
  tendency 
  to 
  this 
  new 
  specialization 
  by 
  way 
  

   of 
  multiplication 
  of 
  calyx 
  plates 
  began 
  earlier 
  than 
  has 
  been 
  sup- 
  

   posed; 
  and 
  it 
  will 
  be 
  shown 
  by 
  the 
  next 
  following 
  species 
  that 
  its 
  

   development 
  to 
  the 
  extreme 
  stage 
  attained 
  by 
  the 
  typical 
  Acrocrinus 
  

   was 
  by 
  a 
  further 
  gradual 
  process. 
  

  

  It 
  may 
  be 
  here 
  observed 
  that 
  along 
  with 
  all 
  these 
  various 
  deriva- 
  

   tives 
  of 
  Dichocrinus 
  the 
  strong 
  parent 
  genus 
  continued 
  to 
  carry 
  on 
  

   to 
  the 
  end 
  of 
  the 
  lower 
  Carboniferous, 
  where 
  it 
  is 
  represented 
  by 
  a 
  

   well 
  defined 
  species, 
  D. 
  superstes, 
  occurring 
  in 
  the 
  latest 
  formation 
  

   of 
  the 
  Chester. 
  The 
  only 
  one 
  that 
  survived 
  it 
  was 
  Acrocrinus, 
  which 
  

   held 
  over 
  into 
  the 
  Coal 
  Measures 
  with 
  a 
  degenerate 
  species 
  having 
  

   only 
  six 
  bands 
  of 
  supplementary 
  plates, 
  actually 
  less 
  like 
  the 
  typical 
  

   form 
  than 
  is 
  the 
  species 
  just 
  described. 
  

  

  Horizon 
  and 
  locality. 
  — 
  Mississippian, 
  Upper 
  Burlington 
  lime- 
  

   stone; 
  Burlington, 
  Iowa. 
  

  

  ACROCRINUS 
  INTERMEDIUS, 
  new 
  species 
  

  

  Plate 
  12, 
  figs. 
  2-5 
  

  

  In 
  this 
  species, 
  presenting 
  a 
  further 
  immature 
  or 
  rudimentary 
  

   stage 
  of 
  the 
  genus, 
  we 
  are 
  not 
  obliged 
  to 
  rely 
  upon 
  an 
  isolated 
  indi- 
  

   vidual, 
  but 
  are 
  fortunate 
  in 
  the 
  possession 
  of 
  a 
  series 
  of 
  excellent 
  

   specimens 
  from 
  a 
  single 
  colony, 
  by 
  which 
  all 
  the 
  characters 
  of 
  this 
  

   type 
  are 
  thoroughly 
  illustrated. 
  They 
  were 
  found 
  by 
  Frederick 
  

   Braun 
  while 
  collecting 
  for 
  me 
  in 
  the 
  season 
  of 
  1913 
  21 
  in 
  Monroe 
  

   County, 
  Illinois, 
  as 
  a 
  part 
  of 
  a 
  considerable 
  colony 
  of 
  well 
  preserved 
  

   crinoids 
  from 
  a 
  formation 
  in 
  the 
  lower 
  part 
  of 
  the 
  Chester 
  now 
  des- 
  

   ignated 
  by 
  the 
  Geological 
  Survey 
  of 
  Illinois 
  as 
  the 
  Renault 
  for- 
  

   mation. 
  

  

  The 
  material 
  consists 
  of 
  two 
  nearly 
  complete 
  specimens, 
  with 
  arms 
  

   and 
  stem 
  well 
  preserved, 
  and 
  two 
  calices 
  which 
  contribute 
  important 
  

   information. 
  All 
  agree 
  in 
  having 
  a 
  band 
  of 
  supplementary 
  plates, 
  

   of 
  either 
  tw 
  r 
  o 
  or 
  of 
  three 
  rings, 
  which 
  diminish 
  downwards, 
  inter- 
  

   calated 
  between 
  the 
  bisected 
  base 
  and 
  the 
  radials: 
  the 
  plates 
  of 
  the 
  

   successive 
  rings 
  above 
  the 
  basals 
  alternate 
  regularly 
  except 
  at 
  the 
  

   posterior 
  side, 
  where 
  there 
  is 
  a 
  vertical 
  series 
  below 
  the 
  anal. 
  Above 
  

  

  21 
  Explorations 
  and 
  Fioldwork 
  of 
  the 
  Smithsonian 
  Institution 
  in 
  1913. 
  Smithson. 
  Misc. 
  

   Coll., 
  vol. 
  63, 
  no. 
  8, 
  pp. 
  14-16. 
  

  

  