﻿44 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol.67 
  

  

  the 
  anal 
  plate, 
  and 
  back 
  from 
  the 
  edge, 
  is 
  a 
  pyramid 
  of 
  small 
  plates 
  

   marking 
  an 
  opening 
  through 
  the 
  tegmen. 
  As 
  in 
  the 
  preceding 
  

   species, 
  the 
  radials 
  are 
  relatively 
  short, 
  their 
  average 
  height 
  in 
  the 
  

   four 
  specimens 
  being 
  32 
  per 
  cent 
  out 
  of 
  a 
  total 
  of 
  6 
  to 
  T 
  mm. 
  The 
  

   calyx 
  is 
  well 
  elongated 
  and 
  conical, 
  with 
  curved 
  sides; 
  height 
  to 
  

   width 
  at 
  the 
  top 
  being 
  as 
  1.5 
  to 
  1, 
  and 
  at 
  the 
  base 
  as 
  1.5 
  to 
  .27. 
  The 
  

   species 
  is 
  small, 
  the 
  four 
  specimens 
  varying 
  but 
  little 
  from 
  6 
  mm. 
  

   in 
  height 
  of 
  cup. 
  

  

  The 
  arms 
  are 
  relatively 
  strong, 
  biserial, 
  erect, 
  and 
  closely 
  fringed 
  

   with 
  long 
  pinnules. 
  The 
  stem 
  is 
  composed 
  of 
  rounded, 
  nearly 
  equal 
  

   columnals, 
  about 
  half 
  as 
  long 
  as 
  wide, 
  increasing 
  somewhat 
  in 
  width 
  

   distally; 
  this 
  is 
  not 
  so 
  evident 
  in 
  one 
  of 
  the 
  figures 
  because 
  not 
  well 
  

   exposed 
  in 
  the 
  matrix. 
  

  

  The 
  striking 
  feature 
  of 
  this 
  series 
  of 
  specimens 
  is 
  the 
  extreme 
  

   regularity 
  of 
  their 
  construction, 
  with 
  the 
  single 
  exception 
  of 
  the 
  

   number 
  of 
  ranges 
  of 
  the 
  supplementary 
  plates 
  ; 
  two 
  of 
  the 
  specimens 
  

   having 
  two 
  ranges, 
  one 
  three, 
  and 
  the 
  fourth, 
  which 
  is 
  considerably 
  

   crushed, 
  apparently 
  has 
  partly 
  both. 
  Thus 
  the 
  addition 
  of 
  the 
  

   inserted 
  plates 
  is 
  a 
  definite 
  structure, 
  somewhat 
  plastic, 
  heralding 
  

   their 
  great 
  development 
  to 
  the 
  twenty 
  circlets 
  of 
  the 
  typical 
  

   Acrocrinus. 
  Presumably 
  the 
  new 
  species 
  preceded 
  the 
  latter 
  in 
  

   time, 
  but 
  our 
  knowledge 
  of 
  the 
  stratigraphy 
  of 
  their 
  occurrence 
  is 
  

   not 
  sufficiently 
  minute 
  to 
  furnish 
  the 
  proof 
  of 
  it. 
  A. 
  shumardi, 
  

   from 
  the 
  Glen 
  Dean 
  formation 
  of 
  the 
  Upper 
  Chester, 
  is, 
  of 
  course, 
  

   later 
  than 
  the 
  present 
  species. 
  A. 
  amphora, 
  from 
  the 
  Ohara 
  forma- 
  

   tion, 
  may 
  be 
  of 
  approximately 
  equivalent 
  age; 
  both 
  of 
  these 
  species 
  

   being 
  from 
  a 
  different 
  region. 
  But 
  there 
  is 
  a 
  third 
  very 
  rare 
  species, 
  

   A. 
  wrnaeformis, 
  in 
  the 
  same 
  mature 
  stage, 
  described 
  by 
  Hall 
  from 
  

   the 
  same 
  region 
  in 
  Illinois 
  as 
  our 
  species, 
  22 
  of 
  which 
  there 
  can 
  be 
  no 
  

   doubt, 
  as 
  I 
  found 
  a 
  well 
  marked 
  specimen 
  of 
  it 
  in 
  the 
  same 
  layer 
  

   which 
  produced 
  the 
  types 
  herein 
  described; 
  so 
  that 
  while 
  the 
  ma- 
  

   tured 
  form 
  continued 
  into 
  the 
  later 
  formations, 
  with 
  our 
  present 
  

   knowledge 
  it 
  must 
  be 
  considered 
  that 
  both 
  forms 
  existed 
  together 
  

   at 
  one 
  period. 
  

  

  As 
  a 
  result 
  of 
  the 
  facts 
  brought 
  out 
  imder 
  the 
  last 
  two 
  heads, 
  

   we 
  have 
  in 
  the 
  concurrent 
  development 
  in 
  the 
  genus 
  Dichocrinus 
  

   and 
  its 
  derivatives 
  of 
  two 
  such 
  striking 
  characters 
  as 
  the 
  recumbent 
  

   arms 
  and 
  an 
  added 
  calyx 
  element, 
  a 
  remarkable 
  example 
  of 
  long 
  

   preparation 
  for 
  an 
  eventual 
  culmination 
  which 
  is 
  to 
  mark 
  the 
  ex- 
  

   tinction 
  of 
  the 
  order 
  to 
  which 
  it 
  belongs. 
  

  

  Horizon 
  and 
  locality. 
  — 
  Mississippian, 
  lower 
  Chester, 
  Renault 
  for- 
  

   mation 
  ; 
  Burksville, 
  Monroe 
  County. 
  Illinois. 
  

  

  ^Geol. 
  Iowa, 
  pt. 
  2, 
  1858, 
  p. 
  090, 
  pi. 
  25, 
  figs. 
  11«. 
  b. 
  

  

  