﻿INVEETEBEATES. 
  281 
  

  

  Arms 
  composed 
  of 
  rounded, 
  nearly 
  quadrangular 
  joints, 
  and 
  after 
  

   the 
  first 
  division 
  on 
  the 
  second 
  brachial 
  they 
  divide 
  twice 
  more 
  in 
  

   the 
  posterior 
  rays, 
  first 
  on 
  the 
  seventh, 
  and 
  each 
  division 
  again 
  on 
  

   the 
  tenth 
  plate, 
  making 
  eight 
  arms 
  to 
  each 
  of 
  these 
  rays. 
  The 
  

   other 
  rays 
  are 
  partially 
  embedded 
  in 
  the 
  matrix, 
  minute 
  projec- 
  

   tions 
  are 
  visible 
  with 
  a 
  good 
  glass 
  on 
  the 
  outer 
  side 
  of 
  the 
  arm 
  

   plates, 
  that 
  give 
  a 
  roughened 
  aspect 
  to 
  the 
  whole 
  specimen, 
  and 
  has 
  

   suggested 
  the 
  specific 
  name, 
  and 
  will 
  also 
  serve 
  to 
  distinguish 
  it 
  

   from 
  nearly 
  related 
  species. 
  

  

  Anal 
  plates 
  so 
  closely 
  anchylosed 
  that 
  their 
  forms 
  cannot 
  be 
  

   clearly 
  determined, 
  but 
  the 
  first 
  one 
  rests 
  between 
  two 
  of 
  the 
  sub- 
  

   radials, 
  and 
  is 
  succeeded 
  by 
  a 
  double 
  series 
  of 
  smaller 
  plates 
  that 
  

   apparently 
  go 
  to 
  form 
  the 
  base 
  of 
  the 
  ventral 
  tube. 
  Traces 
  of 
  

   coarse 
  striae, 
  or 
  rows 
  of 
  granules, 
  extending 
  across 
  the 
  brachial 
  

   plates, 
  as 
  well 
  as 
  the 
  radials, 
  can 
  be 
  seen 
  with 
  a 
  good 
  glass. 
  

  

  Column 
  round, 
  composed 
  of 
  rather 
  even 
  joints, 
  with 
  numerous 
  

   delicate 
  cirrhi 
  attached 
  to 
  it 
  at 
  short 
  intervals. 
  

  

  Position 
  and 
  locality: 
  Upper 
  part 
  of 
  the 
  geodiferous 
  shales 
  of 
  

   the 
  Keokuk 
  group 
  ; 
  one 
  mile 
  below 
  Keokuk. 
  

  

  No. 
  234 
  of 
  Mr. 
  L. 
  A. 
  Cox's 
  collection. 
  

  

  PoTERIOCRINUS 
  ARACHNiEFORMIS, 
  Worthen. 
  

  

  PI. 
  XXVIII. 
  Fig. 
  12. 
  

  

  Poteriocrinus 
  arachnasforniis, 
  Woethen, 
  February, 
  1882. 
  

  

  Bulletin 
  No. 
  1, 
  of 
  the 
  Illinois 
  State 
  Museum 
  of 
  Natural 
  History, 
  p. 
  13. 
  

  

  Body 
  small, 
  obconical, 
  length 
  and 
  breadth 
  about 
  equal, 
  gradually 
  

   swelling 
  from 
  the 
  base 
  to 
  the 
  summit 
  of 
  the 
  radial 
  series. 
  Basals 
  

   projecting 
  more 
  than 
  half 
  their 
  length 
  beyond 
  the 
  column, 
  and 
  

   forming 
  by 
  themselves 
  a 
  low, 
  pentagonal 
  cup. 
  Subradials 
  compar- 
  

   atively 
  large, 
  three 
  hexagonal, 
  and 
  two 
  heptagonal. 
  Eadials 
  smaller 
  

   than 
  the 
  subradials, 
  the 
  articulating 
  scar 
  semi-circular, 
  and 
  occu- 
  

   pying 
  only 
  about 
  half 
  the 
  width 
  of 
  the 
  plate, 
  with 
  a 
  projecting 
  

   rim 
  around 
  its 
  outer 
  margin. 
  

  

  Only 
  one 
  anal 
  plate 
  is 
  preserved 
  in 
  the 
  specimen 
  under 
  descrip- 
  

   tion, 
  which 
  is 
  about 
  half 
  as 
  large 
  as 
  the 
  largest 
  of 
  the 
  subradials 
  

   between 
  which 
  it 
  rests, 
  and 
  it 
  extends 
  up 
  to 
  the 
  summit 
  of 
  the 
  

   right 
  posterior 
  radial. 
  The 
  left 
  posterior 
  subradial 
  is 
  truncated 
  

   at 
  the 
  summit 
  for 
  the 
  support 
  of 
  the 
  second 
  anal, 
  which 
  is 
  not 
  

   preserved. 
  

  

  