﻿INVEETEBEATES. 
  295 
  

  

  . 
  This 
  species 
  differs 
  in 
  its 
  arm 
  formula 
  from 
  all 
  other 
  Poterio- 
  

   crinidm 
  known 
  at 
  this 
  time, 
  and 
  its 
  depaiture 
  from 
  the 
  normal 
  

   structure 
  may 
  prove 
  of 
  generic 
  importance. 
  

  

  Position 
  and 
  locality: 
  St. 
  Louis 
  limestone, 
  Monroe 
  county, 
  111. 
  

  

  No. 
  2,456, 
  Illinois 
  State 
  collection. 
  

  

  POTEKIOCIUNUS 
  SIMILIS, 
  Wortlieil. 
  

  

  PI. 
  XXX, 
  Fig. 
  12. 
  

   Pot 
  eriocrinus 
  similis, 
  Woethen, 
  February, 
  1882. 
  

   Bulletin 
  No. 
  1, 
  of 
  the 
  Illinois 
  State 
  Museum 
  of 
  Natural 
  History, 
  p. 
  23. 
  

  

  Body 
  small, 
  basin-shaped, 
  twice 
  as 
  wide 
  as 
  high 
  to 
  the 
  top 
  of 
  

   the 
  radial 
  series. 
  The 
  plates 
  of 
  the 
  body 
  are 
  all 
  slightly 
  protu- 
  

   berant 
  in 
  the 
  center, 
  and 
  depressed 
  at 
  the 
  angles. 
  

  

  Base 
  slightly 
  depressed 
  and 
  basals 
  concealed 
  by 
  the 
  first 
  columnar 
  

   joints. 
  

  

  Subradials 
  about 
  as 
  wide 
  as 
  high, 
  hexagonal 
  and 
  heptagonal. 
  

  

  Eadials 
  wider 
  than 
  high, 
  with 
  a 
  well-defined 
  suture 
  between 
  them 
  

   and 
  the 
  brachials. 
  Brachials 
  on 
  the 
  two 
  posterior 
  rays 
  nearly 
  

   three 
  times 
  as 
  long 
  as 
  wide, 
  constricted 
  in 
  the 
  middle 
  and 
  angular 
  

   above, 
  supporting 
  on 
  their 
  sloping 
  sides 
  the 
  two 
  divisions 
  of 
  the 
  

   rays. 
  In 
  the 
  right 
  antero-lateral 
  ray 
  the 
  brachial 
  is 
  only 
  about 
  

   twice 
  as 
  wide 
  as 
  high, 
  its 
  upper 
  angle 
  projecting 
  so 
  as 
  to 
  form 
  a 
  

   short 
  node. 
  

  

  Arms 
  two 
  to 
  each 
  ray 
  on 
  the 
  three 
  rays 
  visible, 
  composed 
  of 
  

   rounded 
  joints 
  that, 
  at 
  first, 
  are 
  nearly 
  twice 
  as 
  long 
  as 
  wide, 
  but 
  

   they 
  gradually 
  become 
  shorter 
  above. 
  They 
  all 
  project 
  laterally, 
  

   and 
  support 
  strong 
  pinnules, 
  that 
  are 
  given 
  off 
  alternately 
  from 
  the 
  

   longest 
  side 
  of 
  the 
  arm 
  joints. 
  The 
  lateral 
  projections 
  of 
  the 
  joints 
  

   give 
  a 
  zigzag 
  arrangement 
  to 
  the 
  arms. 
  

  

  Only 
  one 
  anal 
  plate 
  is 
  visible 
  in 
  the 
  specimen, 
  and 
  this 
  is 
  placed, 
  

   as 
  is 
  usual 
  in 
  this 
  genus, 
  between 
  two 
  of 
  the 
  subradials, 
  and 
  under 
  

   the 
  left 
  side 
  of 
  the 
  right 
  posterior 
  radial. 
  

  

  Column 
  round, 
  and 
  at 
  its 
  upper 
  extremity 
  composed 
  of 
  joints 
  of 
  

   nearly 
  equal 
  thickness. 
  

  

  This 
  little 
  crinoid 
  is 
  related 
  to 
  Pot. 
  Columbiensis, 
  from 
  the 
  same 
  

   locality, 
  but 
  differs 
  from 
  that 
  in 
  its 
  more 
  depressed 
  base, 
  and 
  in 
  

   the 
  slightly 
  protuberant 
  character 
  of 
  its 
  body 
  plates. 
  

  

  Position 
  and 
  locality 
  .- 
  Chester 
  limestone, 
  Monroe 
  county, 
  111. 
  

  

  No. 
  2,446, 
  Illinois 
  State 
  collection. 
  

  

  