﻿INVEETEBKATES. 
  293 
  

  

  Radials 
  pentagonal, 
  nearly 
  twice 
  as 
  wide 
  as 
  high, 
  with 
  a 
  sharp 
  

   node 
  in 
  the 
  center 
  corresponding 
  with 
  those 
  on 
  the 
  subradials, 
  with 
  

   distinct 
  ridges 
  extending 
  obliquely 
  from 
  the 
  nodes 
  to 
  meet 
  those 
  on 
  

   the 
  subradial 
  plates. 
  

  

  Brachials 
  two, 
  the 
  first 
  quadrangular, 
  half 
  as 
  long 
  as 
  wide, 
  the 
  

   second 
  pentagonal, 
  about 
  as 
  long 
  as 
  wide, 
  and 
  sharply 
  angular 
  

   above, 
  supporting 
  on 
  its 
  upper 
  sloping 
  sides 
  the 
  first 
  arm 
  plates, 
  

   which 
  are 
  only 
  preserved 
  to 
  the 
  seventh 
  plate 
  above 
  the 
  last 
  brachial 
  

   in 
  our 
  specimen. 
  The 
  brachials 
  and 
  the 
  arm 
  plates 
  as 
  far 
  as 
  they 
  

   are 
  preserved 
  are 
  spine-bearing. 
  

  

  Two 
  anals 
  are 
  preserved, 
  the 
  first 
  is 
  about 
  half 
  as 
  large 
  as 
  the 
  

   large 
  subradials 
  between 
  which 
  it 
  rests, 
  the 
  second 
  is 
  smaller 
  and 
  

   rests 
  on 
  the 
  truncated 
  upper 
  margin 
  of 
  the 
  left 
  posterior 
  subradial, 
  

   and 
  both 
  ornamented 
  like 
  the 
  subradials. 
  

  

  This 
  species 
  is 
  apparently 
  related 
  to 
  Z. 
  Slimpsoni 
  of 
  S. 
  S. 
  Lyon, 
  

   but 
  differs 
  conspicuously 
  from 
  that, 
  in 
  the 
  number 
  of 
  its 
  brachials, 
  

   as 
  well 
  as 
  in 
  minor 
  details. 
  

  

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

  

  No. 
  2,445, 
  Illinois 
  State 
  collection. 
  

  

  Poteriocrinus 
  Columbiensis, 
  Worthen. 
  

  

  PL 
  XXIX, 
  Pig. 
  6. 
  

   Poteriocrinus 
  Columbiensis, 
  Wokthen, 
  February, 
  1882. 
  

   Bulletin 
  No. 
  1, 
  of 
  the 
  Illinois 
  State 
  Museum 
  of 
  Natural 
  History, 
  p. 
  22. 
  

  

  Body 
  small, 
  turbinate, 
  rapidly 
  spreading 
  from 
  the 
  base 
  to 
  the 
  

   summit 
  of 
  the 
  radial 
  plates. 
  

  

  Basal 
  plates 
  small, 
  projecting 
  about 
  half 
  their 
  length 
  beyond 
  the 
  

   column. 
  Subradials 
  about 
  as 
  high 
  as 
  wide, 
  three 
  hexagonal, 
  and 
  

   two 
  on 
  the 
  posterior 
  side 
  a 
  little 
  larger 
  and 
  heptagonal. 
  

  

  Badials 
  pentagonal, 
  a 
  little 
  wider 
  than 
  long, 
  except 
  the 
  one 
  on 
  

   the 
  right 
  posterior 
  ray, 
  which 
  is 
  narrower 
  and 
  nearly 
  quadrangular. 
  

   Brachials 
  about 
  twice 
  as 
  wide 
  as 
  long, 
  rounded 
  and 
  constricted 
  in 
  

   the 
  middle, 
  pentagonal, 
  supporting 
  on 
  their 
  upper 
  sloping 
  sides 
  two 
  

   arms 
  that 
  continue 
  single 
  to 
  their 
  extremities. 
  

  

  Arms 
  composed 
  at 
  their 
  base 
  of 
  long, 
  rounded 
  joints, 
  twice 
  as 
  

   long 
  as 
  wide, 
  but 
  decreasing 
  in 
  length 
  above, 
  the 
  joints 
  projecting 
  

   laterally 
  for 
  the 
  support 
  of 
  strong 
  pinnules 
  which 
  are 
  given 
  off 
  from 
  

   each 
  plate 
  on 
  alternate 
  sides. 
  Arm 
  plates 
  slightly 
  cuneate 
  in 
  form, 
  

   and 
  the 
  lateral 
  projections 
  give 
  a 
  zigzag 
  appearance 
  to 
  the 
  arms. 
  

   Pinnules 
  composed 
  of 
  round 
  joints 
  that 
  are 
  twice 
  as 
  long 
  as 
  wide 
  

   where 
  they 
  join 
  the 
  arms. 
  

  

  