﻿INVEBTEBBATES. 
  271 
  

  

  radials, 
  and 
  this 
  is 
  succeeded 
  by 
  a 
  double 
  series 
  of 
  minute 
  plates 
  

   that 
  form 
  the 
  lower 
  extremity 
  of 
  the 
  ventral 
  tube. 
  This 
  organ 
  is 
  

   composed 
  of 
  minute 
  plates, 
  is 
  balloon 
  shaped, 
  and 
  shows 
  a 
  small 
  

   opening 
  about 
  two-thirds 
  the 
  distance 
  from 
  the 
  base 
  to 
  the 
  summit. 
  

   Column 
  at 
  the 
  top 
  composed 
  of 
  round, 
  nearly 
  uniform 
  plates, 
  but 
  a 
  

   short 
  distance 
  below 
  thicker 
  joints 
  are 
  intercalated 
  at 
  irregular 
  inter- 
  

   vals. 
  It 
  decreases 
  in 
  diameter 
  nearly 
  one-half 
  in 
  a 
  distance 
  of 
  two 
  

   inches 
  from 
  the 
  top, 
  and 
  has 
  delicate 
  cyrrhi 
  attached 
  on 
  either 
  side 
  

   at 
  irregular 
  intervals. 
  

  

  The 
  depressions 
  at 
  the 
  angles 
  of 
  the 
  body 
  plates, 
  give 
  to 
  the 
  ex- 
  

   terior 
  of 
  the 
  cup-shaped 
  body 
  a 
  somewhat 
  rough 
  appearance. 
  

  

  This 
  species 
  is 
  named 
  in 
  honor 
  of 
  Mr. 
  N. 
  K. 
  Burket, 
  of 
  Keokuk, 
  

   Iowa, 
  who 
  discovered 
  the 
  interesting 
  locality 
  at 
  Hamilton 
  where 
  

   this 
  and 
  several 
  other 
  new 
  forms 
  have 
  been 
  obtained. 
  

  

  Position 
  and 
  locality: 
  Top 
  of 
  the 
  Keokuk 
  limestone, 
  Hamilton, 
  

   Illinois. 
  

  

  Nos. 
  260 
  and 
  402 
  of 
  Mr. 
  L. 
  A. 
  Cox's 
  collection. 
  

  

  POTERIOORINUS 
  TENUIDACTYLUS, 
  Worthen. 
  

  

  PL 
  XXVIII, 
  Pig. 
  13. 
  

  

  Poieriocrinus 
  tenuidactylus, 
  Woethen, 
  February, 
  1882. 
  

  

  Bulletin 
  No. 
  1, 
  of 
  the 
  Illinois 
  State 
  Museum 
  of 
  Natural 
  History, 
  p. 
  6. 
  

  

  Body 
  obconical, 
  length 
  and 
  breadth 
  to 
  the 
  top 
  of 
  the 
  radial 
  plates 
  

   about 
  equal. 
  Basals 
  well 
  developed, 
  expanding 
  upward 
  from 
  the 
  

   column, 
  forming 
  a 
  pentagonal 
  cup 
  rather 
  wider 
  than 
  long. 
  Subra- 
  

   dials 
  about 
  as 
  wide 
  as 
  high, 
  the 
  one 
  on 
  the 
  posterior 
  side 
  supporting 
  

   in 
  part 
  two 
  quadrangular 
  anal 
  plates, 
  that 
  are 
  succeeded 
  by 
  a 
  

   double 
  series 
  of 
  smaller 
  plates 
  of 
  the 
  same 
  form, 
  forming 
  the 
  base 
  

   of 
  the 
  ventral 
  tube. 
  

  

  Badials 
  pentagonal, 
  wider 
  than 
  high. 
  Brachials 
  four 
  to 
  each 
  of 
  

   the 
  posterior 
  rays, 
  three 
  of 
  them 
  quadrangular 
  and 
  the 
  fourth 
  pent- 
  

   angular, 
  supporting 
  on 
  its 
  upper 
  sloping 
  sides 
  the 
  first 
  divisions 
  of 
  

   the 
  rays. 
  The 
  arms 
  in 
  the 
  left 
  posterior 
  ray, 
  after 
  dividing 
  on 
  the 
  

   last 
  brachial, 
  give 
  off 
  a 
  branch 
  from 
  each 
  division, 
  about 
  the 
  twelfth 
  

   to 
  the 
  twentieth 
  plate, 
  beyond 
  which 
  they 
  are 
  simple 
  as 
  far 
  as 
  can 
  

   be 
  seen, 
  giving 
  four 
  arms 
  to 
  this 
  ray. 
  

  

  The 
  arms 
  are 
  long 
  and 
  slender, 
  composed 
  of 
  rather 
  long, 
  rounded 
  

   joints, 
  slightly 
  zigzag 
  in 
  their 
  arrangement, 
  and 
  give 
  off 
  rounded 
  

   pinnules 
  from 
  their 
  projecting 
  sides. 
  

  

  