﻿INVERTEBRATES. 
  £75 
  

  

  PoTERIOCRXNUS 
  LATIDACTYLUS, 
  Worthen. 
  

  

  PI. 
  XXVIII, 
  Fig. 
  6. 
  

  

  JPoteriocrim+s 
  latidactylus, 
  Wobthen, 
  February, 
  1882. 
  

  

  Bulletin 
  No. 
  1, 
  of 
  the 
  Illinois 
  State 
  Museum 
  of 
  Natural 
  History, 
  p. 
  8. 
  

  

  This 
  interesting 
  species 
  is 
  only 
  represented 
  by 
  a 
  single 
  imperfect 
  

   specimen, 
  showing 
  the 
  posterior 
  side 
  of 
  the 
  body, 
  and 
  a 
  part 
  of 
  three 
  

   of 
  the 
  rays. 
  

  

  Body 
  mamillaeform, 
  once 
  and 
  a 
  half 
  as 
  wide 
  as 
  high 
  to 
  the 
  top 
  

   of 
  the 
  first 
  radials, 
  and 
  composed 
  of 
  smooth, 
  closely 
  joined 
  plates. 
  

   Basals 
  unknown. 
  Subradials 
  nearly 
  as 
  large 
  as 
  the 
  radials, 
  length 
  

   and 
  breadth 
  about 
  equal, 
  four 
  of 
  them 
  hexagonal 
  counting 
  three 
  

   angles 
  below, 
  the 
  left 
  posterior 
  one 
  larger 
  than 
  those 
  on 
  the 
  anterior 
  

   side 
  and 
  heptagonal. 
  Badials 
  pentagonal, 
  once 
  and 
  a 
  half 
  as 
  wide 
  

   as 
  high, 
  and 
  truncated 
  straight 
  across 
  their 
  upper 
  margins 
  for 
  the 
  

   reception 
  of 
  the 
  brachials. 
  These 
  are 
  of 
  the 
  same 
  form 
  and 
  a 
  little 
  

   larger 
  than 
  the 
  radials, 
  and 
  support 
  on 
  their 
  upper 
  sloping 
  sides 
  the 
  

   first 
  divisions 
  of 
  the 
  rays. 
  The 
  arms 
  are 
  composed 
  of 
  broad, 
  short, 
  

   quadrangular 
  plates, 
  resembling 
  closely 
  the 
  arms 
  of 
  Woodocrinus 
  

   and 
  Bursacrinus. 
  In 
  two 
  of 
  the 
  rays 
  which 
  are 
  partly 
  preserved 
  in 
  

   the 
  specimen 
  before 
  me, 
  a 
  second 
  bifurcation 
  takes 
  place 
  on 
  the 
  

   fourth 
  plate 
  above 
  the 
  brachials, 
  beyond 
  which 
  the 
  arm 
  structure 
  is 
  

   unknown. 
  The 
  first 
  arm-plates 
  in 
  the 
  two 
  posterior 
  rays 
  are 
  about 
  

   twice 
  as 
  long 
  as 
  the 
  succeeding 
  ones. 
  

  

  Three 
  anal 
  plates 
  are 
  visible 
  ; 
  the 
  first 
  one 
  is 
  larger 
  than 
  the 
  

   others, 
  and 
  rests 
  between 
  the 
  upper 
  angles 
  of 
  the 
  two 
  posterior 
  sub- 
  

   radials. 
  The 
  second 
  rests 
  on 
  top 
  of 
  the 
  left 
  posterior 
  subradial, 
  and 
  

   the 
  third, 
  which 
  is 
  smaller 
  than 
  either 
  of 
  the 
  others, 
  rests 
  upon 
  the 
  

   first, 
  and 
  above 
  this 
  the 
  lower 
  margins 
  of 
  two 
  or 
  more 
  succeeding 
  

   plates 
  can 
  be 
  seen. 
  

  

  Column 
  stout, 
  covering 
  the 
  entire 
  width 
  of 
  the 
  base, 
  and 
  com- 
  

   posed 
  at 
  first 
  of 
  nearly 
  equal 
  joints 
  that 
  decrease 
  in 
  diameter 
  below, 
  

   and 
  pass 
  into 
  a 
  series 
  of 
  alternately 
  thick 
  and 
  thinner 
  joints 
  as 
  the 
  

   distance 
  from 
  the 
  base 
  increases. 
  

  

  Position 
  and 
  locality: 
  Upper 
  part 
  of 
  the 
  Keokuk 
  limestone, 
  Hamil- 
  

   ton, 
  Illinois. 
  

  

  No. 
  401 
  of 
  Mr. 
  L. 
  A. 
  Cox's 
  collection. 
  

  

  