﻿INVEETEBEATES. 
  333 
  

  

  ambulacral 
  plates 
  upon 
  either 
  side. 
  The 
  exposed 
  surface 
  of 
  each 
  

   plate 
  is 
  quadrangular, 
  and 
  usually 
  square, 
  or 
  nearly 
  so. 
  The 
  sur- 
  

   face 
  is 
  covered 
  with 
  small 
  granules 
  for 
  the 
  articulation 
  of 
  minute 
  

   needle-like 
  spines, 
  the 
  articular 
  ends 
  of 
  which 
  are 
  marked 
  by 
  a 
  

   slight 
  swelling. 
  The 
  jaws 
  are 
  remarkably 
  large, 
  and 
  seem 
  to 
  have 
  

   formed 
  an 
  ovate 
  body 
  of 
  ten 
  or 
  twelve 
  pieces, 
  each 
  of 
  which 
  is 
  

   deeply 
  furrowed 
  at 
  the 
  ends, 
  and 
  perforated 
  in 
  the 
  central 
  region. 
  

  

  Other 
  openings 
  and 
  characters 
  of 
  this 
  species 
  cannot 
  be 
  correctly 
  

   defined 
  from 
  our 
  specimens. 
  Those 
  represented 
  by 
  figures 
  5 
  and 
  6 
  

   show 
  only 
  the 
  crushed 
  apical 
  regions 
  and 
  the 
  parts 
  described, 
  and 
  

   were 
  collected 
  from 
  the 
  Chester 
  limestone 
  on 
  Prairie 
  du 
  Long 
  creek, 
  

   in 
  Monroe 
  county, 
  by 
  A. 
  H. 
  Worthen. 
  The 
  one 
  represented 
  by 
  fig. 
  

   7 
  was 
  obtained 
  from 
  the 
  same 
  horizon 
  in 
  Pope 
  county, 
  and 
  may 
  or 
  

   may 
  not 
  belong 
  to 
  this 
  species. 
  The 
  jaws 
  seem 
  to 
  be 
  smaller 
  than 
  

   they 
  are 
  in 
  the 
  other 
  two 
  specimens, 
  but 
  this 
  may 
  result 
  from 
  their 
  

   being 
  more 
  fragmentary. 
  One 
  genital 
  plate, 
  however, 
  appears 
  upon 
  

   this 
  specimen 
  having 
  four 
  pores, 
  and 
  if 
  it 
  belongs 
  to 
  this 
  species 
  it 
  

   will 
  of 
  course 
  add 
  that 
  additional 
  character. 
  

  

  No. 
  2,481. 
  Illinois 
  State 
  collection. 
  

  

  G-enss 
  PERISHODOMUS, 
  McCoy. 
  

   &^ 
  

  

  Pebischodomus 
  Illinoibnsis, 
  Sp. 
  nov. 
  

  

  PI. 
  XXXI. 
  fig. 
  8, 
  yiew 
  of 
  the 
  apical 
  end 
  of 
  a 
  distorted 
  specimen, 
  natural 
  size. 
  

  

  General 
  form 
  apparently 
  subspheroidal, 
  and 
  as 
  there 
  are 
  five 
  am- 
  

   bulacral 
  and 
  five 
  interambulacral 
  areas, 
  and 
  the 
  former 
  are 
  so 
  nar- 
  

   row, 
  it 
  probably 
  has 
  a 
  subpentagonal 
  outline 
  as 
  noticed 
  by 
  McCoy 
  

   in 
  diagnosing 
  the 
  genus. 
  

  

  Ambulacral 
  areas 
  narrow, 
  depressed, 
  over-lapped 
  on 
  either 
  side 
  by 
  

   interambulacral 
  plates, 
  and 
  consisting 
  of 
  a 
  double 
  series 
  of 
  inter- 
  

   locking 
  transversely 
  elongated 
  plates, 
  each 
  one 
  of 
  which 
  is 
  pierced 
  

   by 
  a 
  single 
  pair 
  of 
  pores. 
  These 
  plates 
  are 
  not 
  uniform 
  in 
  size 
  nor 
  

   shape, 
  though 
  most 
  of 
  them 
  seem 
  to 
  have 
  imperfectly 
  defined 
  sub- 
  

   pentagonal 
  outlines. 
  They 
  slightly 
  overlap 
  from 
  above 
  downward. 
  

  

  Interambulacral 
  areas 
  covering 
  nearly 
  the 
  entire 
  surface, 
  but 
  as 
  

   the 
  equatorial 
  region 
  is 
  not 
  preserved, 
  it 
  is 
  impossible 
  to 
  state 
  the 
  

   number 
  of 
  ranges 
  of 
  plates 
  in 
  each 
  area. 
  There 
  may 
  be 
  five 
  and 
  

   there 
  may 
  be 
  seven 
  only 
  five 
  can 
  be 
  distinctly 
  determined 
  from 
  the 
  

   apical 
  view 
  of 
  our 
  specimen. 
  The 
  plates 
  are 
  very 
  irregular 
  in 
  size, 
  

  

  