﻿INVERTEBRATES. 
  331 
  

  

  the 
  ambulacral 
  plates, 
  the 
  concave 
  sides 
  of 
  the 
  adambulacrals, 
  and 
  

   the 
  attaching 
  ends 
  of 
  the 
  succeeding 
  plates, 
  so 
  that 
  they 
  are 
  bounded 
  

   by 
  four 
  plates 
  while 
  nearly 
  surrounded 
  by 
  two. 
  

  

  The 
  oral 
  plates 
  form, 
  in 
  pairs, 
  elongated 
  triangles, 
  with 
  the 
  acute 
  

   angle 
  directed 
  toward 
  the 
  center 
  of 
  the 
  visceral 
  cavity. 
  The 
  apices 
  

   are 
  joined 
  by 
  a 
  suture 
  with 
  the 
  basal 
  portions, 
  which 
  are 
  partly 
  

   separated 
  by 
  a 
  dividing 
  furrow. 
  A 
  single 
  genital 
  pore 
  perforates 
  

   each 
  basal 
  section 
  of 
  the 
  oral 
  plates. 
  Dorsal 
  side 
  unknown. 
  

  

  Locality 
  and 
  position; 
  Prairie 
  du 
  Long 
  cteek, 
  Monroe 
  county, 
  

   Chester 
  limestone. 
  

  

  No. 
  2479, 
  Illinois 
  State 
  collection 
  of 
  1878. 
  

  

  ORDER 
  PERISCHCECHINID^E. 
  

  

  Family 
  LEPIDECHIOTD^E. 
  

  

  Hybochinus, 
  n. 
  gen. 
  

  

  (Ety 
  hubos, 
  hump-backed; 
  echinus, 
  the 
  sea 
  urchin.) 
  

  

  Test 
  flexible 
  subspheroidal, 
  and 
  consisting 
  of 
  five 
  (?) 
  ambulacral, 
  

   and 
  the 
  same 
  number 
  of 
  interambulacral 
  areas. 
  Ambulacral 
  areas 
  

   composed 
  of 
  numerous 
  ranges 
  of 
  interlocking 
  and 
  overlapping 
  plates, 
  

   each 
  of 
  which 
  is 
  perforated 
  in 
  the 
  central 
  part, 
  by 
  a 
  single 
  pair 
  of 
  

   pores. 
  The 
  plates 
  imbricate 
  from 
  below 
  upward. 
  Interambulacral 
  

   areas 
  narrower, 
  and 
  consisting 
  in 
  the 
  equatorial 
  region 
  of 
  five 
  or 
  

   more 
  ranges 
  of 
  overlapping 
  plates 
  some 
  of 
  which 
  disappear 
  before 
  

   reaching 
  the 
  poles. 
  The 
  plates 
  imbricate 
  from 
  above 
  downward 
  and 
  

   from 
  the 
  central 
  range 
  outward. 
  Surface 
  covered 
  with 
  small 
  gran- 
  

   ules 
  — 
  not 
  large 
  tubercles 
  — 
  for 
  the 
  articulation 
  of 
  minute 
  spines. 
  

   Jaws 
  consisting 
  of 
  large, 
  subtriangular, 
  truncated 
  conical 
  pieces, 
  

   deeply 
  furrowed 
  towards 
  the 
  ends, 
  and 
  perforated 
  in 
  the 
  central 
  part. 
  

  

  This 
  genus 
  is 
  related 
  in 
  many 
  respects 
  to 
  Lepidesthes, 
  but' 
  differs 
  

   in 
  the 
  important 
  particular 
  of 
  having 
  the 
  imbrication 
  of 
  the 
  plates 
  

   in 
  the 
  opposite 
  direction, 
  as 
  in 
  the 
  Echinothuridce 
  or 
  more 
  modern 
  

   family 
  of 
  flexible 
  Echinoids. 
  The 
  humps 
  and 
  protuberances 
  of 
  the 
  

   upper 
  end 
  of 
  the 
  type 
  specimen, 
  which 
  suggested 
  the 
  generic 
  name, 
  

   are 
  supposed 
  to 
  indicate 
  the 
  great 
  flexibility 
  of 
  the 
  test, 
  and 
  an 
  ab- 
  

   normal 
  development 
  of 
  the 
  number 
  of 
  ambulacral 
  plates. 
  An 
  ab- 
  

  

  