﻿THE 
  ECHINOTHURID-S: 
  AND 
  THE 
  PEEISCHOECHINID.E. 
  309 
  

  

  distinction 
  between 
  the 
  ambulacral 
  and 
  interambulacral 
  areas 
  becomes 
  

   entirely 
  lost. 
  

  

  I 
  have 
  quoted 
  thus 
  largely 
  from 
  Prof. 
  "Wyville 
  Thomson's 
  work 
  

   that 
  the 
  succeeding 
  remarks 
  on 
  the 
  fossil 
  forms 
  may 
  be 
  thoroughly 
  

   understood. 
  

  

  The 
  first 
  of 
  these, 
  Echinothuria, 
  S. 
  P. 
  "Woodward, 
  together 
  with 
  

   the 
  two 
  preceding 
  recent 
  genera, 
  compose 
  Thomson's 
  family 
  of 
  the 
  

   EcMnoihuridce 
  ; 
  and 
  it 
  is 
  to 
  the 
  consideration 
  of 
  the 
  relation 
  of 
  this 
  

   family 
  to 
  the 
  Perischoechinidce, 
  M'Coy, 
  that 
  I 
  purpose 
  to 
  devote 
  the 
  

   remainder 
  of 
  these 
  remarks. 
  

  

  Echinothuria 
  was 
  obtained 
  some 
  years 
  ago 
  by 
  Mr. 
  Wickham 
  Flower 
  

   from 
  the 
  Upper 
  Chalk 
  of 
  Higham, 
  near 
  Kochester*, 
  and 
  in 
  its 
  prin- 
  

   cipal 
  characters 
  agrees 
  completely 
  with 
  Calveria 
  and 
  Phormosoma. 
  It 
  

   differs 
  from 
  the 
  first 
  named, 
  however, 
  in 
  the 
  wider 
  interambulacral 
  

   and 
  ambulacral 
  plates, 
  in 
  the 
  smaller 
  amount 
  of 
  overlapping 
  of 
  the 
  

   same, 
  and 
  in 
  the 
  abseuce 
  of 
  membranous 
  intervals 
  ; 
  from 
  the 
  latter 
  

   by 
  having 
  the 
  structure 
  of 
  the 
  oral 
  and 
  apical 
  surfaces 
  the 
  same. 
  

   {Thomson.) 
  

  

  So 
  far 
  we 
  have 
  examined 
  the 
  characters 
  of 
  those 
  genera 
  which 
  

   constitute 
  the 
  Echinothuridce 
  ; 
  and 
  if 
  we 
  now 
  turn 
  to 
  the 
  palaeozoic 
  

   forms 
  of 
  the 
  Echinoidea 
  we 
  shall 
  find 
  an 
  interesting 
  repetition 
  of 
  

   many 
  of 
  the 
  foregoing 
  facts. 
  

  

  Palaeozoic 
  Echini 
  are 
  represented 
  by 
  a 
  limited 
  number 
  of 
  species, 
  

   comprised 
  in 
  the 
  following 
  genera 
  : 
  — 
  Archceocidaris, 
  M'Coy, 
  Palce- 
  

   chinus, 
  Scouler, 
  Perischodomus, 
  M'Coy, 
  Lepidechinus, 
  Hall, 
  Eocidaris, 
  

   Desor, 
  Melonites, 
  D. 
  D. 
  Owen, 
  Lcpidocentrus, 
  Miiller, 
  and 
  Oligoporus, 
  

   Meek 
  & 
  Worthen. 
  

  

  In 
  1849 
  Professor 
  M'Coy 
  f 
  proposed 
  the 
  order 
  Perischoechinidce, 
  to 
  

   include 
  palaeozoic 
  Echini 
  possessing 
  more 
  than 
  two 
  rows 
  of 
  plates 
  in 
  

   each 
  interambulacrum, 
  as 
  distinguished 
  from 
  those 
  of 
  Secondary, 
  

   Tertiary, 
  and 
  living 
  genera, 
  where 
  the 
  same 
  area? 
  are 
  composed 
  only 
  

   of 
  two 
  rows. 
  

  

  This 
  complexity 
  of 
  the 
  interambulacral 
  system 
  was 
  found 
  by 
  M'Coy 
  

   to 
  be 
  present 
  in 
  the 
  genera 
  Archceocidaris, 
  Palcechinus, 
  and 
  Peris- 
  

   chodomus. 
  Since 
  the 
  first 
  enunciation 
  of 
  these 
  characters 
  sufficient 
  

   information 
  has 
  been 
  obtained 
  to 
  enable 
  us 
  to 
  add 
  the 
  remainder 
  of 
  

   the 
  palaeozoic 
  genera 
  mentioned 
  (I 
  am 
  not 
  so 
  certain 
  of 
  Lepidocen- 
  

   trus, 
  Miiller, 
  and 
  therefore 
  do 
  not 
  wish 
  these 
  remarks 
  to 
  be 
  applied 
  

   to 
  that 
  genus 
  with 
  so 
  much 
  certainty 
  as 
  to 
  the 
  others) 
  to 
  M'Coy's 
  

   Perischoechinidce. 
  

  

  Mr. 
  Young's 
  J 
  remarks 
  on 
  the 
  interambulacral 
  plates 
  of 
  Archceo- 
  

   cidaris 
  clearly 
  indicate 
  that 
  a 
  certain 
  amount 
  of 
  overlapping 
  took 
  

   place 
  in 
  that 
  genus. 
  These, 
  as 
  originally 
  pointed 
  out 
  by 
  M'Coy 
  §, 
  are 
  

   of 
  two 
  kinds, 
  a 
  pentagonal 
  and 
  a 
  hexagonal 
  series, 
  arranged 
  in 
  the 
  

   test 
  in 
  three 
  or 
  more 
  rows. 
  The 
  larger 
  pentagonal 
  plates 
  have 
  two 
  

   of 
  the 
  sides 
  bevelled 
  (PI. 
  XXIV. 
  a, 
  fig. 
  1), 
  against 
  which 
  would 
  fit 
  

   the 
  corresponding 
  bevelled 
  surface 
  of 
  the 
  next 
  plate, 
  but 
  in 
  this 
  case 
  

  

  * 
  W. 
  Thomson, 
  I 
  c. 
  p. 
  162. 
  

  

  t 
  Annals 
  & 
  Mag. 
  Nat. 
  Hist. 
  1849, 
  vol. 
  iii. 
  \ 
  Loc. 
  cit. 
  p. 
  302. 
  

  

  § 
  Synopsis 
  Carb. 
  Foss. 
  Ireland, 
  p. 
  173. 
  

   Q. 
  J. 
  G. 
  S. 
  No. 
  119. 
  z 
  

  

  