﻿204 
  Prof. 
  P. 
  M. 
  Duncan 
  on 
  the 
  

  

  The 
  model 
  at 
  the 
  Museum 
  of 
  Practical 
  Geology 
  shows 
  a 
  

   double 
  row 
  of 
  pairs 
  of 
  pores 
  here 
  and 
  there, 
  in 
  an 
  ambulacrum, 
  

   but 
  the 
  suturing 
  of 
  the 
  plates 
  is 
  not 
  visible. 
  Nevertheless 
  

   the 
  plates 
  were 
  evidently 
  arranged 
  after 
  the 
  system 
  of 
  the 
  

   primaries 
  of 
  P. 
  spacer 
  icus 
  (fig. 
  II.). 
  Forbes 
  gives 
  a 
  diagram 
  of 
  

   the 
  plates 
  and 
  the 
  pairs 
  ; 
  but 
  the 
  inner 
  pair 
  of 
  pores 
  is 
  placed 
  

   too 
  close 
  to 
  the 
  median 
  line 
  of 
  the 
  ambulacrum 
  and 
  its 
  plate 
  is 
  

   too 
  much 
  blocked 
  out 
  from 
  the 
  ambulacro-interradial 
  suture. 
  

   (Compare 
  Forbes, 
  1848, 
  Mem. 
  Geol. 
  Survey 
  United 
  King- 
  

   dom, 
  vol. 
  ii. 
  pt. 
  i. 
  p. 
  384, 
  pi. 
  xxis. 
  fig. 
  1, 
  and 
  the 
  fig. 
  II. 
  of 
  

   this 
  communication.) 
  

  

  The 
  Grouping 
  of 
  the 
  Species. 
  — 
  It 
  will 
  have 
  been 
  observed 
  

   that 
  no 
  less 
  than 
  five 
  species 
  of 
  Palceechinus 
  present 
  two 
  ver- 
  

   tical 
  rows 
  of 
  pairs 
  of 
  pores 
  on 
  each 
  side 
  of 
  the 
  ambulacra. 
  

   The 
  species 
  group 
  themselves 
  into 
  two 
  divisions. 
  In 
  one, 
  con- 
  

   taining 
  P. 
  gigas 
  and 
  P. 
  ellipticus, 
  the 
  outer 
  pairs 
  of 
  pores 
  of 
  

   the 
  vertical 
  rows 
  are 
  in 
  demi-plates, 
  and 
  in 
  the 
  other 
  division, 
  

   containing 
  P. 
  sphcericus, 
  P. 
  intermedins, 
  and 
  P. 
  Phiilipsiw, 
  the 
  

   plates 
  ot 
  both 
  sets 
  of 
  pairs 
  of 
  pores 
  are 
  primaries, 
  some 
  being- 
  

   blocked 
  out 
  from 
  the 
  ambulacro-interradial 
  sutures. 
  Very 
  

   rarely 
  there 
  is 
  some 
  crowding 
  and 
  union 
  of 
  plates 
  to 
  form 
  

   compound 
  ones, 
  as 
  in 
  P. 
  sphtericus. 
  

  

  It 
  is 
  very 
  remarkable 
  how 
  distinct 
  some 
  of 
  the 
  plates 
  are 
  

   from 
  their 
  neighbours, 
  and 
  it 
  is 
  evident 
  that 
  their 
  great 
  thick- 
  

   ness 
  would 
  keep 
  them 
  from 
  overlapping. 
  It 
  would 
  seem 
  at 
  

   first 
  sight 
  as 
  it 
  there 
  had 
  always 
  been 
  a 
  space 
  between 
  the 
  

   edges 
  of 
  the 
  plates 
  ; 
  but 
  in 
  the 
  YVoodwardian 
  specimens 
  and 
  

   in 
  some 
  elsewhere 
  the 
  plates 
  are 
  often 
  so 
  close 
  that 
  there 
  is 
  

   no 
  vestige 
  of 
  a 
  sutural 
  line 
  between 
  them. 
  Hence 
  the 
  draw- 
  

   ings 
  given 
  by 
  M'Coy 
  and 
  W. 
  Keeping, 
  which 
  have 
  been 
  

   noticed. 
  

  

  The 
  Characters 
  of 
  Palueechinus 
  elegans 
  and 
  P. 
  quadriseri- 
  

   alis. 
  — 
  The 
  question 
  arises, 
  Where 
  are 
  Palceechinus 
  elegans, 
  

   M'Coy, 
  P. 
  quadriserialis, 
  J. 
  Wright 
  (Journ. 
  R. 
  Geol. 
  iSoc. 
  

   Irel. 
  vol. 
  i. 
  p. 
  62, 
  pi. 
  iii.), 
  and 
  a 
  small-plated 
  Palceechinus 
  in 
  the 
  

   Woodwardian 
  Collection, 
  which 
  does 
  not 
  present 
  the 
  charac- 
  

   ters 
  of 
  any 
  of 
  the 
  five 
  species 
  mentioned 
  above, 
  to 
  be 
  placed 
  ? 
  

  

  In 
  a 
  classification 
  of 
  recent 
  or 
  Mesozoic 
  Echinoidea 
  no 
  one 
  

   would 
  place 
  species 
  with 
  a 
  single 
  vertical 
  row 
  of 
  pores 
  on 
  

   each 
  side 
  of 
  an 
  ambulacrum 
  in 
  the 
  same 
  genus 
  as 
  species 
  

   presenting 
  two 
  vertical 
  series 
  of 
  pairs 
  on 
  each 
  side 
  of 
  an 
  

   ambulacrum, 
  the 
  plates 
  differing 
  markedly 
  in 
  their 
  shape 
  and 
  

   arrangement 
  also. 
  Cidaris 
  is 
  separated 
  on 
  these 
  lines 
  from 
  

   Diplocidaris. 
  Now 
  P. 
  elegans 
  has 
  all 
  its 
  ambulacral 
  plates 
  

  

  