﻿200 
  Prof. 
  P. 
  M. 
  Duncan 
  on 
  the 
  

  

  (a'). 
  It 
  does 
  not 
  appear 
  that 
  this 
  demi-plate 
  and 
  its 
  actinal 
  

   or 
  abactinal 
  primary 
  ever 
  form 
  a 
  true 
  composite 
  plate. 
  There 
  

   is 
  no 
  overlap, 
  of 
  the 
  ambulacral 
  plates. 
  

  

  Love'n 
  (' 
  Etudes,' 
  1874, 
  p. 
  41) 
  notices 
  P. 
  gigas. 
  He 
  ob- 
  

   serves 
  that 
  the 
  figure 
  given 
  of 
  it 
  by 
  M'Coy 
  (op. 
  cit. 
  pi. 
  xxiv. 
  

   fig. 
  A 
  c) 
  shows 
  ambulacral 
  plates 
  each 
  with 
  two 
  pairs 
  of 
  pores 
  ; 
  

   this 
  he 
  properly 
  considered 
  to 
  be 
  erroneous. 
  W. 
  Keeping 
  

   (Quart. 
  Journ. 
  Geol. 
  Soc. 
  1876, 
  vol. 
  xxxii. 
  pi. 
  iii. 
  figs. 
  12, 
  

   13) 
  gives 
  M'Coy's 
  figure 
  and 
  a 
  correction. 
  Now, 
  although 
  

   these 
  drawings 
  show 
  a 
  marked 
  difference 
  from 
  the 
  structures 
  

   visible 
  in 
  the 
  specimens 
  I 
  have 
  described, 
  still 
  in 
  specimens 
  

   where 
  obliteration 
  of 
  the 
  sutures 
  has 
  taken 
  place 
  and 
  where 
  

   the 
  pores 
  are 
  greatly 
  crowded 
  vertically, 
  both 
  of 
  the 
  appear- 
  

   ances 
  drawn 
  by 
  M'Coy 
  and 
  Keeping 
  are 
  to 
  be 
  recognized. 
  

   But 
  they 
  are 
  deceptive, 
  and 
  the 
  truth 
  is 
  explained 
  in 
  

   weathered 
  parts 
  of 
  the 
  same 
  test. 
  Nevertheless 
  there 
  were 
  

   no 
  intercalated 
  demi-plates 
  noticed 
  in 
  M'Coy's 
  definition 
  of 
  

   the 
  genus 
  or 
  species. 
  Loven 
  places 
  P. 
  gigas 
  in 
  a 
  group 
  of 
  

   Palceechini 
  which 
  have 
  the 
  ambulacral 
  plates 
  differing 
  alter- 
  

   nately, 
  having 
  the 
  same 
  height 
  at 
  the 
  median 
  suture 
  and 
  

   one 
  entire 
  (a 
  primary), 
  and 
  the 
  other 
  reduced 
  to 
  a 
  cuneiform 
  

   demi-plate, 
  its 
  point 
  directed 
  outwards. 
  In 
  the 
  specimens 
  of 
  

   P. 
  gigas 
  in 
  London 
  and 
  Cambridge 
  the 
  demi-plate 
  has 
  its 
  

   point 
  directed 
  towards 
  the 
  median 
  line, 
  which 
  it 
  does 
  not 
  

   reach. 
  It 
  will 
  be 
  seen, 
  however, 
  that 
  the 
  structure 
  noticed 
  

   by 
  Loven 
  is 
  found 
  in 
  another 
  species, 
  P. 
  sphcericus. 
  

  

  Ambulacra 
  of 
  Palseechinus 
  ellipticus 
  (fig. 
  VIII.) 
  present 
  

   ambulacral 
  plates 
  somewhat 
  similar 
  to 
  those 
  of 
  P. 
  gigas, 
  and 
  

   the 
  pairs 
  of 
  pores 
  are 
  biserial 
  on 
  each 
  side 
  of 
  an 
  ambulacrum. 
  

   Hence 
  there 
  is 
  an 
  error 
  in 
  the 
  specific 
  diagnosis 
  and 
  drawing 
  of 
  

   M'Coy 
  (in 
  Foss. 
  Mount. 
  Limestone 
  Ireland, 
  1842, 
  pi. 
  xxiv. 
  

   tig. 
  3 
  ; 
  P. 
  gigas 
  is 
  incorrectly 
  drawn, 
  fig. 
  4). 
  

  

  The 
  specimen 
  in 
  the 
  Museum 
  of 
  Practical 
  Geology 
  

   (xin 
  | 
  y^) 
  shows 
  very 
  convex 
  interporiferous 
  areas. 
  The 
  

   plates 
  with 
  the 
  outer 
  pairs 
  of 
  pores 
  are 
  small 
  demi-plates 
  ; 
  the 
  

   primary 
  plates 
  with 
  the 
  inner 
  rows 
  of 
  pairs 
  of 
  pores 
  are 
  nar- 
  

   rowed 
  between 
  the 
  demi-plates 
  and 
  usually 
  do 
  not 
  reach 
  the 
  

   ambulacro-interradial 
  suture 
  (fig. 
  VIII.). 
  

  

  Ambulacra 
  of 
  Palasechinus 
  sphsericus 
  (figs. 
  n.-Vli.). 
  — 
  

   There 
  are 
  two 
  specimens 
  of 
  this 
  species, 
  one 
  a 
  crushed 
  mass 
  

   and 
  the 
  other 
  more 
  or 
  less 
  prolately 
  spheroidal, 
  in 
  the 
  British 
  

   Museum 
  which 
  show 
  the 
  structure 
  of 
  the 
  ambulacra 
  very 
  

   well. 
  The 
  arrangement 
  of 
  the 
  pores 
  in 
  two 
  vertical 
  rows 
  of 
  

  

  