﻿198 
  Prof. 
  P. 
  M. 
  Duncan 
  on 
  the 
  

  

  suture 
  of 
  its 
  plate 
  on 
  the 
  outside 
  of 
  the 
  test, 
  and 
  is 
  equally 
  

   remote 
  from 
  it 
  when 
  seen 
  from 
  within 
  the 
  test. 
  The 
  pairs 
  

   alternate, 
  are 
  in 
  a 
  slightly 
  sunken 
  zone, 
  and 
  the 
  interporiferous 
  

   areas 
  are 
  rather 
  wide 
  and 
  more 
  or 
  less 
  convex. 
  This 
  arrange- 
  

   ment 
  of 
  the 
  pairs 
  is 
  seen 
  in 
  P. 
  gigas, 
  P. 
  sphcsrwus, 
  P. 
  inter- 
  

   medins, 
  P. 
  ellipticus, 
  and 
  P. 
  Philh'psice. 
  

  

  II. 
  The 
  plates 
  of 
  the 
  ambulacra 
  are 
  very 
  numerous 
  and 
  

   variable 
  in 
  shape 
  in 
  the 
  same 
  ambulacrum 
  ; 
  in 
  shape 
  

   and 
  arrangement 
  they 
  differ 
  in 
  the 
  species, 
  but 
  there 
  may 
  

   be 
  a 
  great 
  sameness 
  in 
  the 
  plates 
  of 
  P. 
  gigas. 
  Simple 
  

   primary 
  plates, 
  one 
  large 
  at 
  the 
  ambulacro-interradial 
  end, 
  

   and 
  the 
  next 
  small 
  there 
  or 
  blocked 
  out, 
  with 
  or 
  without 
  

   alternate 
  demi-plates, 
  are 
  the 
  commonest, 
  and 
  true 
  composite 
  

   plates 
  are 
  very 
  rare 
  in 
  all 
  species 
  and 
  absolutely 
  absent 
  in 
  

   some. 
  A 
  succession 
  of 
  similar 
  primary 
  plates, 
  the 
  pairs 
  of 
  

   pores 
  being 
  uniserial, 
  is 
  not 
  observed 
  in 
  the 
  species 
  noticed 
  

   above. 
  

  

  III. 
  The 
  ambulacral 
  plates 
  are 
  low 
  and 
  yet 
  very 
  thick, 
  and 
  

   when 
  in 
  place 
  present 
  a 
  zigzag 
  at 
  their 
  interradial 
  edges 
  — 
  

   there 
  being 
  a 
  vertical 
  series 
  of 
  alternate 
  salient 
  and 
  reentering 
  

   angles 
  there, 
  the 
  one 
  being 
  at 
  the 
  edge 
  of 
  the 
  plates 
  which 
  

   are 
  perforated 
  by 
  the 
  pairs 
  of 
  pores 
  of 
  the 
  outer 
  vertical 
  row, 
  

   and 
  the 
  other 
  corresponding 
  more 
  or 
  less 
  to 
  the 
  plate 
  of 
  the 
  

   inner 
  row 
  of 
  pairs 
  of 
  pores. 
  The 
  projection 
  of 
  the 
  salient 
  

   angles 
  towards 
  the 
  interradia 
  is 
  very 
  decided 
  as 
  a 
  rule, 
  and 
  

   may 
  be 
  more 
  or 
  less 
  rounded 
  (figs. 
  I., 
  II., 
  V., 
  Viii., 
  X.). 
  

  

  This 
  ambulacro-interradial 
  line 
  of 
  suturing 
  of 
  course 
  brings 
  

   the 
  zigzag 
  of 
  the 
  ambulacral 
  edge 
  in 
  contact 
  with 
  the 
  adam- 
  

   bnlacral 
  edge 
  of 
  the 
  interradium, 
  and 
  this 
  will 
  be 
  found 
  to 
  be 
  

   a 
  zigzag, 
  and 
  its 
  reentering 
  angles 
  fit 
  the 
  salient 
  angles 
  of 
  

   the 
  ambulacral 
  edge. 
  Isolated 
  plates 
  may 
  be 
  seen 
  showing 
  

   these 
  projections 
  and 
  depressions, 
  and 
  it 
  is 
  evident 
  that 
  the 
  

   plates 
  of 
  the 
  test 
  were 
  readily 
  separable 
  at 
  this 
  suture, 
  for 
  

   the 
  ambulacra 
  are 
  often 
  displaced, 
  the 
  adambulacral 
  plates 
  of 
  

   the 
  interradia 
  being 
  more 
  prominent 
  or 
  the 
  reverse 
  than 
  

   the 
  ambulacra. 
  When 
  the 
  opposed 
  edges 
  of 
  the 
  two 
  series 
  

   of 
  plates 
  are 
  in 
  their 
  normal 
  condition, 
  the 
  more 
  or 
  less 
  

   rounded 
  outer 
  angles 
  of 
  the 
  ambulacral 
  plates 
  are 
  very 
  visible 
  

   and 
  never 
  assume 
  the 
  simple 
  character 
  figured 
  by 
  M'Coy 
  

   and 
  reproduced 
  by 
  E. 
  Etheridge 
  (Quart. 
  Journ. 
  Geol. 
  Soc. 
  

   vol. 
  xxx. 
  pi. 
  xxiv. 
  fig. 
  2, 
  from 
  M'Coy, 
  Synop. 
  Carb. 
  

   Foss. 
  Irel. 
  pi. 
  xxiv.). 
  There 
  is 
  no 
  overlap 
  of 
  the 
  interradia 
  

   over 
  the 
  ambulacra, 
  and 
  the 
  test 
  was 
  as 
  rigid 
  there 
  as 
  in 
  an 
  

   Echinus 
  and 
  not 
  very 
  unlike 
  one 
  in 
  the 
  suturing. 
  

  

  The 
  Ambulacra 
  o/Talasechinus 
  gigas 
  (fig. 
  l, 
  p. 
  206). 
  — 
  These 
  

  

  