﻿Anatomy 
  o/TalEeechinus 
  (Scouler), 
  APCoy. 
  201 
  

  

  pairs 
  is 
  the 
  same 
  as 
  in 
  P. 
  gigas 
  y 
  but 
  the 
  shape 
  and 
  the 
  nature 
  

   of 
  the 
  plates 
  differ. 
  The 
  commonest 
  arrangement 
  of 
  the 
  plates 
  

   is 
  seen 
  at 
  a 
  little 
  distance 
  from 
  the 
  apical 
  system 
  (fig. 
  II.). 
  

   The 
  pores 
  of 
  the 
  pairs, 
  which 
  are 
  in 
  two 
  vertical 
  series 
  and 
  

   alternate, 
  are 
  large 
  and 
  often 
  rather 
  broad 
  ; 
  they 
  are 
  separated 
  

   by 
  a 
  septum, 
  as 
  in 
  the 
  other 
  species. 
  As 
  in 
  P. 
  gigas, 
  the 
  pair 
  

   of 
  pores 
  of 
  the 
  outer 
  vertical 
  row 
  (b) 
  are 
  in 
  plates 
  which 
  form 
  

   the 
  salient 
  angle 
  of 
  the 
  zigzag 
  at 
  the 
  ambulacral 
  edge 
  ; 
  and 
  

   the 
  pair 
  of 
  pores 
  of 
  the 
  inner 
  row 
  (a) 
  are 
  in 
  relation 
  with 
  the 
  

   reentering 
  angles. 
  The 
  plates 
  are 
  of 
  two 
  principal 
  kinds, 
  

   and 
  the 
  first 
  to 
  be 
  noticed 
  are 
  those 
  which 
  are 
  perforated 
  by 
  

   the 
  inner 
  pairs 
  of 
  pores. 
  These 
  plates 
  are 
  either 
  perfect, 
  

   low, 
  broad 
  primaries, 
  highest 
  at 
  the 
  ambulacral 
  median 
  line 
  

   (a), 
  having 
  much 
  transverse 
  suture, 
  and 
  narrowing 
  and 
  be- 
  

   coming 
  pointed 
  at 
  the 
  ambulacro-interradial 
  suture, 
  or 
  imper- 
  

   fect 
  primaries, 
  with 
  their 
  outer 
  part, 
  near 
  the 
  ambulacro- 
  

   interradial 
  edge, 
  reduced 
  to 
  a 
  point 
  which 
  does 
  not 
  reach 
  

   outwards 
  to 
  the 
  outward 
  edge 
  (a 
  1 
  ). 
  Both 
  of 
  these 
  plates 
  

   are 
  usually 
  high 
  at 
  the 
  median 
  line 
  and 
  have 
  a 
  single 
  hori- 
  

   zontal 
  row 
  of 
  minute 
  scrobiculate 
  tubercles, 
  with 
  a 
  boss 
  and 
  

   some 
  granules 
  (fig. 
  II., 
  a, 
  a!). 
  

  

  The 
  other 
  principal 
  kind 
  of 
  plate 
  (fig. 
  II., 
  b) 
  is 
  a 
  low 
  pri- 
  

   mary, 
  as 
  broad 
  as 
  those 
  just 
  described, 
  but 
  slightly 
  lower 
  at 
  

   the 
  median 
  line 
  and 
  larger 
  externally, 
  where 
  there 
  is 
  a 
  large 
  

   pair 
  of 
  pores 
  of 
  the 
  outer 
  vertical 
  series. 
  The 
  plate 
  is 
  ex- 
  

   panded 
  around 
  the 
  pores 
  and 
  highest 
  there. 
  The 
  ornamen- 
  

   tation 
  is 
  the 
  same 
  as 
  that 
  of 
  the 
  other 
  plates. 
  It 
  will 
  be 
  

   observed 
  that 
  this 
  arrangement 
  is 
  different 
  from 
  that 
  of 
  P. 
  

   gigas 
  and 
  that 
  both 
  plates 
  are 
  primaries, 
  there 
  being 
  no 
  demi- 
  

   plate. 
  Nearer 
  the 
  apical 
  system 
  the 
  size 
  of 
  the 
  pair 
  of 
  pores 
  is 
  

   not 
  sufficient 
  to 
  produce 
  much 
  enlargement 
  of 
  the 
  outer 
  part 
  

   of 
  the 
  plate, 
  which 
  has 
  an 
  outer 
  pair 
  of 
  pores 
  (fig. 
  ill., 
  b). 
  

   J5ut 
  the 
  slight 
  increase 
  of 
  height 
  is 
  readily 
  seen, 
  and 
  it 
  has 
  

   the 
  effect 
  of 
  diminishing 
  the 
  size, 
  of 
  the 
  next 
  plate 
  {a), 
  with 
  an 
  

   inner 
  pair 
  of 
  pores 
  (fig. 
  in.). 
  It 
  certainly 
  appears, 
  when 
  the 
  

   sutures 
  of 
  the 
  plates 
  are 
  invisible, 
  as 
  if 
  there 
  were 
  two 
  pairs 
  

   of 
  pores 
  to 
  a 
  plate, 
  as 
  M'Coy 
  drew 
  and 
  Keeping 
  criticized. 
  

  

  One 
  composite 
  plate 
  is 
  seen 
  in 
  a 
  specimen 
  near 
  the 
  apical 
  

   system 
  and 
  the 
  components 
  are 
  primaries 
  ; 
  the 
  inner 
  edge 
  

   of 
  the 
  combined 
  plates 
  forms 
  an 
  angle, 
  and 
  the 
  larger 
  plate 
  is 
  

   with 
  a 
  large 
  outer 
  and 
  inner 
  part 
  (fig. 
  IV., 
  b) 
  } 
  while 
  the 
  smaller 
  

   plate 
  (a) 
  is 
  low 
  at 
  both 
  ends 
  and 
  has 
  a 
  suture 
  curved 
  adorally. 
  

  

  A 
  common 
  arrangement 
  of 
  the 
  plates 
  of 
  the 
  ambulacra 
  

   (fig. 
  V.) 
  is 
  as 
  follows 
  : 
  — 
  

  

  A 
  plate 
  (fig. 
  v., 
  b') 
  is 
  large 
  where 
  it 
  is 
  pierced 
  by 
  the 
  

   pores, 
  and 
  forms 
  a 
  projecting 
  angle 
  at 
  the 
  ambulacral 
  edge 
  ; 
  

  

  