﻿202 
  Prof. 
  P. 
  M. 
  Duncan 
  on 
  the 
  

  

  it 
  is 
  broad, 
  and 
  whilst 
  its 
  height 
  is 
  the 
  same 
  as 
  that 
  of 
  the 
  

   other 
  plates 
  at 
  the 
  median 
  ambulacral 
  line, 
  it 
  is 
  nipped 
  in, 
  

   vertically, 
  at 
  its 
  middle 
  in 
  consequence 
  of 
  the 
  shape 
  of 
  the 
  

   plates 
  above 
  and 
  below 
  it 
  (a 
  1 
  and 
  a 
  2 
  ) 
  . 
  The 
  first 
  of 
  these 
  (a 
  1 
  ) 
  

   is 
  a 
  low 
  primary, 
  smallest 
  at 
  the 
  extremities, 
  pierced 
  by 
  an 
  

   inner 
  pair 
  of 
  pores, 
  and 
  it 
  has 
  a 
  transverse 
  curved 
  adoral 
  

   edge 
  and 
  also 
  a 
  convex 
  aboral 
  sutural 
  edge 
  to 
  connect 
  it 
  with 
  

   the 
  primary 
  (b'). 
  

  

  The 
  other 
  plate, 
  a 
  primary 
  (a 
  2 
  ), 
  is 
  aboral 
  to 
  the 
  primary 
  

   b', 
  is 
  tall 
  at 
  the 
  ambulacral 
  median 
  line, 
  low 
  close 
  to 
  the 
  

   pair 
  of 
  pores, 
  and 
  externally 
  it 
  is 
  not 
  quite 
  reduced 
  to 
  such 
  a 
  

   mere 
  point 
  as 
  in 
  the 
  instance 
  of 
  the 
  first 
  component 
  (a 
  1 
  ), 
  which 
  

   just 
  reaches 
  the 
  ambulacro-interradial 
  edge 
  at 
  a 
  reentering 
  

   angle. 
  The 
  adoral 
  sutural 
  edge 
  of 
  the 
  plate 
  (a 
  2 
  ) 
  is 
  convex 
  

   and 
  the 
  aboral 
  edge 
  is 
  straight 
  or 
  slightly 
  concave 
  in 
  the 
  

   opposite 
  direction. 
  The 
  arrangement 
  of 
  these 
  three 
  unequal 
  

   primary 
  plates 
  has 
  geometrical 
  outlines 
  neither 
  externally 
  nor 
  

   at 
  the 
  median 
  ambulacral 
  line, 
  and 
  is 
  repeated 
  in 
  the 
  vertical 
  

   succession 
  (fig. 
  v., 
  a 
  1 
  , 
  b, 
  a). 
  

  

  Amongst 
  the 
  plates 
  of 
  this 
  species 
  are 
  some 
  which 
  are 
  not 
  

   only 
  characteristic 
  but 
  very 
  suggestive 
  of 
  the 
  reduced 
  and 
  

   isolated 
  plates 
  of 
  other 
  Palasechinoidea 
  and 
  of 
  the 
  later 
  

   Echinothuridge. 
  

  

  A 
  composite 
  plate 
  (fig. 
  VI.) 
  has 
  a 
  small 
  aboral 
  primary 
  a 
  

   and 
  a 
  large 
  middle 
  primary 
  b 
  ; 
  but 
  the 
  third 
  component 
  is 
  

   adoral 
  and 
  at 
  the 
  ambulacro-interradial 
  edge, 
  and 
  is 
  a 
  small 
  

   demi-plate 
  looking 
  like 
  a 
  peripodium 
  without 
  a 
  plate, 
  and 
  it 
  

   is 
  crushed 
  a 
  little 
  out 
  of 
  place. 
  This 
  demi-plate 
  (a 
  1 
  ) 
  should 
  

   be 
  compared 
  with 
  that 
  seen 
  in 
  a 
  doubtful 
  composite 
  plate 
  

   (fig. 
  VII., 
  a'). 
  It 
  is 
  evident 
  that 
  the 
  crush 
  of 
  growth 
  has 
  

   enabled 
  the 
  large 
  primary 
  plate 
  (If) 
  belonging 
  to 
  the 
  outer 
  

   vertical 
  row 
  of 
  pores 
  to 
  crowd 
  out, 
  with 
  the 
  assistance 
  of 
  its 
  

   adoral 
  neighbour 
  (b 
  1 
  ), 
  the 
  primary 
  of 
  the 
  inner 
  row. 
  This 
  

   has 
  lost 
  all 
  its 
  inner 
  part, 
  has 
  become 
  a 
  demi-plate 
  (a'), 
  and 
  

   but 
  a 
  trace 
  of 
  the 
  outer 
  portion 
  remains 
  ; 
  the 
  rest 
  is 
  like 
  a 
  

   thick 
  peripodium. 
  

  

  The 
  occurrence 
  of 
  small 
  isolated 
  plates 
  along 
  the 
  lines 
  of 
  

   transverse 
  sutures, 
  a 
  phenomenon 
  especially 
  Echinothurian 
  

   in 
  the 
  recent 
  fauna, 
  has 
  therefore 
  a 
  certain 
  explanation. 
  The 
  

   plates 
  thus 
  placed 
  and 
  small 
  are 
  not 
  additional 
  and 
  intercalated 
  

   plates, 
  but 
  are 
  the 
  results 
  of 
  growth, 
  crowding, 
  crushing 
  out 
  

   and 
  in, 
  and 
  atavism 
  of 
  original 
  plates. 
  

  

  It 
  does 
  not 
  appear 
  unreasonable 
  to 
  account 
  for 
  the 
  presence 
  

   of 
  four 
  vertical 
  rows 
  of 
  ambulacral 
  plates 
  in 
  some 
  Palasechi- 
  

   noidea 
  by 
  an 
  early 
  movement 
  of 
  the 
  plates 
  perforated 
  by 
  the 
  

   inner 
  row 
  of 
  pairs 
  of 
  pores 
  inwards 
  whilst 
  the 
  growth 
  pro- 
  

   ceeded. 
  

  

  