﻿116 
  

  

  DE. 
  J. 
  W. 
  GEEGOEY 
  ON 
  THE 
  

  

  [Feb. 
  1897,. 
  

  

  Fig. 
  2. 
  — 
  Ambital 
  ambulacra! 
  

   plate 
  of 
  Pelanechinus. 
  

   (After 
  Groom.) 
  

  

  from 
  both 
  of 
  the 
  vertical 
  sutures, 
  and 
  lying 
  only 
  on 
  the 
  horizontal 
  

   sutures 
  (see 
  fig. 
  1), 
  as 
  in 
  Phormosoma 
  Uranus. 
  JN 
  r 
  ow 
  in 
  Pelanechinus 
  

   the 
  ambital 
  ambulacral 
  plates 
  (fig. 
  2) 
  each 
  consist 
  of 
  three 
  primary 
  

   ambulacral 
  plates 
  and 
  three 
  demi-plates 
  and 
  three 
  klasma-plates, 
  or 
  

   nine 
  constituents 
  in 
  all. 
  This 
  ar- 
  

   rangement 
  is 
  apparently 
  so 
  complex 
  

   that 
  Mr. 
  Groom 
  suggests 
  that 
  it 
  is 
  

   to 
  be 
  regarded 
  as 
  a 
  new 
  type 
  of 
  

   ambulacral 
  plate. 
  1 
  But 
  the 
  plates 
  

   near 
  the 
  end 
  of 
  an 
  ambulacrum 
  

   consist 
  of 
  triplets, 
  each 
  composed 
  

   of 
  a 
  central 
  large 
  primary 
  with 
  a 
  

   demi-plute 
  (or 
  klasma-plate) 
  above 
  

   and 
  below 
  it 
  (fig. 
  3). 
  This 
  is 
  the 
  

   arrangement 
  of 
  a 
  typical 
  ' 
  diade- 
  

   moid 
  ' 
  plate. 
  If 
  we 
  follow 
  along 
  

   an 
  ambulacrum 
  of 
  a 
  Pelanechinus, 
  

   we 
  find 
  a 
  gradual 
  passage 
  from 
  

   the 
  simple 
  diademoid 
  triplets 
  to 
  

   the 
  large 
  compound 
  plates 
  of 
  the 
  

   ambitus. 
  These 
  plates 
  may, 
  there- 
  

   fore, 
  be 
  regarded 
  as 
  formed 
  by 
  the 
  

   fusion 
  of 
  three 
  diademoid 
  plates. 
  

  

  In 
  the 
  living 
  Echinothuridae 
  the 
  

   ambulacral 
  triplets 
  are 
  usually 
  de- 
  

   scribed 
  as 
  formed 
  of 
  a 
  large 
  aboral 
  

   primary 
  and 
  two 
  small 
  demi-plates 
  

   in 
  the 
  adoral 
  suture 
  of 
  the 
  primary 
  

   (fig. 
  1, 
  p. 
  115). 
  This 
  view 
  obscures 
  

   the 
  diademoid 
  structure 
  of 
  the 
  plates. 
  

   If 
  we 
  examine 
  the 
  ambital 
  plates 
  

   the 
  usual 
  view 
  appears 
  the 
  natural 
  

   one. 
  But 
  if 
  we 
  examine 
  the 
  ambu- 
  

   lacral 
  plates 
  near 
  the 
  apex, 
  we 
  find 
  

  

  that 
  they 
  are 
  composed 
  of 
  a 
  central 
  primary 
  between 
  an 
  adoral 
  and 
  

   an 
  aboral 
  primary, 
  as, 
  for 
  example, 
  in. 
  Phormosoma 
  bursarium 
  

   (fig. 
  4), 
  and 
  near 
  the 
  peristome 
  the 
  same 
  arrangement 
  recurs 
  (fig. 
  5) 
  ; 
  

   that 
  is 
  to 
  say, 
  the 
  plates 
  are 
  typically 
  diademoid, 
  and 
  agree 
  in 
  

   arrangement 
  with 
  the 
  equivalent 
  

   plates 
  of 
  Pelanechinus. 
  From 
  

   these 
  upper 
  plates 
  we 
  may 
  follow 
  

  

  Fig. 
  3. 
  — 
  Peristomal 
  ambula- 
  

   cral 
  plates 
  of 
  Pelanechinus 
  r 
  

   showing 
  division 
  into 
  trip- 
  

   lets 
  of 
  the 
  diademoid 
  type. 
  

  

  the 
  series 
  downward, 
  seeing 
  the 
  

  

  gradual 
  increase 
  of 
  the 
  central 
  

   plate, 
  and 
  the 
  reduction 
  of 
  the 
  

   demi-plates 
  until 
  they 
  occur 
  as 
  

   small 
  klasma-plates 
  in 
  the 
  hori- 
  

   zontal 
  suture. 
  The 
  develop- 
  

   ment 
  of 
  the 
  plates 
  shows, 
  therefore 
  

  

  Fig. 
  4. 
  — 
  Ambulacral 
  plates 
  of 
  

   Phormosoma 
  bursarium 
  near 
  

   the 
  apex. 
  (After 
  '^A. 
  Agassiz.) 
  

  

  <®ZZ^> 
  

  

  that 
  the 
  ambulacral 
  plates 
  

  

  1 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xliii. 
  (1887) 
  p. 
  707. 
  The 
  author, 
  however, 
  

   there 
  regards 
  the 
  triplets 
  as 
  of 
  the 
  echinoid, 
  and 
  not 
  of 
  the 
  diademoid 
  type. 
  

  

  