﻿118 
  

  

  DK. 
  J. 
  W. 
  GKEGORY 
  ON 
  THE 
  

  

  [Feb. 
  1897, 
  

  

  agree 
  in 
  having 
  perforate, 
  non-crenulate 
  tubercles, 
  and 
  low, 
  broad 
  

   interambulacral 
  plates, 
  each 
  with 
  a 
  row 
  of 
  tubercles. 
  But 
  the 
  

   genera 
  differ 
  widely 
  by 
  the 
  structure 
  of 
  the 
  ambulacral 
  plates. 
  In 
  

   Memipedina 
  the 
  compound 
  

  

  ambulacral 
  plates 
  are 
  always 
  Fig. 
  6. 
  — 
  Ambulacral 
  plates 
  of 
  

  

  formed 
  of 
  three 
  primaries, 
  Pedina. 
  {After 
  Duncan.) 
  

  

  and 
  the 
  pore-pairs 
  are 
  a 
  simple 
  

   vertical 
  series. 
  But 
  in 
  Pelan- 
  

   echinus 
  the 
  pore-pairs 
  are 
  

   triserial. 
  We 
  have 
  therefore 
  

   to 
  look 
  for 
  its 
  ancestor 
  

   among 
  the 
  triserial 
  Pedinidae, 
  

   and 
  not 
  among 
  the 
  uniserial 
  

   Diadematidae 
  to 
  which 
  Hemi- 
  

   pedina 
  belongs. 
  

  

  Before 
  following 
  this 
  clue, 
  

   however, 
  we 
  must 
  remember 
  that 
  Duncan 
  adopted 
  a 
  course 
  which, 
  

   if 
  agreed 
  upon, 
  would 
  prevent 
  the 
  derivation 
  of 
  the 
  Echinothuridae 
  

   from 
  such 
  specialized 
  Diademoida 
  as 
  the 
  Pedinidae. 
  Duncan 
  divided 
  

   the 
  order 
  Diademoida 
  into 
  two 
  suborders 
  — 
  the 
  Streptosomata, 
  

   including 
  the 
  Echinothuridae, 
  and 
  the 
  Stereosomata, 
  including 
  the 
  

   rest 
  of 
  the 
  order. 
  He 
  denned 
  the 
  Streptosomata 
  as 
  follows 
  ! 
  : 
  — 
  

   ' 
  Test 
  more 
  or 
  less 
  flexible, 
  with 
  external 
  and 
  internal 
  branchiae. 
  

   Ambulacral 
  plates 
  alone 
  continued 
  beyond 
  the 
  peristome 
  to 
  the 
  

   stoma.' 
  But 
  all 
  these 
  characters 
  also 
  occur 
  among 
  the 
  other 
  

   suborder, 
  the 
  Stereosomata. 
  In 
  the 
  pedinid 
  Astropyga 
  and 
  the 
  

   diademid 
  Helxkodiadema 
  (see 
  infra, 
  p. 
  121) 
  the 
  test 
  is 
  more 
  or 
  less 
  

   flexible. 
  Both 
  external 
  and 
  internal 
  branchiae 
  are 
  as 
  well 
  developed 
  

   in 
  Diadema 
  as 
  they 
  are 
  in 
  PJwnnosoma. 
  2 
  In 
  those 
  of 
  the 
  Stereo- 
  

   somata 
  which 
  have 
  plates 
  on 
  the 
  peristomal 
  membrane, 
  these 
  are 
  

   always 
  ambulacral. 
  

  

  Hence 
  Duncan's 
  diagnosis 
  is 
  useless, 
  and 
  there 
  is 
  no 
  evidence 
  to 
  

   show 
  that 
  the 
  Echinothuridae 
  diverged 
  from 
  the 
  main 
  diademoid 
  

   stem 
  at 
  a 
  very 
  early 
  period. 
  We 
  may 
  search 
  therefore 
  among 
  the 
  

   triserial 
  members 
  of 
  that 
  order 
  for 
  a 
  suitable 
  ancestor 
  for 
  the 
  

   echinothurids. 
  Of 
  all 
  the 
  Pedinidae, 
  Pedina 
  is 
  most 
  like 
  Pelan- 
  

   echinus. 
  The 
  interambulacral 
  plates 
  have 
  the 
  same 
  tuberculation,, 
  

   for 
  in 
  both 
  genera 
  they 
  are 
  multiserial, 
  perforate, 
  and 
  non- 
  

   crenulate. 
  

  

  The 
  ambulacral 
  plates 
  of 
  Pedina 
  show 
  a 
  series 
  of 
  stages 
  between 
  

   simple 
  diademoid 
  plates 
  and 
  those 
  of 
  Pelanechinus. 
  The 
  ambital 
  

   plates 
  of 
  Pedina 
  usually 
  consist 
  of 
  three 
  primaries, 
  of 
  very 
  unequal 
  

   size. 
  In 
  some 
  cases 
  the 
  middle 
  primary 
  increases 
  so 
  much 
  in 
  size 
  that 
  

   it 
  crowds 
  out 
  the 
  other 
  two 
  constituents 
  and 
  reduces 
  them 
  to 
  demi- 
  

   plates 
  (fig. 
  6). 
  These 
  demi-plates 
  may 
  even 
  lose 
  any 
  connexion 
  

   with 
  the 
  vertical 
  suture 
  between 
  the 
  ambulacral 
  and 
  interambulacral 
  

   areas 
  ; 
  they 
  are 
  thus 
  reduced 
  to 
  the 
  position 
  of 
  klasma-plates 
  on 
  

   the 
  horizontal 
  sutures 
  between 
  the 
  primaries. 
  Plates 
  in 
  this 
  

  

  1 
  Journ. 
  Linn, 
  Soc. 
  Zool. 
  vol. 
  xxiii. 
  (1890) 
  p. 
  25. 
  

  

  2 
  F. 
  J. 
  Bell, 
  Ann. 
  Mag. 
  Nat. 
  Hist. 
  ser. 
  6, 
  vol. 
  iv. 
  (1889) 
  p. 
  437. 
  

  

  