﻿132 
  DR. 
  J. 
  W. 
  GREGORY 
  ON 
  [Feb. 
  I 
  897, 
  

  

  These 
  plates 
  are, 
  in 
  all 
  probability, 
  ambulacral 
  in 
  origin, 
  and 
  each 
  

   plate 
  is 
  then 
  homologous 
  with 
  the 
  half-pyramid 
  of 
  Echinocystis. 
  In 
  

   order, 
  therefore, 
  to 
  form 
  the 
  pyramids 
  of 
  Echinocystis, 
  it 
  is 
  only 
  

   necessary 
  to 
  assume 
  that 
  the 
  two 
  free 
  ends 
  of 
  the 
  half-pyramids 
  of 
  

   Palceodiscus 
  bent 
  inward, 
  so 
  that 
  the 
  symphysis 
  was 
  continued 
  

   along 
  the 
  whole 
  length, 
  instead 
  of 
  being 
  limited 
  to 
  the 
  aboral 
  end. 
  

   At 
  the 
  same 
  time, 
  increase 
  in 
  the 
  strength 
  of 
  the 
  muscular 
  attach- 
  

   ments 
  led 
  to 
  the 
  development 
  of 
  ridges 
  and 
  depressions 
  on 
  the 
  

   proximal 
  end. 
  

  

  Palceodiscus 
  therefore 
  gives 
  useful 
  information 
  as 
  to 
  the 
  origin 
  of 
  

   the 
  masticatory 
  apparatus 
  of 
  the 
  gnathostomate 
  echinids. 
  For, 
  just 
  

   as 
  in 
  the 
  Stelleroidea 
  the 
  adoral 
  ambulacral 
  plates 
  are 
  modified 
  to 
  

   form 
  the 
  ambulacral 
  oral 
  projections 
  of 
  asterids, 
  and 
  the 
  jaws 
  (that 
  

   is, 
  the 
  distal 
  portion 
  of 
  the 
  syngnaths) 
  of 
  ophiurids, 
  so 
  it 
  is 
  probable 
  

   that 
  the 
  pyramids 
  of 
  the 
  masticatory 
  apparatus 
  of 
  echinids 
  arose 
  as 
  

   a 
  modification 
  of 
  the 
  peristomal 
  ambulacral 
  plates. 
  As 
  it 
  is, 
  how- 
  

   ever, 
  possible 
  that 
  the 
  whole 
  apparatus 
  was 
  formed 
  around 
  the 
  

   oesophagus, 
  and 
  not 
  as 
  part 
  of 
  the 
  exoskeleton, 
  it 
  would 
  not 
  be 
  safe 
  

   to 
  definitely 
  claim 
  the 
  pyramids 
  as 
  the 
  homologues 
  of 
  the 
  ambulacral 
  

   oral 
  projections 
  of 
  asterids 
  and 
  of 
  the 
  jaws 
  of 
  ophiurids. 
  

  

  The 
  absence 
  of 
  calycinal 
  plates 
  is 
  of 
  great 
  interest, 
  as 
  it 
  appears 
  

   fatal 
  to 
  the 
  theory 
  assigning 
  a 
  crinoid 
  ancestry 
  to 
  the 
  Echinoidea. 
  

   The 
  insignificance 
  of 
  these 
  plates 
  in 
  the 
  oldest 
  echinids, 
  when 
  com- 
  

   pared 
  with 
  specialized 
  forms 
  such 
  as 
  Tiarechinus 
  and 
  Lysechinus 
  or 
  

   larval 
  Strongyloccntrotus, 
  shows 
  that 
  the 
  so-called 
  ' 
  calycinal 
  

   plates 
  ' 
  are 
  not 
  primitive. 
  MacBride 
  [6, 
  p. 
  436] 
  has 
  thrown 
  doubt 
  

   on 
  the 
  homology 
  of 
  these 
  plates 
  in 
  asterids 
  and 
  crinids, 
  and 
  Echino- 
  

   cystis 
  suggests 
  similar 
  doubts 
  for 
  echinids. 
  In 
  the 
  early 
  echinids 
  

   'these 
  plates 
  appear 
  as 
  a 
  ring 
  round 
  the 
  anus. 
  In 
  Echinocystis 
  the 
  

   anus 
  opens 
  at 
  the 
  centre 
  of 
  a 
  circle 
  of 
  five 
  valvular 
  plates. 
  This 
  

   Silurian 
  echinid 
  is 
  followed 
  by 
  others 
  in 
  the 
  Devonian 
  and 
  Carboni- 
  

   ferous, 
  such 
  as 
  Palcechinus 
  and 
  Perischodomus, 
  in 
  which 
  the 
  anus 
  

   likewise 
  opens 
  in 
  the 
  centre 
  of 
  five 
  rather 
  large 
  plates. 
  I 
  see 
  no 
  

   reason 
  why 
  these 
  five 
  circumanal 
  plates 
  of 
  Palcechinus 
  are 
  not 
  

   homologous 
  with 
  the 
  five 
  circumanal 
  plates 
  of 
  Echinocystis. 
  Such 
  at 
  

   least 
  is 
  by 
  no 
  means 
  so 
  far-fetched 
  an 
  idea 
  as 
  that 
  which 
  makes 
  

   these 
  plates 
  the 
  homologues 
  of 
  the 
  basal 
  circlet 
  of 
  the 
  crinoid 
  calyx. 
  

   It 
  is 
  therefore 
  at 
  least 
  possible, 
  and 
  perhaps 
  even 
  probable, 
  that 
  the 
  

   apical 
  or 
  so-called 
  ' 
  calycinal 
  plates 
  ' 
  of 
  echinids 
  are 
  homologous 
  

   with 
  the 
  anal 
  valvular 
  pyramid 
  of 
  Cystoidea, 
  and 
  not 
  with 
  the 
  plates 
  

   of 
  the 
  calyx. 
  1 
  

  

  VIII. 
  Classification 
  of 
  the 
  Cystocidarolda. 
  

  

  As 
  Echinocystis 
  and 
  Palceodiscus 
  are 
  therefore 
  accepted 
  as 
  two 
  

   genera 
  of 
  Echinoidea, 
  it 
  may 
  be 
  advisable 
  in 
  conclusion 
  to 
  attempt 
  

   a 
  synopsis 
  of 
  the 
  order 
  to 
  which 
  they 
  belong. 
  

  

  1 
  It 
  may 
  be 
  suggested 
  that 
  the 
  genital 
  pores 
  mark 
  out 
  the 
  basal 
  circlet, 
  and 
  

   that 
  these 
  pores 
  probably 
  opened, 
  in 
  Echinocystis, 
  round 
  tbe 
  apex, 
  and 
  not 
  

   round 
  tbe 
  anus. 
  Eut 
  the 
  connexion 
  of 
  these 
  pores, 
  as 
  of 
  the 
  water-pore, 
  with 
  

   any 
  plates 
  is 
  a 
  purely 
  secondary 
  feature, 
  and 
  cannot 
  be 
  used 
  for 
  the 
  determina- 
  

   tion 
  of 
  a 
  primary 
  homology. 
  

  

  