﻿Mr. 
  J. 
  W. 
  Gregory 
  on 
  Zeuglopleurus. 
  490 
  

  

  disk 
  has 
  the 
  same 
  arrangement, 
  but 
  is 
  more 
  regular. 
  Zeuglo- 
  

   pleurus 
  differs 
  from 
  this 
  by 
  its 
  imperforate 
  tubercles, 
  the 
  

   distribution 
  of 
  the 
  epistroma, 
  and 
  the 
  absence 
  of 
  the 
  obli- 
  

   quity 
  of 
  the 
  apical 
  disk 
  so 
  well 
  marked 
  in 
  Dictyopleurus. 
  In 
  

   Zeuglopleurus 
  the 
  arrangement 
  agrees 
  with 
  that 
  of 
  Evechimts* 
  

   and 
  other 
  genera, 
  with 
  which, 
  however, 
  it 
  has 
  but 
  little 
  

   in 
  common, 
  while 
  that 
  of 
  Echinocyphus 
  may 
  be 
  compared 
  

   to 
  that 
  of 
  a 
  Pygaster 
  in 
  which 
  the 
  posterior 
  basal 
  has 
  

   not 
  been 
  completely 
  absorbed 
  by 
  the 
  backward 
  passage 
  of 
  

   the 
  anus. 
  The 
  three 
  genera 
  in 
  fact 
  form 
  a 
  series 
  in 
  which 
  

   we 
  may 
  see 
  the 
  same 
  tendency 
  towards 
  the 
  retrogression 
  of 
  

   the 
  anus 
  that 
  is 
  so 
  noticeable 
  in 
  the 
  Petalosticha 
  and 
  the 
  

   Clypeastroidea. 
  In 
  Glyphocyphus 
  the 
  anus 
  is 
  central 
  ; 
  in 
  

   Zeuglopleurus 
  it 
  has 
  travelled 
  backwards 
  and 
  the 
  antero-late- 
  

   ral 
  basals 
  have 
  expanded 
  to 
  fill 
  the 
  space 
  thus 
  caused. 
  In 
  

   Echinocyphus 
  the 
  same 
  process 
  has 
  continued 
  and 
  the 
  postero- 
  

   lateral 
  basals 
  have 
  also 
  met 
  across 
  the 
  centre, 
  and 
  thus 
  the 
  

   anal 
  ring, 
  instead 
  of 
  being 
  constituted 
  by 
  the 
  ten 
  plates 
  as 
  in 
  

   Glyphocyphus 
  , 
  or 
  seven 
  as 
  in 
  Zeugopleurus, 
  is 
  limited 
  to 
  three. 
  

   The 
  process, 
  however, 
  seems 
  to 
  have 
  stopped 
  here, 
  and 
  in 
  the 
  

   Tertiary 
  genera 
  the 
  apical 
  disk 
  is 
  on 
  the 
  Zeugopleurus 
  type, 
  

   though 
  the 
  arrangement 
  is 
  far 
  more 
  regular. 
  

  

  The 
  evidence 
  for 
  the 
  validity 
  of 
  this 
  new 
  genus 
  seems 
  quite 
  

   sufficient, 
  and 
  Prof. 
  Duncan 
  proposes 
  to 
  accept 
  it 
  in 
  his 
  forth- 
  

   coming 
  'Revision 
  of 
  the 
  Genera 
  of 
  Echinoidea.' 
  The 
  only 
  

   discovery 
  that 
  could 
  shake 
  it 
  would 
  be 
  that 
  the 
  apical 
  disk 
  of 
  

   Echinocyphus 
  is 
  normally 
  different 
  from 
  that 
  of 
  the 
  specimen 
  

   of 
  E. 
  mespilia 
  in 
  the 
  British 
  Museum 
  and 
  the 
  figure 
  of 
  E. 
  

   difficilis 
  given 
  by 
  Wright. 
  This 
  is 
  the 
  only 
  evidence 
  avail- 
  

   able 
  at 
  present, 
  as 
  M. 
  Cotteau 
  informs 
  me 
  that 
  the 
  apical 
  

   disk 
  has 
  not 
  yet 
  been 
  discovered 
  in 
  any 
  French 
  specimen, 
  

   and, 
  except 
  for 
  a 
  doubt 
  as 
  to 
  the 
  specific 
  determination 
  of 
  

   Wright's 
  figure, 
  there 
  seems 
  no 
  reason 
  to 
  distrust 
  it. 
  The 
  

   evidence 
  in 
  the 
  case 
  of 
  Zeuglopleurus 
  is 
  much 
  stronger 
  ; 
  

   the 
  specimens 
  in 
  the 
  British 
  Museum 
  collection 
  are 
  admirably 
  

   preserved 
  and 
  come 
  from 
  different 
  localities 
  and 
  horizons, 
  and 
  

   are 
  in 
  different 
  stages 
  of 
  growth, 
  from 
  some 
  as 
  small 
  as 
  any 
  of 
  

   Glyphocyphus 
  radiatus 
  to 
  the 
  type, 
  which 
  is 
  larger 
  than 
  any 
  

   of 
  that 
  species, 
  and 
  they 
  agree 
  absolutely 
  in 
  the 
  important 
  

   points 
  of 
  structure. 
  

  

  * 
  JEvechinus 
  cMoriticus 
  (Val.), 
  Al. 
  Agassiz, 
  " 
  Revision 
  of 
  the 
  Echini," 
  

   Mem. 
  Mus. 
  Comp. 
  Zool. 
  Cambridge, 
  Mass., 
  1873, 
  pp. 
  502, 
  503, 
  pi. 
  iv. 
  b. 
  

   tig. 
  7. 
  E. 
  rarituberculatus, 
  Bell, 
  ''Description 
  of 
  a 
  new 
  Species 
  of 
  Eve- 
  

   chinus," 
  Ann. 
  & 
  Mag. 
  Nat. 
  Hist. 
  ser. 
  5, 
  vol. 
  xx. 
  1887, 
  pp. 
  403-405, 
  

   pi. 
  xvii. 
  figs. 
  7 
  and 
  8. 
  

  

  