﻿642 
  ME. 
  J. 
  W. 
  GKREG0EY 
  ON" 
  CYSTECHINUS 
  CEASSUS 
  

  

  Interradii. 
  The 
  plates 
  are 
  broader 
  than 
  those 
  of 
  the 
  ambulacra 
  ; 
  

   in 
  shape 
  they 
  are 
  hexagonal, 
  but 
  elongated. 
  Horizontal 
  sutures 
  

   straight 
  or 
  very 
  slightly 
  curved. 
  In 
  thickness, 
  number 
  in 
  vertical 
  

   series, 
  aud 
  distribution 
  of 
  epistroma 
  they 
  resemble 
  the 
  ambulacral 
  

   plates. 
  The 
  postero-lateral 
  plates 
  are 
  narrower 
  than 
  the 
  antero- 
  

   lateral 
  pair. 
  

  

  Apical 
  system 
  central, 
  small 
  ; 
  the 
  details 
  of 
  the 
  arrangement 
  as 
  } 
  7 
  et 
  

   unknown, 
  but 
  it 
  can 
  only 
  have 
  been 
  slightly 
  disjoint 
  or 
  produced. 
  

  

  The 
  actinal 
  surface 
  with 
  the 
  periproct 
  &c. 
  is 
  not 
  shown 
  in 
  the 
  

   specimen. 
  

  

  Dimensions 
  of 
  type 
  specimen. 
  

  

  mm. 
  

  

  Length 
  150 
  

  

  Width 
  130 
  

  

  Thickness 
  of 
  plates 
  1-5 
  

  

  Ratio 
  of 
  depth 
  to 
  breadth 
  of 
  ambulacral 
  plates 
  ; 
  about 
  .... 
  2:3 
  

   ,, 
  ,, 
  plates 
  of 
  interambulacrum 
  

  

  No. 
  1 
  ; 
  about 
  4 
  : 
  5 
  

   „ 
  „ 
  „ 
  No. 
  3 
  ; 
  about 
  3 
  : 
  5 
  

  

  Locality 
  and 
  Stratigraphical 
  Position. 
  The 
  specimen 
  (B.M. 
  No. 
  

   E 
  318) 
  was 
  obtained 
  by 
  Thos. 
  D. 
  Hill, 
  Esq., 
  in 
  a 
  drab 
  Radiolarian 
  

   marl 
  at 
  a 
  depth 
  of 
  166 
  feet 
  on 
  the 
  Haynesfield 
  Estate, 
  Barbados. 
  

   The 
  specimen 
  was 
  presented 
  to 
  the 
  British 
  Museum 
  (Nat. 
  Hist.) 
  

   by 
  G. 
  E. 
  Thomas, 
  Esq., 
  of 
  Haynesfield. 
  

  

  The 
  age 
  of 
  the 
  marl 
  is 
  probably 
  Pleistocene 
  or 
  late 
  Pliocene, 
  but 
  

   possibly 
  Miocene, 
  as 
  will 
  be 
  subsequently 
  discussed. 
  

  

  Affinities 
  of 
  Cystechinus 
  crassus. 
  — 
  The 
  specimen 
  is 
  unfortunately 
  

   in 
  a 
  very 
  imperfect 
  state 
  of 
  preservation, 
  as 
  the 
  actinal 
  side 
  is 
  not 
  

   shown, 
  and 
  the 
  structure 
  of 
  the 
  apical 
  system 
  can 
  only 
  be 
  inferred. 
  

   Even 
  on 
  the 
  abactinal 
  side 
  many 
  of 
  the 
  plates 
  have 
  been 
  lost, 
  though 
  

   they 
  have 
  left 
  clear 
  impressions 
  of 
  the 
  sutural 
  lines 
  and 
  casts 
  of 
  the 
  

   pores. 
  Nevertheless 
  none 
  of 
  the 
  ' 
  Challenger 
  ' 
  collections 
  of 
  Cyst- 
  

   echinus 
  in 
  the 
  Natural 
  History 
  Museum 
  are 
  more 
  perfect, 
  except 
  a 
  

   couple 
  of 
  very 
  young 
  specimens 
  of 
  0. 
  Wyvillii. 
  The 
  specimen 
  is 
  in 
  

   better 
  perservation 
  than 
  the 
  collection 
  of 
  scattered 
  plates 
  that 
  forms 
  

   the 
  type 
  of 
  C. 
  clypeatus 
  ; 
  a 
  paleontologist 
  need 
  not 
  therefore 
  be 
  afraid 
  

   to 
  deal 
  with 
  the 
  materials 
  when 
  the 
  zoologists 
  have 
  not 
  hesitated 
  to 
  

   use 
  worse. 
  

  

  The 
  specimen 
  was 
  originally 
  identified 
  as 
  Oalymne, 
  no 
  doubt 
  

   largely 
  owing 
  to 
  the 
  fact 
  that 
  Sir 
  Wyville 
  Thomson 
  had 
  placed 
  * 
  in 
  

   that 
  genus 
  the 
  fragments 
  of 
  a 
  form 
  found 
  at 
  Tristan 
  da 
  Cunha, 
  with 
  

   which 
  this 
  species 
  is 
  allied, 
  and 
  on 
  which 
  A. 
  Agassiz 
  has 
  subsequently 
  

   founded 
  the 
  species 
  Cystechinus 
  clypeatus. 
  Erom 
  Calymne, 
  how- 
  

   ever, 
  it 
  is 
  distinguished 
  by 
  the 
  structure 
  of 
  the 
  apical 
  system, 
  which 
  

   is 
  here 
  less 
  produced 
  and 
  disjoint, 
  has 
  fewer 
  intercalated 
  perisomatic 
  

   plates, 
  and 
  the 
  basals 
  more 
  uniform 
  ; 
  the 
  plates 
  of 
  the 
  ambulacra, 
  

   moreover, 
  are 
  less 
  uniform 
  and 
  regular 
  in 
  Calymne 
  than 
  in 
  Cystechi- 
  

  

  * 
  Sir 
  Wyville 
  Thomson, 
  " 
  The 
  Voyage 
  of 
  the 
  * 
  Challenger.' 
  The 
  Atlantic," 
  

   vol. 
  i. 
  (London, 
  1877), 
  p. 
  399. 
  

  

  