﻿644 
  ME. 
  J. 
  W. 
  GEEGOEY 
  ON" 
  CYSTECHIXTJS 
  CEASSTJS 
  

  

  nus 
  crassus. 
  In 
  general 
  shape, 
  so 
  far 
  as 
  the 
  fragments 
  of 
  Calymne 
  

   enable 
  any 
  comparison 
  to 
  be 
  instituted, 
  it 
  more 
  resembles 
  Cystechinus. 
  

   The 
  other 
  characters 
  of 
  Calymne, 
  viz. 
  the 
  subanal 
  fasciole 
  and 
  pro- 
  

   nounced 
  caudal 
  beak, 
  are 
  useless 
  for 
  comparison, 
  owing 
  to 
  our 
  ignor- 
  

   ance 
  of 
  the 
  structure 
  of 
  the 
  actinal 
  surface 
  of 
  the 
  fossil. 
  

  

  Cystecliinus 
  crassus 
  differs 
  from 
  the 
  other 
  three 
  species 
  of 
  the 
  

   genus 
  in 
  the 
  greater 
  thickness 
  of 
  the 
  plates 
  of 
  the 
  test. 
  In 
  most 
  

   cases 
  these 
  are 
  of 
  extreme 
  tenuity, 
  and 
  the 
  test 
  must 
  have 
  been 
  very 
  

   flexible. 
  The 
  thickness 
  of 
  those 
  plates 
  of 
  C. 
  crassus 
  that 
  can 
  be 
  

   measured 
  is 
  1-5 
  mm. 
  Owing 
  to 
  this 
  flexibility 
  of 
  the 
  tests, 
  shape 
  

   can 
  only 
  be 
  used 
  for 
  specific 
  distinctions 
  within 
  certain 
  narrow 
  

   limits. 
  In 
  other 
  respects 
  C. 
  crassus 
  differs 
  from 
  C. 
  vesica 
  in 
  that 
  

   in 
  the 
  latter 
  the 
  number 
  of 
  the 
  ambulacral 
  plates 
  is 
  greater 
  than 
  

   that 
  of 
  the 
  interambulacral, 
  and 
  there 
  are 
  differences 
  in 
  the 
  shape 
  of 
  

   the 
  plates. 
  From 
  C. 
  Wyvillii 
  the 
  new 
  species 
  differs 
  in 
  that 
  (]) 
  the 
  

   plates 
  lack 
  the 
  radiating 
  ridges 
  that 
  ornament 
  that 
  species 
  (see, 
  e.g., 
  

   Chall. 
  Rep. 
  vol. 
  iii. 
  pi. 
  xxix. 
  b, 
  fig. 
  9) 
  : 
  (2) 
  there 
  are 
  fewer 
  plates 
  

   between 
  the 
  apical 
  system 
  and 
  the 
  ambitus 
  ; 
  they 
  are 
  7 
  or 
  8 
  in 
  number 
  

   in 
  G. 
  crassus 
  and 
  10 
  or 
  11 
  in 
  C. 
  Wyvillii 
  : 
  (3) 
  the 
  plates 
  are 
  deeper 
  

   in 
  proportion 
  to 
  their 
  width 
  ; 
  thus 
  the 
  plates 
  of 
  the 
  posterior 
  ambu- 
  

   lacra 
  of 
  C. 
  crassus 
  are 
  almost 
  regular 
  hexagons, 
  whereas 
  those 
  of 
  

   the 
  largest 
  specimens 
  of 
  C. 
  Wyvillii 
  are 
  nearly 
  twice 
  as 
  wide 
  as 
  

   deep 
  : 
  (4) 
  the 
  horizontal 
  sutures 
  are 
  less 
  sinuous 
  in 
  this 
  species 
  : 
  (5) 
  

   the 
  ambulacral 
  plates 
  of 
  the 
  same 
  area 
  are 
  more 
  uniform 
  in 
  size 
  and 
  

   more 
  regular 
  in 
  shape 
  ; 
  thus 
  in 
  following 
  down 
  the 
  vertical 
  series 
  

   in 
  an 
  ambulacrum 
  of 
  C. 
  Wyvillii, 
  long, 
  somewhat 
  narrow 
  sinuous 
  

   plates 
  may 
  be 
  seen 
  between 
  hexagonal 
  and 
  pentagonal 
  plates. 
  The 
  

   new 
  species 
  is 
  probably 
  most 
  closely 
  allied 
  to 
  C. 
  clypeatus, 
  and 
  

   especially 
  to 
  the 
  variety 
  represented 
  by 
  those 
  specimens 
  found 
  be- 
  

   tween 
  Manilla 
  and 
  Hongkong 
  ; 
  these, 
  however, 
  are 
  so 
  fragmentary 
  

   that 
  no 
  precise 
  comparison 
  can 
  be 
  made 
  between 
  them 
  and 
  C. 
  crassus. 
  

   Prom 
  the 
  type 
  specimen 
  of 
  C. 
  dypeatus, 
  the 
  new 
  species 
  is 
  clearly 
  

   distinguished 
  (1) 
  by 
  the 
  smallness 
  of 
  the 
  apical 
  system, 
  (2) 
  the 
  

   greater 
  number 
  of 
  the 
  ambulacral 
  plates, 
  (3) 
  the 
  straighter 
  horizontal 
  

   margins 
  of 
  the 
  plates. 
  If 
  therefore 
  the 
  Philippine 
  fragments 
  belong- 
  

   to 
  the 
  same 
  species 
  as 
  the 
  fossil 
  from 
  Barbados 
  they 
  must 
  be 
  removed 
  

   from 
  C. 
  clypeatus. 
  It 
  is 
  quite 
  possible 
  that 
  the 
  closer 
  affinity 
  of 
  the 
  

   Manilla 
  and 
  Barbados 
  specimens 
  is 
  only 
  a 
  superficial 
  resemblance 
  due 
  

   to 
  the 
  fact 
  that 
  they 
  are 
  both 
  from 
  a 
  less 
  depth 
  than 
  the 
  others 
  ; 
  

   the 
  discovery 
  of 
  more 
  perfect 
  specimens 
  of 
  the 
  Philippine 
  species 
  

   might 
  ally 
  it 
  with 
  either 
  species, 
  or 
  prove 
  its 
  distinctness 
  from 
  both. 
  

  

  Resemblances 
  of 
  the 
  Plates 
  to 
  those 
  of 
  the 
  Palazechinoiclea. 
  — 
  When 
  

   comparing 
  this 
  species 
  with 
  its 
  allies, 
  one 
  cannot 
  but 
  be 
  struck 
  by 
  

   the 
  remarkable 
  resemblances 
  between 
  their 
  plates 
  and 
  those 
  of 
  the 
  

   Palseechinoidea, 
  which 
  are 
  especially 
  obvious 
  in 
  this 
  thick-plated 
  

   form. 
  Almost 
  the 
  whole 
  of 
  the 
  Echinoidea 
  intervene 
  between 
  the 
  

   Exocyclic 
  Spatangoidea 
  and 
  the 
  Endocyclic 
  PalEeechinoidea, 
  and 
  the 
  

   plates 
  of 
  the 
  only 
  Palaeozoic 
  Exocyclica 
  (Echinocystis 
  and 
  Pakeodiscus) 
  

   are 
  too 
  imperfectly 
  known 
  for 
  useful 
  comparison. 
  Nevertheless 
  the 
  

   similarity 
  in 
  the 
  structure 
  of 
  the 
  test 
  is 
  most 
  surprising. 
  In 
  both 
  

  

  