﻿Anatomy 
  of 
  Palaeechinus 
  (Scolder), 
  M 
  l 
  Goy. 
  203 
  

  

  The 
  thickness 
  of 
  the 
  ambulacral 
  plates 
  of 
  P. 
  gigas 
  and 
  P. 
  

   sphcericus 
  is 
  remarkable, 
  and 
  is 
  much 
  greater 
  than 
  their 
  ver- 
  

   tical 
  height 
  ; 
  there 
  is 
  no 
  overlap 
  of 
  the 
  edges 
  and 
  the 
  test 
  was 
  

   rigid. 
  

  

  The 
  interradial 
  plates 
  of 
  the 
  species 
  are 
  very 
  thick 
  and 
  the 
  

   edges 
  present 
  no 
  evidence 
  of 
  flexibility. 
  The 
  ambulacral 
  

   rows 
  of 
  plates 
  have 
  their 
  ambulacral 
  edges 
  as 
  zigzags 
  and 
  the 
  

   angles 
  fit 
  into 
  the 
  corresponding 
  reentering 
  angles 
  of 
  the 
  

   ambulacral 
  plates. 
  The 
  resemblance 
  to 
  many 
  Mesozoic 
  and 
  

   recent 
  species 
  of 
  regular 
  Echinoidea 
  is 
  exact 
  in 
  this 
  respect. 
  

  

  Ambulacra 
  o/Paheechinus 
  intermedius,TF. 
  Keeping 
  (Quart. 
  

   Journ. 
  Geol. 
  Soc. 
  vol 
  xxxii. 
  p. 
  37, 
  pi. 
  iii. 
  figs. 
  9-11). 
  — 
  

   There 
  are 
  specimens 
  of 
  this 
  species 
  in 
  the 
  Wood 
  ward 
  ian 
  

   Museum 
  and 
  also 
  in 
  the 
  Museum 
  of 
  Practical 
  Geology. 
  The 
  

   late 
  Mr. 
  W. 
  Keeping 
  doubted 
  whether 
  the 
  species 
  could 
  be 
  

   placed 
  with 
  Palceechinus 
  ; 
  but 
  he 
  was 
  misled, 
  apparently, 
  by 
  

   the 
  diagnosis 
  of 
  M'Coy, 
  especially 
  of 
  the 
  part 
  relating 
  to 
  

   the 
  construction 
  of 
  the 
  poriferous 
  zones. 
  The 
  ambulacra 
  of 
  

   P. 
  intermedins 
  have 
  biserial 
  pairs 
  of 
  pores 
  and 
  the 
  plates 
  are 
  

   very 
  numerous, 
  low, 
  broad, 
  and 
  thick 
  (figs. 
  IX. 
  and 
  x.). 
  The 
  

   plates 
  are 
  all 
  primaries 
  and 
  are 
  in 
  regular 
  vertical 
  succession, 
  so 
  

   that 
  there 
  is 
  a 
  single 
  vertical 
  row 
  of 
  them 
  on 
  each 
  side 
  of 
  the 
  

   median 
  ambulacral 
  suture. 
  But 
  the 
  plates 
  are 
  different 
  in 
  shape 
  

   in 
  the 
  same 
  vertical 
  row, 
  although 
  not 
  to 
  the 
  amount 
  shown 
  in 
  

   P. 
  gigas. 
  The 
  plates 
  with 
  the 
  outer 
  pairs 
  of 
  pores 
  are 
  largest 
  

   at 
  their 
  outer 
  part, 
  which 
  forms 
  the 
  salient 
  angle 
  of 
  the 
  ambu- 
  

   lacral 
  edge 
  of 
  the 
  ambulacro-interradial 
  suture, 
  and 
  they 
  reach 
  

   the 
  median 
  ambulacral 
  line, 
  being 
  lower 
  there 
  than 
  near 
  the 
  

   pait 
  which 
  bears 
  the 
  pair 
  of 
  pores. 
  The 
  plates 
  which 
  bear 
  

   the 
  inner 
  series 
  of 
  pairs 
  of 
  pores 
  are 
  narrow 
  externally 
  and 
  

   may 
  or 
  may 
  not 
  reach 
  the 
  interradial 
  suture, 
  and 
  if 
  they 
  do 
  

   so 
  it 
  is 
  by 
  a 
  pointed 
  angle. 
  They 
  are 
  low, 
  broad, 
  and 
  are 
  

   often 
  higher 
  at 
  the 
  ambulacral 
  median 
  line 
  than 
  the 
  other 
  

   primaries 
  with 
  the 
  outer 
  series 
  of 
  pairs 
  of 
  pores. 
  Fig. 
  IX. 
  

   from 
  a 
  specimen 
  in 
  the 
  Museum 
  of 
  Practical 
  Geology 
  and 
  

   fig. 
  x. 
  from 
  the 
  Woodwardian 
  specimen. 
  

  

  Ambulacra 
  of 
  Palaeechinus 
  Phillipsiae, 
  Forbes. 
  — 
  Palce- 
  

   echinus 
  Phillipsice, 
  Forbes, 
  is 
  to 
  be 
  seen 
  in 
  the 
  form 
  of 
  models 
  

   which 
  have 
  been 
  made 
  by 
  running 
  in 
  a 
  melted 
  substance 
  

   into 
  the 
  hollow 
  fossil. 
  This 
  substance, 
  when 
  solid, 
  shows 
  

   the 
  inside 
  of 
  part 
  of 
  the 
  test, 
  and 
  therefore 
  the 
  ambulacral 
  

   and 
  other 
  plate3 
  are 
  represented 
  on 
  the 
  model 
  so 
  far 
  as 
  

   their 
  inner 
  surfaces 
  are 
  concerned. 
  

  

  