﻿196 
  Prof. 
  P. 
  M. 
  Duncan 
  on 
  the 
  

  

  XX. 
  — 
  On 
  some 
  Points 
  in 
  the 
  Anatomy 
  of 
  the 
  Species 
  of 
  

   Palajechinus 
  {Scolder), 
  M'Coij, 
  and 
  a 
  proposed 
  Classifi- 
  

   cation. 
  By 
  Prof. 
  P. 
  Martin 
  Duncan, 
  F.K.S., 
  F.L.S., 
  &c. 
  

  

  Contents. 
  

  

  The 
  Dorso-central 
  System 
  and 
  its 
  Variations. 
  

  

  The 
  Anatomy 
  of 
  the 
  Ambulacral 
  Plates. 
  General 
  Observations. 
  

   The 
  Nature 
  of 
  the 
  Interradial-ambulaeral 
  Edges. 
  The 
  Descrip- 
  

   tion 
  of 
  the 
  Ambulacra 
  of 
  Palceechinus 
  r/igas, 
  P. 
  ellipticus, 
  P. 
  sphce- 
  

   ricus, 
  P. 
  intermedins, 
  and 
  P. 
  Phillipsice. 
  

  

  The 
  Grouping 
  of 
  the 
  five 
  Species. 
  

  

  The 
  Characters 
  of 
  P. 
  elegans, 
  P. 
  quadriserialis, 
  and 
  a 
  Specimen 
  in 
  the 
  

   Woodwardian 
  Museum. 
  

  

  The 
  Relations 
  of 
  Palceechinus 
  and 
  Phoeehimts, 
  W. 
  Keeping. 
  

  

  The 
  Synonymy. 
  

  

  There 
  are 
  some 
  very 
  well-preserved 
  specimens 
  of 
  several 
  

   species 
  of 
  the 
  genus 
  Palwechinus 
  (Scouler), 
  M'Coy, 
  in 
  the 
  

   British 
  Museum, 
  in 
  the 
  Woodwardian 
  Museum, 
  Cambridge, 
  

   and 
  in 
  the 
  Museum 
  of 
  Practical 
  Geology, 
  Jermyn 
  Street, 
  

   and 
  their 
  study 
  yields 
  some 
  very 
  definite 
  information 
  upon 
  

   some 
  important 
  points 
  in 
  their 
  anatomy. 
  Some 
  of 
  the 
  speci- 
  

   mens 
  show 
  the 
  dorso-central 
  system 
  nearly 
  perfectly, 
  and 
  

   others 
  exhibit 
  the 
  ambulacral 
  structures, 
  so 
  that 
  the 
  plates 
  

   can 
  be 
  drawn 
  and 
  studied 
  ; 
  portions 
  of 
  the 
  jaws 
  and 
  teetli 
  

   remain 
  and, 
  together 
  with 
  the 
  interradial 
  plates, 
  are 
  very 
  

   interestingly 
  preserved. 
  

  

  The 
  Apical 
  or 
  Dorso-central 
  System 
  and 
  its 
  Variations. 
  

  

  It 
  is 
  evident 
  that 
  there 
  is 
  considerable 
  variability 
  in 
  the 
  

   construction 
  of 
  the 
  apical 
  system 
  in 
  species 
  of 
  Palceechinus. 
  

   The 
  largest 
  specimen 
  of 
  Palceechinus 
  spha?ricus 
  at 
  the 
  British 
  

   Museum 
  has 
  the 
  apical 
  system 
  with 
  five 
  large, 
  tumid, 
  basal 
  

   plates, 
  which 
  are 
  separate, 
  and 
  with 
  five 
  radial 
  plates, 
  which 
  

   are 
  placed 
  between 
  the 
  basal 
  plates 
  and 
  form 
  with 
  them 
  a 
  

   ring 
  around 
  the 
  periproct. 
  There 
  is 
  a 
  circle 
  of 
  small, 
  thick, 
  

   and 
  somewhat 
  irregularly 
  shaped 
  anal 
  plates 
  immediately 
  

   within 
  the 
  ring 
  of 
  the 
  periproct. 
  The 
  basal 
  and 
  radial 
  plates 
  

   are 
  large 
  and 
  tumid. 
  

  

  Another 
  specimen 
  of 
  the 
  same 
  species 
  has 
  the 
  apical 
  system 
  

   perfect, 
  except 
  in 
  the 
  anal 
  plates. 
  The 
  periproct 
  is 
  penta- 
  

   gonal 
  and 
  is 
  surrounded 
  by 
  a 
  circle 
  of 
  thick, 
  tumid, 
  basal 
  

   plates 
  ; 
  the 
  five 
  radial 
  plates 
  are 
  triangular 
  and 
  are 
  only 
  inter- 
  

   calated 
  between 
  the 
  basal 
  plates 
  on 
  the 
  outside 
  of 
  the 
  system, 
  

   and 
  they 
  do 
  not 
  form 
  a 
  part 
  of 
  the 
  ring 
  or 
  margin 
  of 
  the 
  peri- 
  

  

  