﻿814 
  K. 
  ETHEBIDGE 
  ON 
  THE 
  RELATIONSHIP 
  BETWEEN 
  

  

  trace 
  of 
  pores, 
  at 
  other 
  times 
  a 
  single 
  pore 
  may 
  be 
  present 
  in 
  two 
  of 
  

   them. 
  The 
  genital 
  plates 
  vary 
  even 
  more 
  than 
  the 
  oculars 
  ; 
  for 
  in 
  

   some 
  instances 
  three 
  of 
  the 
  plates 
  are 
  pierced 
  by 
  four 
  pores, 
  the 
  

   other 
  two 
  by 
  five 
  ; 
  again, 
  three 
  of 
  the 
  plates 
  may 
  carry 
  four 
  pores, 
  

   and. 
  the 
  other 
  two 
  plates 
  three 
  pores 
  only*, 
  showing 
  how 
  great 
  may 
  

   be 
  the 
  variation 
  in 
  this 
  respect 
  amongst 
  individuals 
  of 
  this 
  genus. 
  

  

  Genus 
  Oligoportts, 
  Meek 
  & 
  Worthen, 
  1860 
  f. 
  PI. 
  XXIV. 
  fig. 
  4. 
  

  

  In 
  the 
  size 
  and 
  arrangement 
  of 
  the 
  plates 
  composing 
  its 
  ambula- 
  

   cral 
  and 
  interambulacral 
  areae 
  Oligoporus 
  agrees 
  very 
  closely 
  with 
  

   Melonites, 
  but 
  differs 
  from 
  it 
  in 
  possessing 
  only 
  four 
  rows 
  of 
  plates 
  

   and 
  four 
  double 
  rows 
  of 
  pores 
  in 
  each 
  ambulacrum, 
  in 
  the 
  place 
  of 
  

   ten. 
  The 
  ambulacra 
  are 
  deeply 
  and 
  doubly 
  furrowed. 
  

  

  Oligoporus 
  appears 
  to 
  be 
  intermediate 
  between 
  Melonites 
  and 
  

   Palcechinus, 
  possessing 
  half 
  the 
  number 
  of 
  ambulacral 
  plates 
  and 
  

   pores 
  of 
  the 
  former, 
  and 
  double 
  the 
  number 
  of 
  the 
  latter. 
  It 
  also 
  

   agrees 
  with 
  the 
  former 
  in 
  the 
  deeply 
  furrowed 
  nature 
  of 
  the 
  ambu- 
  

   lacra 
  J. 
  

  

  Confined 
  to 
  Carboniferous 
  rocks. 
  

  

  In 
  the 
  foregoing 
  remarks 
  some 
  of 
  the 
  more 
  general 
  characters 
  of 
  

   the 
  various 
  genera 
  have 
  been 
  touched 
  upon. 
  I 
  have 
  now 
  to 
  notice 
  

   a 
  few 
  deductions 
  to 
  be 
  drawn 
  from 
  them. 
  

  

  1. 
  We 
  have 
  seen 
  that 
  the 
  group 
  Perischoechinidce 
  was 
  established 
  

   by 
  M'Coy 
  for 
  the 
  reception 
  of 
  those 
  palaeozoic 
  Echini 
  with 
  more 
  than 
  

   two 
  rows 
  of 
  plates 
  in 
  each 
  interambulacrum, 
  — 
  a 
  character 
  common 
  

   to 
  all 
  the 
  genera 
  previously 
  noticed, 
  and 
  thereby 
  distinguishing 
  

   them 
  from 
  Kecent 
  and 
  Secondary 
  forms. 
  

  

  2. 
  Certain 
  members 
  of 
  the 
  Perischoechinidce 
  possessed 
  overlapping 
  

   plates, 
  both 
  in 
  the 
  ambulacra 
  and 
  interambulacra, 
  in 
  one 
  at 
  least, 
  

   Lepiclechinus, 
  on 
  a 
  plan 
  similar 
  to 
  that 
  seen 
  in 
  the 
  recent 
  genus 
  

   Calveria. 
  

  

  3. 
  We 
  have 
  no 
  conclusive 
  evidence 
  of 
  the 
  presence 
  of 
  mem- 
  

   branous 
  interspaces 
  in 
  conjunction 
  with 
  the 
  overlapping 
  plates 
  as 
  

   in 
  the 
  Echinothuridce 
  ; 
  but 
  the 
  fragmentary 
  condition 
  in 
  which 
  the 
  

   remains 
  of 
  Archcsocidaris 
  are 
  usually 
  found 
  would 
  lead 
  us 
  to 
  the 
  con- 
  

   clusion 
  that 
  such 
  did 
  exist. 
  

  

  4. 
  There 
  does 
  not 
  appear 
  to 
  have 
  been 
  any 
  palaeozoic 
  genus, 
  so 
  

   far 
  as 
  our 
  present 
  knowledge 
  goes, 
  which 
  exhibits 
  the 
  peculiar 
  want 
  

   of 
  distinction 
  between 
  ambulacra 
  and 
  interambulacra 
  on 
  the 
  ventral 
  

   half 
  of 
  the 
  test 
  seen 
  in 
  the 
  genus 
  Phormosoma. 
  

  

  5. 
  In 
  two 
  closely 
  allied 
  genera, 
  Melonites 
  and 
  Oligoporus, 
  we 
  find 
  

   an 
  increase 
  in 
  the 
  number 
  of 
  rows 
  of 
  plates 
  in 
  each 
  of 
  the 
  ambu- 
  

   lacra, 
  similar 
  to 
  that 
  which 
  takes 
  place 
  in 
  the 
  interambulacra 
  

   throughout 
  the 
  whole 
  group. 
  

  

  6. 
  Lastly, 
  the 
  Perischoechinidce 
  differ 
  from 
  recent 
  and 
  later 
  Echini 
  

   in 
  an 
  increase 
  in 
  the 
  number 
  of 
  perforations 
  of 
  the 
  ocular 
  and 
  genital 
  

  

  * 
  Pal. 
  Illinois, 
  vol. 
  ii. 
  p. 
  228. 
  

  

  t 
  Proc. 
  Acad. 
  Nat. 
  Sciences 
  Philad. 
  1860, 
  p. 
  474. 
  

  

  X 
  Pal. 
  Illinois, 
  vol. 
  ii. 
  p. 
  249. 
  

  

  