﻿312 
  E. 
  ETHERIDGE 
  ON 
  THE 
  KELATIONSH1P 
  BETWEEN 
  

  

  racters, 
  viz. 
  the 
  triple 
  perforation 
  of 
  the 
  genital 
  plates 
  and 
  the 
  double 
  

   perforation 
  of 
  the 
  oculars. 
  It 
  has 
  usually 
  been 
  considered 
  that 
  

   Palcechinus 
  possessed 
  spines 
  only 
  of 
  one 
  kind, 
  small 
  ones, 
  analogous 
  

   to 
  the 
  secondary 
  spines 
  of 
  Archceocidaris 
  ; 
  butBaily 
  has 
  shown* 
  that 
  

   both 
  primary 
  and 
  secondary 
  spines 
  were 
  present, 
  the 
  former 
  with 
  

   tubercles 
  perforated 
  for 
  the 
  passage 
  of 
  the 
  ligamentum 
  teres 
  of 
  the 
  

   spine. 
  

  

  Palcechinus 
  is 
  chiefly 
  confined 
  to 
  Carboniferous 
  rocks. 
  It 
  is 
  said 
  

   to 
  occur 
  also 
  in 
  the 
  Silurian. 
  

  

  Genus 
  Pebjschodomt/s, 
  M'Coy, 
  1849 
  f. 
  

  

  Nearly 
  related 
  to 
  Archceocidaris, 
  from 
  which 
  it 
  is 
  principally 
  di- 
  

   stinguished 
  by 
  the 
  presence 
  of 
  primary 
  tubercles 
  only 
  on 
  the 
  mar- 
  

   ginal 
  interambulacral 
  plates 
  bordering 
  the 
  ambulacra. 
  Each 
  ambu- 
  

   lacrum 
  is 
  composed 
  of 
  two 
  rows 
  of 
  small 
  plates, 
  each 
  pierced 
  by 
  a 
  

   pair 
  of 
  small 
  pores 
  ; 
  the 
  interambulacra 
  are 
  wide, 
  and 
  composed 
  of 
  

   five 
  rows 
  of 
  plates 
  of 
  irregular 
  form 
  and 
  shape. 
  

  

  Very 
  little 
  is 
  known 
  regarding 
  this 
  genus. 
  It 
  was 
  described 
  

   originally 
  from 
  two 
  very 
  imperfect 
  specimens, 
  consisting 
  only 
  of 
  

   portions 
  of 
  the 
  test 
  ; 
  and 
  I 
  am 
  not 
  aware 
  that 
  any 
  other 
  specimens 
  

   have 
  since 
  been 
  described. 
  With 
  the 
  small 
  amount 
  of 
  evidence 
  at 
  

   our 
  command 
  it 
  is 
  impossible 
  to 
  form 
  an 
  opinion 
  regarding 
  the 
  con- 
  

   struction 
  of 
  the 
  test 
  ; 
  all 
  that 
  can 
  be 
  said 
  at 
  present 
  is, 
  that 
  it 
  is 
  

   closely 
  allied 
  to 
  Archceocidaris, 
  although 
  the 
  interambulacral 
  plates 
  

   appear 
  to 
  be 
  more 
  irregular 
  in 
  form 
  and 
  size. 
  

  

  Perischodomus, 
  so 
  far 
  as 
  is 
  known, 
  is 
  confined 
  to 
  Carboniferous 
  

   rocks. 
  

  

  Genus 
  Lepidechinus, 
  Hall, 
  1861 
  J. 
  

  

  " 
  The 
  form 
  and 
  arrangement 
  of 
  the 
  ambulacral 
  and 
  interambula- 
  

   cral 
  series, 
  as 
  in 
  Palcechinus, 
  with 
  the 
  plates 
  of 
  the 
  interambulacral 
  

   series 
  imbricating 
  from 
  the 
  dorsal 
  side, 
  and 
  the 
  lower 
  edges 
  of 
  each 
  

   range 
  overlapping 
  those 
  below, 
  while 
  the 
  plates 
  of 
  the 
  ambulacral 
  

   areas 
  are 
  imbricating 
  in 
  the 
  opposite 
  direction, 
  narrow 
  and 
  deeply 
  in- 
  

   terlocking 
  at 
  their 
  joining 
  edges, 
  each 
  plate 
  pierced 
  near 
  the 
  opposite 
  

   extremity 
  by 
  two 
  pores 
  " 
  §. 
  

  

  Of 
  all 
  the 
  palaeozoic 
  genera 
  of 
  Echinoidea 
  the 
  present 
  one 
  shows 
  

   in 
  the 
  most 
  complete 
  manner 
  the 
  imbricating 
  character 
  of 
  the 
  Echino- 
  

   thuridce. 
  According 
  to 
  Prof. 
  Hall, 
  the 
  interambulacral 
  arese 
  are 
  

   many 
  times 
  as 
  wide 
  as 
  the 
  ambulacral, 
  and 
  contain 
  a 
  large 
  number 
  

   of 
  plates 
  of 
  somewhat 
  irregular 
  form, 
  judging 
  from 
  the 
  figures 
  j| 
  . 
  The 
  

   imbricating 
  nature 
  of 
  the 
  plates 
  of 
  the 
  test 
  closely 
  connects 
  this 
  genus 
  

   with 
  Archceocidaris, 
  from 
  which 
  it 
  may 
  be 
  distinguished 
  by 
  the 
  form 
  

   of 
  the 
  ambulacral 
  and 
  other 
  plates, 
  and 
  probably, 
  according 
  to 
  Meek 
  

   and 
  "Worthen^f, 
  by 
  the 
  fact 
  of 
  there 
  being 
  no 
  primary 
  tubercles 
  on 
  

  

  *'- 
  Baily, 
  he. 
  cit. 
  f 
  Annals 
  & 
  Mag. 
  Nat. 
  Hist. 
  1849, 
  2nd 
  ser. 
  vol. 
  iii. 
  p. 
  253. 
  

   ' 
  | 
  Deser. 
  New 
  Sp. 
  Crinoidea, 
  Prelim. 
  Notice, 
  Albany, 
  1861, 
  p. 
  18; 
  also, 
  20th 
  

   Rep. 
  State 
  Cabinet 
  N. 
  Y. 
  1867, 
  p. 
  295. 
  

  

  § 
  See 
  Note, 
  p. 
  310. 
  || 
  Ibid. 
  pi. 
  9. 
  fig. 
  10. 
  If 
  Pal. 
  Illinois, 
  vol. 
  ii. 
  p. 
  295. 
  

  

  