﻿THE 
  ECHIXOTHTJRIDiE 
  ASTD 
  THE 
  PEEISCHOECHIjSTID^:. 
  313 
  

  

  any 
  of 
  the 
  interambulacral 
  plates 
  on 
  the 
  underside 
  of 
  the 
  test, 
  except 
  

   on 
  the 
  marginal 
  plates 
  bordering 
  the 
  ambulacra. 
  If 
  we 
  accept 
  this 
  

   statement, 
  founded 
  on 
  imperfect 
  specimens, 
  as 
  given 
  by 
  the 
  latter 
  

   authors, 
  it 
  must 
  be 
  borne 
  in 
  mind 
  that 
  Perischodomus 
  has 
  pri- 
  

   mary 
  tubercles 
  only 
  on 
  the 
  marginal 
  interambulacral 
  plates 
  ; 
  so, 
  

   presuming 
  for 
  one 
  moment 
  that 
  that 
  genus 
  is 
  ultimately 
  proved 
  to 
  

   possess 
  imbricating 
  plates, 
  the 
  two 
  genera, 
  Lepidecliinus 
  and 
  Peris- 
  

   chodomus, 
  may 
  perhaps 
  be 
  considered 
  identical. 
  

  

  Lepidechinus 
  has 
  hitherto 
  been 
  found 
  only 
  in 
  America, 
  in 
  the 
  

   Chemung 
  (Devonian) 
  and 
  Burlington-Limestone 
  (sub-Carboniferous) 
  

   groups. 
  

  

  Genus 
  Eocidaeis, 
  Desor, 
  1858*. 
  

  

  Ambulacra 
  composed 
  of 
  a 
  double 
  series 
  of 
  plates, 
  perforated 
  near 
  

   their 
  outer 
  extremities 
  by 
  two 
  small 
  pores 
  ; 
  interambulacra 
  of 
  five 
  

   or 
  more 
  rows 
  of 
  plates, 
  those 
  bordering 
  the 
  ambulacra, 
  as 
  in 
  the 
  other 
  

   genera, 
  pentagonal, 
  the 
  remainder 
  hexagonal, 
  some 
  of 
  the 
  rows 
  of 
  

   which 
  become 
  obsolete 
  before 
  reaching 
  the 
  poles 
  ; 
  each 
  interambu- 
  

   lacral 
  plate 
  carries 
  a 
  primary 
  tubercle, 
  but 
  without 
  the 
  usual 
  miliary 
  

   ring 
  surrounding 
  it. 
  

  

  The 
  above 
  characters 
  are 
  taken 
  from 
  Prof. 
  Hall's 
  emended 
  de- 
  

   scription 
  f 
  ; 
  and 
  from 
  them 
  it 
  appears 
  tolerably 
  certain 
  that 
  this 
  genus 
  

   did 
  not 
  possess 
  imbricating 
  plates, 
  but 
  is 
  distinguished 
  from 
  the 
  

   other 
  palaeozoic 
  genera 
  by 
  the 
  absence 
  of 
  the 
  ring 
  surrounding 
  the 
  

   primary 
  tubercles, 
  and 
  the 
  decrease 
  in 
  the 
  number 
  of 
  the 
  rows 
  of 
  

   interambulacral 
  plates 
  towards 
  the 
  poles. 
  

  

  Eocidaris 
  occurs 
  in 
  the 
  Devonian, 
  Carboniferous, 
  and 
  Permian 
  

   formations. 
  

  

  Genus 
  Melonites, 
  D. 
  D. 
  Owen, 
  1846 
  +. 
  PI. 
  XX1Y. 
  figs. 
  5 
  & 
  6. 
  

  

  Body 
  ovoid, 
  spherical 
  ; 
  interambiilacral 
  plates 
  hexagonal 
  ; 
  ambu- 
  

   laoral 
  plates 
  of 
  two 
  kinds 
  — 
  one 
  elongated 
  hexagonal 
  series 
  composed 
  

   of 
  two 
  rows 
  in 
  the 
  centre 
  of 
  the 
  ambulacral 
  areae, 
  elevated 
  into 
  a 
  

   prominent 
  ridge, 
  giving 
  rise 
  to 
  a 
  furrow 
  on 
  each 
  side 
  at 
  the 
  junction 
  

   of 
  each 
  row 
  with 
  the 
  second 
  kind 
  of 
  plates, 
  which 
  are 
  smaller, 
  poly- 
  

   gonal, 
  ranged 
  in 
  four 
  vertical 
  rows 
  on 
  each 
  side 
  the 
  larger 
  central 
  

   rows, 
  thus 
  giving 
  to 
  each 
  ambulacrum 
  no 
  less 
  than 
  ten 
  rows 
  of 
  

   plates 
  ; 
  each 
  ambulacral 
  plate 
  perforated 
  by 
  a 
  pair 
  of 
  pores, 
  which, 
  

   in 
  the 
  smaller 
  polygonal 
  plates, 
  are 
  said 
  by 
  Owen 
  to 
  be 
  central, 
  

   whereas, 
  in 
  the 
  larger 
  hexagonal 
  plates, 
  they 
  are 
  placed 
  on 
  one 
  side, 
  

   that 
  furthest 
  from 
  the 
  central 
  ridge. 
  

  

  In 
  Melonites 
  we 
  find 
  a 
  still 
  further 
  departure 
  from 
  the 
  regular 
  

   Echinid 
  type, 
  in 
  the 
  great 
  development 
  of 
  the 
  rows 
  of 
  plates 
  in 
  the 
  

   ambulacral 
  arese, 
  five 
  times 
  as 
  many 
  as 
  those 
  seen 
  in 
  Calveria, 
  Palce- 
  

   chinus, 
  or 
  Archceocidaris. 
  The 
  apical 
  disk 
  in 
  this 
  genus 
  presents 
  

   some 
  peculiarities 
  worthy 
  of 
  notice. 
  Both 
  ocular 
  and 
  genital 
  plates 
  

   are 
  present, 
  as 
  usual 
  ; 
  but 
  in 
  some 
  cases 
  the 
  former 
  are 
  without 
  any 
  

  

  * 
  Synopsis 
  des 
  Echinides, 
  p. 
  155. 
  

  

  t 
  20th 
  Eeport 
  State 
  Cab. 
  New 
  York, 
  1867, 
  p. 
  297. 
  

  

  I 
  American 
  Journal, 
  1846, 
  2nd 
  ser. 
  vol. 
  ii. 
  pp. 
  225 
  et 
  seq. 
  

  

  