﻿E. 
  ETHEEIDGE 
  ON 
  ECBTINOTHURID.E 
  AND 
  PEEISCHOECHINID^. 
  307 
  

  

  25. 
  On 
  the 
  Relationship 
  existing 
  between 
  the 
  Echinothtjeid^), 
  Wy 
  ville 
  

   Thomson, 
  and 
  the 
  Peeischoechinid^;, 
  M'Coy. 
  By 
  E. 
  Etheeidge, 
  

   Jim., 
  Esq., 
  F.G.S. 
  (Eead 
  March 
  11, 
  1874.) 
  

  

  [Plate 
  XXIV.] 
  

  

  A 
  considerable 
  degree 
  of 
  light 
  has 
  lately 
  been 
  thrown 
  on 
  the 
  rela- 
  

   tionship 
  existing 
  between 
  palaeozoic 
  and 
  recent 
  Echini, 
  from 
  mate- 
  

   rials 
  obtained 
  during 
  the 
  cruise 
  of 
  the 
  ' 
  Porcupine 
  ' 
  in 
  1869. 
  

  

  Prof. 
  Wy 
  ville 
  Thomson, 
  in 
  his 
  ' 
  Depths 
  of 
  the 
  Sea 
  '*, 
  described 
  

   two 
  new 
  and 
  peculiar 
  genera 
  of 
  Echinoidea, 
  Calveria 
  and 
  Phormo- 
  

   soma, 
  which, 
  in 
  certain 
  peculiarities 
  of 
  structure, 
  such 
  as 
  the 
  over- 
  

   lapping 
  of 
  the 
  constituent 
  plates 
  of 
  the 
  test, 
  appear 
  to 
  unite 
  the 
  

   Echini 
  of 
  the 
  present 
  seas 
  with 
  certain 
  of 
  those 
  genera 
  which 
  existed 
  

   during 
  palaeozoic 
  times. 
  

  

  Although 
  the 
  structure 
  referred 
  to 
  had 
  previously 
  been 
  noticed, 
  in 
  

   the 
  case 
  of 
  Cretaceous 
  Echini, 
  by 
  the 
  late 
  Dr. 
  S. 
  P. 
  "Woodward, 
  and 
  

   in 
  that 
  of 
  a 
  Palaeozoic 
  genus 
  by 
  Prof. 
  James 
  Hall, 
  of 
  Albany, 
  yet 
  

   it 
  was 
  not 
  until 
  Mr. 
  J. 
  Young, 
  of 
  Glasgow, 
  drew 
  attention 
  to 
  the 
  

   nature 
  of 
  the 
  plates 
  in 
  the 
  Carboniferous 
  genus 
  Archceocidaris 
  that 
  

   such 
  structure 
  had 
  been 
  observed 
  in 
  any 
  palaeozoic 
  genus 
  found 
  in 
  

   this 
  country 
  f. 
  

  

  Prof. 
  W. 
  Thomson 
  states 
  J 
  that 
  in 
  the 
  test 
  of 
  Calveria 
  (woodcuts, 
  

   figs. 
  1 
  & 
  2) 
  the 
  ambulacral 
  and 
  interambulacral 
  plates, 
  instead 
  of 
  

   abutting 
  directly 
  against 
  one 
  another, 
  and 
  thus 
  forming 
  a 
  rigid 
  and 
  

   compact 
  test, 
  overlap 
  each 
  other, 
  those 
  of 
  the 
  ambulacra 
  from 
  below 
  

   upwards 
  (that 
  is, 
  from 
  the 
  mouth 
  to 
  the 
  apical 
  pole), 
  those 
  of 
  the 
  

   interambulacra 
  from 
  above 
  downwards, 
  or 
  from 
  the 
  apical 
  to 
  the 
  

   oral 
  pole. 
  The 
  overlapping 
  of 
  both 
  series 
  of 
  plates 
  takes 
  place 
  at 
  

   their 
  inner 
  ends, 
  whilst 
  the 
  outer 
  ends 
  of 
  the 
  interambulacral 
  plates 
  

   are 
  separated, 
  the 
  one 
  from 
  the 
  other, 
  by 
  intervening 
  membrane, 
  thus 
  

   assisting 
  in 
  the 
  general 
  mobility 
  of 
  the 
  framework. 
  At 
  their 
  outer 
  

   extremities 
  the 
  interambulacral 
  plates 
  carry 
  each 
  a 
  primary 
  tuber- 
  

   cle, 
  and 
  also 
  overlap 
  the 
  outer 
  extremities 
  of 
  the 
  ambulacral 
  plates, 
  

   " 
  so 
  that 
  the 
  ambulacral 
  areae 
  are 
  essentially 
  within 
  the 
  interambu- 
  

   lacral" 
  §. 
  The 
  pores 
  of 
  the 
  ambulacral 
  plates 
  are 
  arranged 
  in 
  a 
  pe- 
  

   culiar 
  manner, 
  in 
  arcs 
  of 
  three, 
  specially 
  placed. 
  

  

  The 
  second 
  genus, 
  Phormosoma 
  (P. 
  placenta, 
  W. 
  T.), 
  resembles 
  the 
  

   preceding 
  in 
  the 
  arrangement 
  of 
  the 
  ambulacral 
  and 
  interambula- 
  

   cral 
  plates 
  and 
  flexibility 
  of 
  peristome 
  ; 
  but 
  the 
  overlapping 
  of 
  the 
  

   plates 
  is 
  not 
  carried 
  to 
  so 
  great 
  an 
  extent 
  as 
  in 
  Calveria, 
  and 
  no 
  

   membranous 
  spaces 
  are 
  left 
  ; 
  so 
  that 
  the 
  test 
  is 
  continuous, 
  as 
  in 
  the 
  

   normal 
  forms 
  of 
  the 
  Echinoidea. 
  Phormosoma 
  is 
  chiefly 
  remarkable 
  

   for 
  the 
  peculiar 
  characters 
  of 
  the 
  under 
  or 
  ventral 
  surface, 
  where 
  the 
  

  

  * 
  ' 
  The 
  Depths 
  of 
  the 
  Sea, 
  an 
  account 
  of 
  the 
  General 
  Results 
  of 
  the 
  Dredging- 
  

   cruise 
  of 
  H.M.SS. 
  "Porcupine" 
  and 
  "Lightning," 
  during 
  the 
  Summers 
  of 
  

   1868, 
  '69, 
  70.' 
  London. 
  8vo. 
  1873. 
  

  

  f 
  Geol. 
  Mag. 
  1873, 
  vol. 
  x. 
  p. 
  301. 
  J 
  Loc. 
  cit. 
  p. 
  158. 
  § 
  Loc. 
  cit. 
  

  

  