﻿114 
  DR. 
  J. 
  W. 
  GEEG0EY 
  ON" 
  THE 
  [Feb. 
  1897, 
  

  

  Duncan, 
  who 
  carefully 
  considered 
  the 
  Sarasins' 
  arguments 
  during 
  

   the 
  preparation 
  of 
  his 
  ' 
  Revision 
  of 
  the 
  Genera 
  and 
  Great 
  Groups 
  of 
  

   the 
  Echinoidea,' 
  did 
  not 
  accept 
  their 
  conclusions. 
  He 
  pointed 
  out 
  

   the 
  differences 
  between 
  the 
  Palaeozoic 
  echinids 
  and 
  the 
  Echino- 
  

   thuridae, 
  and 
  concluded 
  that 
  ' 
  it 
  appears 
  more 
  reasonable 
  to 
  place 
  

   the 
  Echinothuridae 
  near 
  the 
  Diadematidae, 
  granting 
  some 
  atavism, 
  

   than 
  to 
  station 
  them 
  at 
  the 
  end 
  of 
  the 
  Palaechinoidea.' 
  l 
  Like 
  Bell, 
  

   however, 
  he 
  separated 
  them 
  from 
  the 
  rest 
  of 
  his 
  order, 
  the 
  Diadema- 
  

   toida 
  (which 
  included 
  all 
  Neozoic 
  regular 
  echinids 
  except 
  the 
  

   Cidaridae), 
  as 
  a 
  special 
  suborder, 
  the 
  Streptosomata. 
  

  

  Hence 
  there 
  have 
  been, 
  and 
  still 
  are, 
  two 
  opposite 
  theories 
  as 
  to 
  

   the 
  affinities 
  of 
  the 
  Echinothuridae. 
  According 
  to 
  one 
  school, 
  the 
  

   characters 
  of 
  this 
  family 
  are 
  primitive 
  and 
  ancestral 
  ; 
  according 
  to 
  

   the 
  other, 
  they 
  are 
  degenerate 
  and 
  highly 
  specialized. 
  As 
  it 
  is 
  

   impossible 
  to 
  progress 
  with 
  the 
  phylogenetic 
  classification 
  of 
  the 
  

   echinids 
  until 
  it 
  be 
  known 
  whether 
  the 
  Echinothuridae 
  are 
  a 
  root 
  

   or 
  whether 
  they 
  are 
  a 
  branch 
  near 
  the 
  summit, 
  I 
  may 
  be 
  excused 
  

   for 
  again 
  calling 
  attention 
  to 
  this 
  question. 
  

  

  As 
  we 
  have 
  already 
  seen, 
  it 
  has 
  been 
  remarked 
  by 
  A. 
  Agassiz, 
  

   Bell, 
  and 
  Duncan 
  — 
  to 
  whom 
  Neumayr 
  2 
  may 
  be 
  added 
  — 
  that, 
  in 
  

   spite 
  of 
  the 
  resemblances 
  between 
  the 
  Echinothuridae 
  and 
  the 
  

   flexible 
  Palaeozoic 
  echinids, 
  this 
  family 
  is 
  most 
  closely 
  allied 
  to 
  the 
  

   Diadematidae. 
  It 
  was 
  natural 
  first 
  to 
  compare 
  Asthenosoma 
  with 
  

   Astropyga. 
  But, 
  as 
  Wyville 
  Thomson 
  remarked 
  in 
  1874, 
  although 
  

   8 
  some 
  characters 
  would 
  seem 
  to 
  indicate 
  a 
  tendency 
  to 
  a 
  passage 
  

   from 
  the 
  Diadematidae 
  to 
  the 
  Echinothuridae, 
  through 
  such 
  forms 
  as 
  

   Astropyga, 
  the 
  resemblances 
  are 
  for 
  the 
  most 
  part 
  superficial, 
  and 
  

   very 
  important 
  anatomical 
  characters 
  maintain, 
  according 
  to 
  our 
  

   present 
  knowledge, 
  a 
  broad 
  line 
  of 
  distinction 
  between 
  the 
  

   families.' 
  3 
  Astropyga 
  is 
  now 
  recognized 
  as 
  a 
  thin-tested 
  member 
  

   of 
  the 
  family 
  Pedinidae, 
  with 
  no 
  special 
  resemblances 
  to 
  any 
  of 
  the 
  

   Echinothuridae. 
  

  

  In 
  most 
  previous 
  attempts 
  to 
  determine 
  the 
  origin 
  of 
  this 
  family, 
  

   the 
  living 
  echinothurids 
  have 
  been 
  taken 
  as 
  the 
  starting-point. 
  

   The 
  ancestry 
  of 
  the 
  family 
  is 
  inferred 
  from 
  the 
  characters 
  of 
  the 
  

   latest 
  and 
  most 
  specialized, 
  instead 
  of 
  from 
  those 
  of 
  the 
  earliest 
  and 
  

   most 
  primitive 
  members. 
  The 
  best 
  clue 
  as 
  to 
  the 
  origin 
  of 
  the 
  

   Echinothuridae 
  is 
  obtained 
  from 
  Pelanechinus, 
  a 
  Corallian 
  genus 
  

   founded 
  by 
  "W. 
  Keeping, 
  4 
  and 
  ably 
  described 
  by 
  T. 
  T. 
  Groom. 
  5 
  His 
  

   account 
  leaves 
  no 
  doubt 
  that 
  Pelanechinus 
  is 
  an 
  echinothurid, 
  of 
  

   which 
  family 
  it 
  is 
  the 
  oldest 
  known 
  form. 
  

  

  1 
  ' 
  A 
  Kevision 
  of 
  the 
  Genera 
  and 
  Great 
  Groups 
  of 
  the 
  Echinoidea,' 
  Journ. 
  

   Linn. 
  Soc, 
  Zool. 
  vol. 
  xxiii. 
  (1891) 
  pp. 
  39-40. 
  

  

  2 
  'Die 
  Stamme 
  des 
  Thierreiches,' 
  vol. 
  i. 
  (1889) 
  p. 
  377. 
  

  

  3 
  Phil. 
  Trans. 
  Eoy. 
  Soc. 
  vol. 
  ckiv. 
  (1874) 
  p. 
  732. 
  

  

  * 
  ' 
  On 
  Pelanechinus, 
  a 
  New 
  Genus 
  of 
  Sea-Urchins 
  from 
  the 
  Coral 
  Bag,' 
  Quart. 
  

   Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxiv. 
  (1878) 
  pp. 
  924-930. 
  

  

  6 
  ' 
  On 
  some 
  New 
  Features 
  in 
  Pelanechinus 
  corallinus, 
  > 
  ibid. 
  vol. 
  xliii. 
  (1887) 
  

   pp. 
  703-714 
  & 
  pi. 
  xxviii. 
  

  

  