﻿Vol. 
  53.] 
  AFFINITIES 
  OF 
  THE 
  ECHIX0THURID2E. 
  117 
  

  

  ought 
  to 
  be 
  described 
  as 
  composed 
  of 
  a 
  central 
  primary 
  between 
  

   aboral 
  and 
  adoral 
  demi-plates 
  ; 
  and 
  of 
  the 
  pair 
  of 
  pore-pairs 
  below 
  

   each 
  primary, 
  one 
  belongs 
  to 
  the 
  same 
  triplet 
  as 
  the 
  plate 
  above, 
  

   .and 
  the 
  other 
  to 
  that 
  of 
  the 
  plate 
  below. 
  The 
  limits 
  of 
  the 
  triplets 
  

   in 
  Pelanechinus 
  are 
  shown 
  in 
  fig. 
  5. 
  

  

  Pelaneclrinus 
  and 
  the 
  living 
  echinothurids, 
  therefore, 
  begin 
  with 
  

   plates 
  of 
  identically 
  the 
  same 
  

  

  arrangement. 
  In 
  Pelanechinus 
  Fig. 
  5. 
  — 
  Ambulacral 
  plates 
  of 
  

   three 
  of 
  the 
  triplets 
  fuse 
  into 
  a 
  Phormosoma 
  luculentum 
  near 
  

   compound 
  plate 
  ; 
  while 
  in 
  Asthe- 
  the 
  peristome. 
  {After 
  A. 
  

   .nosoma 
  and 
  Phormosoma, 
  on 
  the 
  Agassiz.) 
  

   other 
  hand, 
  the 
  triplets 
  are 
  broken 
  

   up 
  — 
  owing 
  to 
  reduction 
  in 
  the 
  

   calcification 
  of 
  the 
  test. 
  

  

  It 
  appears, 
  therefore, 
  possible 
  

   to 
  explain 
  the 
  peculiar 
  arrange- 
  

   ment 
  of 
  the 
  ambulacral 
  plates 
  of 
  

   living 
  echinothurids 
  as 
  due 
  : 
  1st, 
  

   to 
  the 
  fusion 
  of 
  three 
  diademoid 
  

   triplets 
  ; 
  2nd, 
  to 
  their 
  dissociation 
  

   pari 
  passu 
  with 
  the 
  reduction 
  of 
  

  

  the 
  calcareous 
  matter 
  in 
  the 
  test, 
  as 
  the 
  plates 
  become 
  thinner 
  and 
  

   the 
  test 
  flexible. 
  If 
  this 
  view 
  be 
  correct, 
  then 
  the 
  flexibility 
  of 
  

   the 
  Echinothuridae 
  is 
  a 
  secondary 
  character, 
  and 
  was 
  not 
  due 
  to 
  

   inheritance 
  from 
  a 
  flexible, 
  Palaeozoic 
  ancestor. 
  

  

  The 
  structure 
  of 
  the 
  ambulacral 
  plates 
  seems 
  conclusively 
  to 
  

   prove 
  that 
  the 
  Echinothuridee 
  are 
  members 
  of 
  the 
  order 
  Diademoida. 
  

   The 
  question 
  arises 
  whether 
  the 
  members 
  of 
  this 
  family 
  have 
  

   descended 
  from 
  a 
  diademoid 
  ancestor, 
  or 
  whether 
  the 
  rest 
  of 
  that 
  

   order 
  have 
  descended 
  from 
  an 
  echinothurid. 
  

  

  That 
  the 
  former 
  alternative 
  is 
  the 
  true 
  one 
  is 
  rendered 
  probable 
  by 
  

   several 
  reasons 
  : 
  — 
  lstly, 
  the 
  Echinothuridae 
  are 
  younger 
  than 
  many 
  

   of 
  the 
  Diademoida, 
  for 
  this 
  order 
  begins 
  in 
  the 
  Lower 
  Lias, 
  while 
  

   the 
  family 
  does 
  not 
  appear 
  until 
  the 
  Corallian 
  ; 
  2ndly, 
  the 
  oldest 
  

   form 
  of 
  echinothurid 
  is 
  more 
  nearly 
  allied 
  to 
  the 
  nearest 
  diademid 
  

   than 
  are 
  the 
  later, 
  and 
  apparently 
  more 
  primitive, 
  members 
  of 
  

   the 
  family. 
  To 
  accept 
  the 
  other 
  alternative 
  would 
  make 
  diademids 
  

   found 
  in 
  the 
  Lower 
  Lias, 
  and 
  even 
  some 
  older 
  forms 
  occurring 
  in 
  the 
  

   Trias, 
  descendants 
  from 
  genera 
  not 
  known 
  to 
  occur 
  earlier 
  than 
  the 
  

   Corallian. 
  

  

  Let 
  us 
  therefore 
  next 
  enquire 
  whether 
  there 
  be 
  any 
  member 
  

   of 
  the 
  Diademoida, 
  of 
  Corallian 
  or 
  pre- 
  Corallian 
  age, 
  from 
  which 
  

   Pelanechinus 
  could 
  have 
  been 
  derived. 
  

  

  Neumayr 
  l 
  has 
  already 
  suggested 
  that 
  the 
  evolution 
  of 
  the 
  family 
  

   was 
  through 
  ' 
  a 
  series 
  from 
  Bemipedina 
  by 
  Pelanechinus 
  to 
  the 
  

   Echinothuridae.' 
  It 
  is 
  true 
  that 
  the 
  only 
  species 
  of 
  Pelanechinus 
  

   was 
  originally 
  described 
  as 
  an 
  Hemlpedina 
  2 
  ; 
  and 
  the 
  two 
  genera 
  

  

  1 
  ' 
  Ueber 
  Palcechinus, 
  Typhlechinus, 
  unci 
  die 
  Echinothuriden,' 
  Neues 
  Jahrb* 
  

   1 
  890, 
  vol. 
  i. 
  p. 
  85. 
  

  

  2 
  H. 
  corallina, 
  Wright, 
  ' 
  Monogr. 
  Brit. 
  Foss. 
  Ech. 
  Ool.,' 
  p. 
  163, 
  pi. 
  xii. 
  fig. 
  1. 
  

  

  