﻿Yol. 
  53.] 
  AFFINITIES 
  OF 
  THE 
  ECHINOTHURID2E. 
  115 
  

  

  The 
  Characters 
  of 
  the 
  Ambulacra 
  in 
  the 
  Echinothuridce. 
  — 
  Since 
  

   Duncan's 
  classical 
  paper 
  on 
  the' 
  4 
  Structure 
  of 
  the 
  Ambulacra 
  of 
  

   some 
  Fossil 
  Genera 
  and 
  Species 
  of 
  Regular 
  Echinoidea,' 
  1 
  it 
  has 
  been 
  

   universally 
  admitted 
  that 
  the 
  characters 
  of 
  the 
  ambulacral 
  plates 
  

   offer 
  the 
  best 
  guidance 
  in 
  the 
  classification 
  of 
  that 
  section 
  of 
  the 
  

   class. 
  These 
  plates 
  in 
  Asthenosoma 
  and 
  Phormosoma, 
  are 
  very 
  

   different 
  from 
  those 
  of 
  any 
  other 
  living 
  echinid. 
  It 
  will 
  be 
  

   advisable 
  therefore, 
  in 
  the 
  first 
  place, 
  to 
  endeavour 
  to 
  trace 
  their 
  

   gradual 
  development. 
  In 
  the 
  living 
  echinothurids 
  the 
  ambulacral 
  

   plates 
  are 
  all 
  free 
  ; 
  two-thirds 
  of 
  the 
  pore-pairs 
  occur 
  in 
  small 
  

   plates 
  (' 
  klasma-plates 
  ') 
  lying 
  along 
  the 
  horizontal 
  sutures 
  between 
  

   the 
  remaining 
  plates 
  (fig. 
  1). 
  Both 
  characters 
  remind 
  us 
  of 
  the 
  

   ambulacral 
  plates 
  of 
  some 
  

  

  Palasozoic 
  echinids, 
  which 
  are 
  Y 
  1.— 
  Ambulacral 
  plates 
  of 
  

   multiserial 
  simple 
  and 
  free. 
  Phormosoma 
  Uranus. 
  (After 
  

  

  But 
  m 
  the 
  living 
  echinothurids 
  ^ 
  Agassiz.) 
  

  

  every 
  third 
  plate 
  is 
  a 
  large 
  

  

  primary 
  ; 
  and 
  in 
  spite 
  of 
  the 
  y 
  \ 
  

  

  apparent 
  irregularity 
  of 
  the 
  "^ 
  ^— 
  4T 
  S 
  "* 
  J 
  

  

  plates, 
  they 
  may 
  be 
  recognized 
  <£%? 
  

  

  as 
  occurring 
  in 
  triplets. 
  This 
  

  

  is 
  quite 
  different 
  from 
  anything 
  met 
  with 
  among 
  the 
  Palaeozoic 
  

   echinids, 
  and 
  they 
  agree 
  fundamentally 
  with 
  the 
  typical 
  Diadema- 
  

   toida. 
  If 
  the 
  Echinothuridae 
  are 
  to 
  be 
  regarded 
  as 
  descendants 
  

   from 
  the 
  Palaeozoic 
  echinids, 
  then 
  compound 
  ambulacral 
  plates 
  have 
  

   l)een 
  twice 
  independently 
  developed 
  from 
  the 
  simple 
  primary 
  plates 
  

   of 
  the 
  latter 
  and 
  of 
  the 
  Cidaridae. 
  This 
  is 
  not 
  impossible, 
  for 
  

   heterogenetic 
  homoeomorphy 
  unquestionably 
  occurs 
  sometimes. 
  It 
  

   is 
  highly 
  probable 
  that 
  echinids 
  with 
  ' 
  arbacioid 
  ' 
  ambulacral 
  plates 
  

   have 
  developed 
  from 
  forms 
  with 
  simple 
  ambulacral 
  plates, 
  through 
  

   stages 
  represented 
  by 
  the 
  genera 
  Salenia 
  and 
  Acrosalenia 
  ; 
  while 
  

   those 
  with 
  ' 
  diademoid 
  ' 
  ambulacral 
  plates 
  have 
  developed 
  through 
  

   forms 
  such 
  as 
  Eodiadema 
  and 
  Archceodiadema. 
  It 
  is 
  conceivable 
  

   that 
  there 
  may 
  have 
  been 
  a 
  third 
  line 
  along 
  which 
  the 
  Echino- 
  

   thuridae 
  have 
  developed 
  ; 
  but 
  there 
  is 
  no 
  evidence 
  of 
  any 
  such 
  deve- 
  

   lopment, 
  and 
  we 
  ought 
  not 
  to 
  assume 
  this 
  origin 
  in 
  face 
  of 
  actual 
  

   evidence 
  for 
  one 
  which 
  is 
  inherently 
  far 
  more 
  probable. 
  

  

  It 
  will 
  be 
  remembered 
  that 
  the 
  typical 
  ' 
  diademoid 
  ' 
  ambulacral 
  

   plates 
  consist 
  of 
  three 
  primary 
  plates 
  fused 
  into 
  one 
  (fig. 
  9, 
  p. 
  121). 
  

   The 
  middle 
  primary 
  is 
  usually 
  the 
  largest. 
  The 
  gradual 
  growth 
  of 
  

   this 
  central 
  plate 
  may 
  be 
  traced 
  until 
  the 
  adoral 
  and 
  aboral 
  plates 
  are 
  

   cut 
  off 
  from 
  the 
  vertical 
  suture-line, 
  and 
  thus 
  become 
  demi-plates 
  

   (fig. 
  6<tf, 
  p. 
  118). 
  By 
  a 
  further 
  progress 
  in 
  the 
  same 
  direction, 
  the 
  

   demi-plates 
  are 
  reduced 
  to 
  the 
  condition 
  of 
  'klasma-plates,' 
  2 
  detached 
  

  

  1 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xli. 
  (1885) 
  pp. 
  419-452. 
  

  

  2 
  A 
  name 
  suggested 
  for 
  the 
  small, 
  eye-shaped 
  plates, 
  which 
  are 
  cut 
  off 
  from 
  

   contact 
  with 
  either 
  the 
  vertical 
  suture 
  running 
  down 
  the 
  centre 
  of 
  the 
  ambu- 
  

   lacral 
  area, 
  or 
  that 
  between 
  the 
  ambulacrum 
  and 
  interambulacrum. 
  They 
  

   represent 
  a 
  stage 
  of 
  reduction 
  further 
  than 
  that 
  of 
  demi-plates. 
  From 
  

   x\d<Tfia, 
  a 
  fraction. 
  

  

  i2 
  

  

  