﻿G02 
  Mr. 
  II. 
  L. 
  Clark 
  on 
  the 
  

  

  in 
  position 
  (this 
  is 
  assumed 
  for 
  Anomolanthus 
  , 
  as 
  neither 
  the 
  

   describer 
  of 
  the 
  unique 
  type 
  species 
  nor 
  the 
  zoologist 
  who 
  

   established 
  the 
  genus 
  refer 
  to 
  this 
  most 
  important 
  character), 
  

   there 
  are 
  five 
  genital 
  glands 
  and 
  pores, 
  only 
  slightly 
  petaloid 
  

   ambulacra, 
  and 
  small 
  paired 
  interambulacral 
  plates 
  in 
  contact 
  

   with 
  each 
  genital. 
  In 
  Clypeaster 
  the 
  interambulacra 
  may 
  

   become 
  discontinuous 
  in 
  the 
  adults, 
  i. 
  e. 
  the 
  primordial 
  inter- 
  

   ambulacral 
  plate 
  becomes 
  separated 
  by 
  ambulacral 
  plates 
  

   from 
  the 
  rest 
  of 
  the 
  interambulacrum. 
  It 
  would 
  be 
  very 
  

   interesting 
  to 
  know 
  whether 
  this 
  is 
  also 
  true 
  of 
  Anomolanthus, 
  

   for 
  the 
  high 
  test 
  and 
  marginal 
  anus 
  of 
  that 
  genus 
  show 
  a 
  

   less 
  degree 
  of 
  specialization 
  than 
  is 
  shown 
  by 
  any 
  Recent 
  

   Clypeaster. 
  

  

  With 
  the 
  increasing 
  tendency 
  to 
  a 
  subarenaceous 
  life 
  the 
  

   dcpressionof 
  the 
  test 
  has 
  gone 
  on 
  rapidly 
  in 
  the 
  Clypeastroida, 
  

   the 
  extreme 
  being 
  reached 
  in 
  Arachnoides 
  placenta. 
  Curiously 
  

   enough, 
  however, 
  except 
  for 
  the 
  discontinuous 
  interambu- 
  

   lacra, 
  doubtless 
  associated 
  with 
  the 
  flatness 
  of 
  the 
  test, 
  

   ■Arachnoides 
  is 
  otherwise 
  quite 
  a 
  primitive 
  form, 
  as 
  evidenced 
  

   by 
  the 
  paired 
  ambulacra! 
  auricles 
  and 
  the 
  supra-marginal 
  

   anus. 
  The 
  ambulacra, 
  too, 
  are 
  only 
  slightly 
  petaloid. 
  How 
  

   the 
  real 
  relationships 
  of 
  this 
  remarkable 
  clypeastroid 
  can 
  best 
  

   be 
  shown 
  is, 
  of 
  course, 
  debatable, 
  but 
  it 
  seems 
  to 
  be 
  contrary 
  

   to 
  the 
  evidence 
  to 
  separate 
  it 
  widely 
  from 
  Clypeaster, 
  and 
  yet 
  

   I 
  cannot 
  believe 
  it 
  ought 
  to 
  be 
  placed 
  in 
  the 
  same 
  family 
  with 
  

   that 
  genus. 
  I 
  therefore 
  recognize 
  two 
  distinct 
  but 
  nearly 
  

   related 
  families, 
  the 
  Clypeastridse 
  and 
  the 
  Arachnoididse. 
  

  

  While 
  Arachnoides 
  is 
  thus 
  superficially 
  very 
  unlike 
  Clype- 
  

   aster 
  but 
  really 
  nearly 
  related, 
  Laganum 
  represents 
  the 
  other 
  

   extreme, 
  for 
  it 
  is 
  superficially 
  much 
  like 
  Clypeaster 
  but 
  in 
  

   reality 
  is 
  quite 
  different. 
  In 
  the 
  fused 
  auricles, 
  interradially 
  

   placed, 
  and 
  in 
  the 
  very 
  narrow 
  continuous 
  interambulacra 
  

   with 
  a 
  large 
  and 
  apparently 
  (though 
  not 
  really) 
  unpaired 
  

   abactinal 
  plate 
  adjoining 
  the 
  genital, 
  Laganum 
  shows 
  struc- 
  

   tural 
  progress 
  that 
  makes 
  it 
  improper 
  to 
  place 
  it 
  in 
  the 
  same 
  

   family 
  with 
  Clypeaster. 
  The 
  peculiar 
  character 
  of 
  the 
  inter- 
  

   ambulacra, 
  combined 
  with 
  the 
  straight, 
  simple, 
  actinal 
  

   ambulacral 
  furrows, 
  prevents 
  our 
  placing 
  it 
  in 
  the 
  same 
  

   family 
  with 
  Scutella 
  and 
  its 
  allies, 
  and 
  it 
  is 
  therefore 
  necessary 
  

   to 
  recognize 
  the 
  family 
  Laganida3. 
  

  

  The 
  remainder 
  of 
  the 
  clypeastroids 
  form 
  quite 
  a 
  homo- 
  

   geneous 
  group 
  with 
  more 
  or 
  less 
  discoidal 
  tests, 
  fused 
  

   auricles, 
  interradially 
  placed, 
  interambulacra 
  beginning 
  

   abactinally 
  in 
  a 
  pair 
  of 
  small 
  plates 
  next 
  the 
  genital, 
  and 
  

   with 
  actinal 
  ambulacral 
  furrows 
  more 
  or 
  less 
  forked 
  or 
  

  

  