﻿122 
  AFFINITIES 
  OF 
  THE 
  ECEUNOTHUPvID^. 
  [Feb. 
  1897, 
  

  

  Apical 
  system 
  apparently 
  a 
  single 
  circle 
  of 
  ten 
  plates 
  . 
  Gen 
  ital 
  

   plates 
  pentagonal 
  and 
  large. 
  

  

  Ambulacra. 
  — 
  The 
  plates 
  near 
  the 
  apex 
  are 
  primaries. 
  At 
  the 
  

   ambitus 
  they 
  are 
  of 
  three 
  fused 
  primaries. 
  Near 
  the 
  peristome 
  

   they 
  are 
  crowded, 
  and 
  demi-plates 
  are 
  numerous. 
  

  

  Tubercles 
  perforate 
  and 
  crenulate. 
  

  

  Spines 
  annulated, 
  fluted, 
  hollow. 
  

  

  Distribution. 
  — 
  Chalk, 
  England. 
  

  

  Type 
  Species. 
  — 
  Helikodiadema 
  fragile 
  (Wilts.), 
  of 
  which 
  the 
  

   synonymy 
  is 
  as 
  follows 
  : 
  — 
  

  

  Cidaris, 
  sp., 
  Forbes, 
  1850, 
  in 
  Dixon, 
  ' 
  Geol. 
  Suss.,' 
  Expl. 
  of 
  

  

  pi., 
  p. 
  x, 
  pi. 
  xxv. 
  fig. 
  28. 
  

   Micraster, 
  sp., 
  Forbes, 
  1850, 
  Dec. 
  Geol. 
  Surv. 
  no. 
  iii. 
  pi. 
  x. 
  

  

  fig. 
  15. 
  

   Diadema, 
  sp., 
  S. 
  P. 
  Woodward, 
  1856, 
  Dec. 
  Geol. 
  Surv. 
  no. 
  v. 
  

  

  pi. 
  ii. 
  p. 
  11 
  ; 
  Wright, 
  1868. 
  ' 
  Mon. 
  Cret. 
  Ech.' 
  pi. 
  xiv. 
  fig. 
  2 
  ; 
  

  

  S. 
  P. 
  Woodward, 
  1878, 
  in 
  Dixon, 
  < 
  Geol. 
  Suss.' 
  2nd 
  edit. 
  

  

  p. 
  372, 
  pi. 
  xxv. 
  [28] 
  fig. 
  28. 
  

   Pseudodiadema 
  fragile, 
  Wiltshire, 
  1882, 
  in 
  Wright, 
  op. 
  cit. 
  

  

  pi. 
  lxxx. 
  

  

  Summary 
  of 
  Conclusion's. 
  

  

  1. 
  That 
  the 
  family 
  Echinothuridse 
  is 
  a 
  member 
  of 
  the 
  order 
  

   Diademoida, 
  and 
  is 
  derived 
  from 
  the 
  Pedinidas. 
  

  

  2. 
  That 
  the 
  oldest 
  member 
  of 
  the 
  family 
  is 
  the 
  genus 
  Pelanechinus, 
  

   and 
  that 
  the 
  extreme 
  flexibility 
  and 
  loose 
  articulation 
  of 
  the 
  plates 
  

   of 
  the 
  living 
  genera 
  Astlienosoma 
  and 
  Phormosoma 
  are 
  due 
  to 
  

   diminished 
  calcification 
  of 
  the 
  plates. 
  

  

  3. 
  That 
  the 
  apparently 
  primitive 
  features 
  of 
  the 
  Echinothuridse 
  

   are 
  secondarily 
  acquired 
  and 
  are 
  not 
  primaeval. 
  The 
  recent 
  genera 
  

   are 
  therefore 
  extremely 
  specialized, 
  instead 
  of 
  being 
  primitive 
  forms. 
  

  

  4. 
  A 
  new 
  genus 
  Pedinothuria 
  is 
  a 
  connecting-link 
  between 
  the 
  

   Pedinidas 
  and 
  the 
  Echinothuridse. 
  

  

  5. 
  A 
  new 
  genus 
  HeliJcodiadema, 
  which 
  has 
  a 
  flexible 
  test, 
  is 
  a 
  

   modified 
  form 
  of 
  Pseudodiadema 
  ; 
  it 
  has 
  probably 
  arisen 
  from 
  the 
  

   adoption 
  of 
  deep-sea 
  life 
  having 
  resulted 
  in 
  diminished 
  calcifica- 
  

   tion 
  of 
  the 
  test. 
  

  

  [For 
  Explanation 
  of 
  Plate 
  VII., 
  see 
  p. 
  134.] 
  

  

  