﻿Vol. 
  53-] 
  

  

  AFFINITIES 
  OF 
  THE 
  ECHINOTHTJRID.E. 
  

  

  121 
  

  

  family. 
  S. 
  P. 
  Woodward 
  called 
  attention 
  to 
  these 
  plates 
  in 
  his 
  

   original 
  description 
  of 
  EchinotTiuria. 
  Spines 
  had 
  previously 
  been 
  

   figured 
  by 
  Forbes, 
  first 
  as 
  belonging 
  to 
  Micr 
  aster 
  l 
  and 
  then 
  to 
  

   Cidaris. 
  2 
  S. 
  P. 
  "Woodward 
  3 
  and 
  Wright 
  4 
  showed 
  a 
  truer 
  apprecia- 
  

   tion 
  of 
  their 
  characters 
  by 
  assigning 
  them 
  to 
  Diaclema. 
  At 
  this 
  

   time, 
  however, 
  the 
  echinid 
  was 
  known 
  only 
  from 
  isolated 
  plates. 
  

   In 
  Wright's 
  ' 
  Monograph 
  of 
  the 
  Cretaceous 
  Echinoidea 
  ' 
  a 
  better 
  

  

  Fig. 
  7. 
  — 
  Ambulacra! 
  plates 
  of 
  

   Helikodiadema 
  at 
  summit 
  

   of 
  ambulacrum. 
  

  

  Pig. 
  9. 
  — 
  Ambulacral 
  plates 
  of 
  

   Helikodiadema 
  adjoining 
  

   peristome. 
  

  

  Pig. 
  8. 
  — 
  Ambital 
  ambidacral 
  Pig. 
  

  

  plate 
  of 
  Helikodiadema. 
  

  

  10. 
  — 
  Genital 
  plate 
  of 
  

   Helikodiadema. 
  

  

  specimen 
  was 
  figured, 
  with 
  the 
  plates 
  in 
  their 
  relative 
  positions 
  : 
  5 
  

   it 
  was 
  named 
  Pseudodiadema 
  fragile. 
  The 
  structure 
  of 
  the 
  ambu- 
  

   lacral 
  plates 
  was 
  not 
  shown; 
  but 
  the 
  accompanying 
  figure 
  (7) 
  

   of 
  a 
  specimen 
  in 
  the 
  British 
  Museum 
  (B. 
  M. 
  46,781) 
  shows 
  that 
  the 
  

   plates 
  are 
  diademoid. 
  

  

  The 
  echinid 
  is 
  most 
  nearly 
  allied 
  to 
  Pseudodiadema, 
  but 
  the 
  

   differences 
  between 
  it 
  and 
  that 
  genus 
  seem 
  well 
  worthy 
  of 
  generic 
  

   distinction. 
  I 
  therefore 
  describe 
  it 
  as 
  follows 
  :— 
  

  

  Helikodiadema, 
  6 
  gen. 
  nov. 
  (Pigs. 
  7-10.) 
  

  

  Diagnosis. 
  — 
  Diadematidae 
  with 
  a 
  large, 
  flexible 
  test, 
  composed 
  of 
  

   thin, 
  loosely-fitting 
  plates. 
  Peristome 
  and 
  periproct 
  large. 
  Peri- 
  

   gnathic 
  processes 
  slender. 
  

  

  1 
  E. 
  Forbes, 
  in 
  Dixon's 
  ' 
  Geol. 
  Suss.,' 
  Expl. 
  of 
  pi., 
  p. 
  x, 
  pi. 
  xxv. 
  fig. 
  28. 
  

  

  2 
  Id., 
  Dec. 
  Geol. 
  Surv. 
  no. 
  iii. 
  pi. 
  x. 
  fig. 
  15. 
  

  

  3 
  S. 
  P. 
  Woodward, 
  ibid. 
  no. 
  v. 
  pi. 
  ii. 
  p. 
  11. 
  

  

  4 
  Wright, 
  ' 
  Monogr. 
  Cret. 
  Ech.' 
  pi. 
  xiv. 
  fig. 
  2. 
  

  

  5 
  Ibid. 
  pi. 
  lxxx. 
  

  

  6 
  From 
  eXiicos, 
  twisting. 
  

  

  