﻿124 
  DR. 
  J. 
  W. 
  GREGORY 
  ON 
  [Feb. 
  1897, 
  

  

  Melchior 
  Neumayr 
  kindly 
  urged 
  me 
  to 
  publish 
  then. 
  The 
  hope, 
  

   however, 
  of 
  obtaining 
  access 
  to 
  better 
  specimens 
  has 
  delayed 
  me 
  from 
  

   doing 
  so 
  until 
  now. 
  

  

  II. 
  History 
  of 
  the 
  Genera. 
  

  

  Palceodiscus 
  was 
  the 
  first 
  established 
  of 
  the 
  two 
  genera. 
  It 
  was 
  

   founded 
  by 
  J. 
  W. 
  Salter 
  in 
  1857 
  [9, 
  p. 
  332, 
  pi. 
  ix. 
  fig. 
  6] 
  on 
  a 
  fossil 
  

   from 
  the 
  Ludlow 
  Hags 
  of 
  Leintwardiue. 
  He 
  described 
  it 
  as 
  a 
  

   new 
  starfish, 
  a 
  view 
  which 
  seemed 
  to 
  be 
  established 
  by 
  the 
  flattened, 
  

   pentagonal 
  form 
  of 
  the 
  fossil 
  and 
  its 
  radiating 
  lanceolate 
  ambulacra, 
  

   which 
  were 
  apparently 
  limited 
  to 
  one 
  side 
  of 
  the 
  body. 
  

  

  Echinocystis 
  (or 
  Echinocystites) 
  was 
  founded 
  by 
  Wyville 
  Thomson 
  

   in 
  1861 
  [11] 
  for 
  some 
  fossils 
  from 
  the 
  same 
  horizon 
  as 
  Palceodiscus. 
  

   He 
  recognized 
  that 
  they 
  were 
  echinids 
  with 
  affinities 
  to 
  the 
  cystids, 
  

   and 
  placed 
  Palceodiscus 
  as 
  a 
  flat 
  member 
  of 
  the 
  same 
  family. 
  In 
  

   this 
  course, 
  however, 
  he 
  has 
  not 
  been 
  widely 
  supported. 
  Wright 
  

   [14, 
  p. 
  35J, 
  who 
  next 
  referred 
  to 
  Palceodiscus 
  in 
  1863, 
  accepted 
  it 
  as 
  

   an 
  asterid, 
  and 
  Zittel, 
  in 
  1879 
  [15, 
  p. 
  453], 
  did 
  the 
  same, 
  placing 
  

   it 
  in 
  his 
  suborder 
  Encrinasteriae. 
  Neumayr, 
  in 
  1881, 
  republished 
  

   Wright's 
  figure, 
  and 
  concluded 
  [7, 
  p. 
  156], 
  ' 
  trotzdem 
  tauchte 
  kein 
  

   Bedenken 
  gegen 
  die 
  Seesternnatur 
  dieser 
  Eormen 
  auf.' 
  

  

  Nevertheless, 
  Duncan, 
  in 
  1889, 
  after 
  examining 
  specimens, 
  not 
  

   only 
  regarded 
  Palceodiscus 
  as 
  an 
  echinid, 
  but 
  he 
  made 
  it 
  a 
  synonym 
  

   of 
  the 
  genus 
  Echinocystis 
  [2, 
  p. 
  20]. 
  The 
  latter 
  genus 
  had, 
  mean- 
  

   while, 
  been 
  made 
  by 
  von 
  Zittel 
  [15, 
  p. 
  480] 
  the 
  type 
  of 
  a 
  new 
  

   order 
  of 
  Echinoidea 
  named 
  Oystocidaroida, 
  while 
  Steinmann 
  and 
  

   Hoderleln 
  [10, 
  p. 
  180] 
  used 
  it 
  as 
  the 
  basis 
  of 
  an 
  order 
  of 
  Cystoidea 
  

   which 
  they 
  named 
  Cystechinoidea. 
  Finally, 
  Jackson 
  retains 
  it 
  

   among 
  the 
  Echinoidea 
  [4, 
  p. 
  242]. 
  

  

  Palceodiscus 
  is 
  therefore 
  regarded 
  as 
  an 
  echinid 
  or 
  an 
  asterid, 
  

   and 
  Echinocystis 
  as 
  an 
  echinid 
  or 
  a 
  cystid. 
  As 
  there 
  are 
  good 
  

   specimens 
  of 
  both 
  genera 
  in 
  the 
  British 
  Museum, 
  I 
  propose 
  to 
  

   describe 
  them 
  — 
  in 
  order 
  to 
  attempt 
  the 
  determination 
  of 
  the 
  class 
  or 
  

   classes 
  to 
  which 
  they 
  belong. 
  The 
  specimens 
  all 
  come 
  from 
  the 
  

   Lower 
  Ludlow 
  Elags 
  of 
  Leintwardine 
  ; 
  those 
  of 
  Echinocystis 
  are 
  

   •casts, 
  which 
  yield 
  good 
  impressions 
  in 
  wax 
  of 
  the 
  original 
  animals. 
  

  

  III. 
  The 
  Structure 
  of 
  Echinocystis. 
  

  

  The 
  specimens 
  representing 
  this 
  genus 
  in 
  the 
  British 
  Museum 
  

   are 
  mostly 
  crushed 
  on 
  to 
  a 
  plane, 
  passing 
  through 
  the 
  ambitus. 
  

   Sir 
  Wyville 
  Thomson, 
  however, 
  figured 
  specimens 
  which 
  had 
  been 
  

   flattened 
  along 
  a 
  plane 
  passing 
  vertically 
  upward 
  through 
  the 
  

   mouth, 
  and 
  these 
  show 
  the 
  general 
  form 
  of 
  the 
  animal. 
  Steiu- 
  

   mann's 
  diagrammatic 
  restoration 
  is 
  based 
  on 
  these 
  figures. 
  The 
  

   remaining 
  points 
  in 
  the 
  anatomy 
  are 
  illustrated 
  by 
  different 
  speci- 
  

   mens 
  in 
  the 
  British 
  Museum 
  ; 
  thus 
  the 
  structure 
  of 
  the 
  ambulacra 
  

   and 
  the 
  shape 
  and 
  position 
  of 
  the 
  madreporite 
  are 
  shown 
  in 
  

   No. 
  40,158, 
  and 
  the 
  character 
  of 
  the 
  jaws 
  in 
  Nos. 
  E 
  1256 
  and 
  

   40,156. 
  

  

  The 
  following 
  account 
  of 
  the 
  animal 
  has 
  been 
  compiled 
  from 
  the 
  

  

  