﻿126 
  db. 
  j. 
  w. 
  Gregory 
  on 
  [Feb. 
  1897, 
  

  

  IV. 
  The 
  Affinities 
  of 
  Echinocystis. 
  

  

  Echinocystis 
  has 
  been 
  assigned 
  to 
  both 
  the 
  classes 
  Echinoidea 
  and 
  

   Cystoidea, 
  and 
  there 
  can 
  be 
  little 
  doubt 
  that 
  it 
  belongs 
  to 
  one 
  of 
  

   them. 
  In 
  order 
  to 
  determine 
  in 
  which 
  of 
  the 
  two 
  it 
  is 
  to 
  be 
  

   placed, 
  we 
  must 
  first 
  consider 
  the 
  essential 
  differences 
  between 
  

   them. 
  We 
  are 
  at 
  once 
  confronted 
  with 
  the 
  difficulty 
  that 
  the 
  

   diagnoses 
  of 
  the 
  class 
  Cystoidea 
  are 
  very 
  unsatisfactory, 
  and 
  it 
  is 
  

   easier 
  to 
  condemn 
  than 
  to 
  improve 
  them. 
  Bell, 
  in 
  1891, 
  when 
  

   discussing 
  the 
  inter-relations 
  of 
  the 
  classes 
  of 
  Echinoderma, 
  recoiled 
  

   from 
  the 
  task 
  of 
  drawing 
  up 
  a 
  diagnosis 
  of 
  the 
  Cystoidea, 
  and 
  hoped 
  

   [1, 
  p. 
  215] 
  that 
  ' 
  perhaps 
  a 
  palaeontologist 
  will 
  oblige.' 
  Since 
  that 
  

   date 
  two 
  palaeontologists 
  have 
  ' 
  obliged/ 
  and 
  we 
  must 
  first 
  enquire 
  

   whether 
  Echinocystis 
  is 
  necessarily 
  included 
  by 
  them 
  among 
  the 
  

   cystids. 
  

  

  Yon 
  Zittel 
  [16, 
  p. 
  148], 
  in 
  1895, 
  defined 
  the 
  Cystoidea 
  as 
  

   follows 
  : 
  — 
  ' 
  Extinct, 
  short-stalked, 
  more 
  rarely 
  unstalked 
  Pelma- 
  

   tozoa, 
  with 
  more 
  or 
  less 
  irregularly-arranged 
  calycinal 
  plates, 
  with 
  

   the 
  arms 
  feebly 
  developed, 
  and 
  sometimes 
  quite 
  absent. 
  The 
  

   calycinal 
  plates 
  are 
  frequently 
  perforated 
  by 
  fine 
  canals/ 
  

  

  There 
  is 
  nothing 
  in 
  this 
  diagnosis 
  except 
  the 
  word 
  Pelmatozoa 
  to 
  

   exclude 
  Echinocystis 
  ; 
  for 
  it 
  is 
  an 
  extinct, 
  unstalked 
  form, 
  with 
  

   irregularly- 
  arranged 
  plates, 
  without 
  arms, 
  and 
  having 
  some 
  of 
  the 
  

   plates 
  perforated 
  b)^ 
  fine 
  canals. 
  But 
  Lepidesthes 
  and 
  Melonites 
  

   both 
  possess 
  the 
  same 
  series 
  of 
  characters, 
  for 
  in 
  these 
  two 
  genera 
  

   the 
  plates 
  protecting 
  the 
  body 
  are 
  less 
  regular 
  than 
  in 
  Garyocrinus 
  

   or 
  Porocrinus. 
  We 
  have 
  therefore 
  to 
  refer 
  back 
  to 
  von 
  Zittel's 
  

   diagnosis 
  of 
  Pelmatozoa 
  to 
  see 
  if 
  this 
  will 
  exclude 
  forms 
  like 
  Tiar- 
  

   echinus 
  and 
  yet 
  admit 
  Echinocystis. 
  

  

  The 
  diagnosis 
  of 
  the 
  Pelmatozoa 
  [16, 
  p. 
  113] 
  is 
  as 
  follows 
  ; 
  — 
  

   1 
  Echinoderma 
  which 
  are 
  fixed 
  by 
  a 
  jointed 
  stem 
  or 
  directly 
  by 
  the 
  

   aboral 
  (dorsal) 
  side 
  of 
  the 
  body, 
  either 
  throughout 
  life 
  or 
  in 
  youth. 
  

   A 
  bag- 
  shaped, 
  calyx-shaped, 
  or 
  spherical 
  capsule 
  of 
  calcareous 
  plates 
  

   -encloses 
  the 
  body- 
  cavity. 
  On 
  the 
  upper 
  (oral 
  or 
  ventral) 
  side 
  are 
  

   the 
  mouth 
  and 
  anus, 
  as 
  well 
  as 
  ambulacral 
  vessels 
  leading 
  to 
  the 
  

   mouth. 
  At 
  the 
  distal 
  ends 
  of 
  the 
  ambulacral 
  furrows 
  of 
  the 
  tegmen 
  

   there 
  usually 
  arise 
  jointed 
  arms, 
  or 
  the 
  ambulacral 
  furrows 
  continue 
  

   over 
  the 
  sides 
  of 
  the 
  cup, 
  and 
  are 
  bordered 
  on 
  both 
  sides 
  by 
  pinnules. 
  

   The 
  lower 
  (dorsal, 
  aboral) 
  side 
  is 
  encompassed 
  by 
  one 
  or 
  two 
  circles 
  

   of 
  basal 
  plates, 
  which 
  either 
  rest 
  on 
  the 
  stem 
  or 
  surround 
  a 
  centro- 
  

   dorsal 
  plate.' 
  The 
  positive 
  characters 
  in 
  this 
  definition 
  are: 
  — 
  

   (1) 
  fixation 
  at 
  one 
  period 
  of 
  life 
  ; 
  (2) 
  a 
  capsule 
  of 
  calcareous 
  plates; 
  

   (3) 
  the 
  occurrence 
  of 
  mouth, 
  anus, 
  and 
  ambulacra 
  on 
  the 
  upper 
  

   surface 
  ; 
  (4) 
  the 
  presence 
  of 
  arms 
  or 
  pinnules 
  ; 
  (5) 
  the 
  double 
  circlet 
  

   of 
  calycinal 
  plates. 
  

  

  This 
  combination 
  of 
  characters 
  does 
  not, 
  however, 
  give 
  any 
  test 
  

   by 
  which 
  we 
  can 
  at 
  once 
  say 
  whether 
  a 
  certain 
  fossil 
  is 
  a 
  cystid 
  or 
  

   an 
  echinid. 
  There 
  are 
  exceptions 
  in 
  regard 
  to 
  each 
  character. 
  

   Thus 
  we 
  now 
  know 
  both 
  pelmatozoic 
  and 
  apelmatozoic 
  cystids 
  

   and 
  crinoids, 
  including 
  forms 
  which 
  not 
  only 
  have 
  no 
  stalk, 
  but 
  no 
  

   trace 
  of 
  an 
  aboral 
  attachment. 
  Prof. 
  Bell 
  remarks 
  [1, 
  p. 
  210], 
  

  

  