﻿Yol. 
  53.] 
  ECHrNocrsns 
  and 
  pal^eodiscus. 
  129 
  

  

  symmetry; 
  (2) 
  a 
  radial 
  anthodium 
  ; 
  (3) 
  theca 
  uniaxial 
  or 
  radial; 
  

   (4) 
  usually 
  fixed 
  aborally, 
  but 
  sometimes 
  free 
  ; 
  (5) 
  a 
  scaly 
  or 
  plated 
  

   test 
  ; 
  (6) 
  a 
  central 
  mouth 
  ; 
  (7) 
  an 
  excentric 
  anus 
  on 
  the 
  aboral 
  side 
  ; 
  

   (8) 
  a 
  valvular 
  pyramid 
  ; 
  (9) 
  gonopore 
  and 
  hydropore, 
  when 
  present, 
  

   situated 
  between 
  mouth 
  and 
  anus 
  ; 
  (10) 
  the 
  presence 
  of 
  pinnules, 
  

   arms, 
  or 
  brachioles. 
  This 
  diagnosis 
  clearly 
  separates 
  the 
  cystids 
  

   from 
  the 
  crinids 
  and 
  from 
  the 
  members 
  of 
  Hackel's 
  new 
  class, 
  

   the 
  Amphoralia 
  ; 
  but 
  it 
  does 
  not 
  give 
  any 
  very 
  satisfactory 
  grounds 
  

   for 
  separation 
  from 
  the 
  echinids. 
  The 
  Echinoidea 
  agree 
  with 
  the 
  

   Cystoidea 
  in 
  characters 
  Nos. 
  1,2, 
  3, 
  and 
  5 
  ; 
  some 
  echinids 
  agree 
  

   with 
  some 
  cystids 
  in 
  Nos. 
  4, 
  6, 
  7, 
  and 
  8 
  1 
  ; 
  9 
  and 
  10 
  are 
  not 
  present 
  

   in 
  all 
  cystids, 
  for 
  there 
  is 
  no 
  more 
  reason 
  to 
  regard 
  A.ristocystis 
  as 
  

   having 
  pinnules, 
  arms, 
  or 
  armlets 
  than 
  some 
  Palaeozoic 
  echinids. 
  

   Thus 
  the 
  diagnosis 
  does 
  not 
  yield 
  a 
  single 
  constant, 
  invariable 
  dis- 
  

   tinction 
  between 
  tbe 
  Cystoidea 
  and 
  the 
  Echinoidea. 
  

  

  In 
  order, 
  therefore, 
  to 
  refer 
  Ecliinocystis 
  to 
  its 
  class, 
  we 
  have 
  to 
  

   rely 
  on 
  the 
  sum 
  of 
  its 
  characters. 
  It 
  agrees 
  with 
  both 
  cystids 
  and 
  

   echinids 
  in 
  the 
  possession 
  of 
  characters 
  1, 
  2, 
  3, 
  5, 
  6, 
  7, 
  and 
  8. 
  In 
  

   the 
  remaining 
  characters, 
  4, 
  9, 
  and 
  10, 
  it 
  agrees 
  with 
  typical 
  

   echinids, 
  but 
  differs 
  from 
  typical 
  cystids. 
  Hence 
  it 
  seems 
  more 
  

   reasonable 
  to 
  follow 
  von 
  Zittel 
  and 
  assign 
  the 
  genus 
  to 
  the 
  Echi- 
  

   noidea 
  than, 
  with 
  Neumayr 
  and 
  Steinmann, 
  to 
  regard 
  it 
  as 
  a 
  cystid. 
  

   The 
  absence 
  of 
  calycinal 
  plates 
  or 
  of 
  any 
  aboral 
  attachment 
  ; 
  the 
  

   presence 
  of 
  a 
  jaw-apparatus 
  and 
  of 
  articulator}* 
  spines 
  ; 
  the 
  develop- 
  

   ment 
  of 
  an 
  exoskeleton, 
  which 
  is 
  spherical 
  in 
  shape, 
  and 
  consists 
  of 
  

   closely-fitting 
  plates 
  ; 
  the 
  occurrence 
  of 
  biserial 
  ambulacral 
  pore- 
  

   pairs 
  ; 
  and, 
  finally, 
  the 
  ventral 
  aspect 
  of 
  the 
  mouth, 
  are 
  all 
  

   characters 
  in 
  which 
  Echinocystis 
  agrees 
  with 
  the 
  Echinoidea. 
  

  

  Y. 
  The 
  Structure 
  op 
  Pal^omscus. 
  

  

  Form. 
  — 
  Discoid, 
  possibly 
  subpentagonal. 
  Test 
  thin 
  and 
  probably 
  

   flexible. 
  Mouth 
  central 
  on 
  lower 
  surface. 
  Anus 
  aboral, 
  central. 
  

  

  Ambulacra 
  broad 
  on 
  the 
  oral 
  surface, 
  but 
  gradually 
  diminishing 
  

   in 
  width 
  on 
  the 
  aboral 
  surface. 
  In 
  flat- 
  

   tened 
  specimens 
  they 
  are 
  somewhat 
  petaloid 
  Pig. 
  4. 
  — 
  Ambulacral 
  

   (a 
  character, 
  however, 
  greatly 
  exaggerated 
  plates 
  of 
  Palaeodiscus. 
  

   in 
  Wright's 
  figure). 
  

  

  On 
  the 
  oral 
  surface 
  the 
  ambulacral 
  

   plates 
  are 
  simple, 
  thin, 
  and 
  bar-shaped 
  

   (fig. 
  4). 
  

  

  On 
  the 
  aboral 
  surface 
  each 
  ambulacrum 
  

   consists 
  of 
  two 
  alternate 
  series 
  of 
  simple, 
  

   thin, 
  bar-shaped 
  plates. 
  There 
  are 
  no 
  

   pores 
  through 
  the 
  plates, 
  but 
  the 
  podia 
  

   must 
  have 
  been 
  extruded 
  through 
  the 
  

   sutures 
  between 
  them. 
  

  

  On 
  the 
  aboral 
  surface 
  the 
  ambulacra 
  are 
  narrower 
  (see, 
  for 
  

   example, 
  E 
  1482) 
  ; 
  and, 
  according 
  to 
  Wyville 
  Thomson, 
  there 
  is 
  

   a 
  series 
  of 
  single 
  pores 
  on 
  each 
  side. 
  

  

  1 
  A 
  valvular 
  pyramid 
  occurs 
  in 
  Palaostoma. 
  

   Q. 
  J. 
  G. 
  S. 
  No. 
  209. 
  k 
  

  

  