﻿Vol. 
  53-] 
  ECHINOCYSTIS 
  AND 
  PAL2EODISCU6. 
  133 
  

  

  Order 
  Cystocidaroida, 
  Zittel, 
  1879. 
  

  

  Diagnosi 
  s. 
  — 
  Echinoidea 
  with 
  a 
  test 
  composed 
  of 
  irregular, 
  thin, 
  

   spine-bearing 
  plates 
  ; 
  the 
  mouth 
  is 
  central 
  ; 
  the 
  anus 
  is 
  aboral 
  ; 
  

   the 
  madreporite 
  is 
  large 
  and 
  single, 
  and 
  occurs 
  in 
  the 
  same 
  inter- 
  

   ambulacrum 
  as 
  the 
  anus 
  (unknown 
  on 
  one 
  genus) 
  ; 
  there 
  are 
  no 
  

   calycinal 
  plates 
  ; 
  the 
  ambulacral 
  plates 
  are 
  simple, 
  and 
  may 
  be 
  

   perforate 
  or 
  imperforate 
  ; 
  there 
  is 
  a 
  jaw-apparatus 
  composed 
  of 
  five 
  

   pyramids. 
  

  

  Family 
  1. 
  Pal^odiscid^e. 
  

  

  Diagnosi 
  s. 
  — 
  Cystocidaroida 
  with 
  a 
  depressed 
  pentagon 
  al 
  body. 
  

   The 
  ambulacral 
  plates 
  on 
  the 
  oral 
  surface 
  are 
  imperforate. 
  The 
  

   anus 
  is 
  central. 
  

  

  Genus 
  Palceodiscus, 
  Salter, 
  1857 
  [9]. 
  

  

  Type 
  -specie 
  s. 
  — 
  P. 
  ferox, 
  Salter. 
  Leintwardine 
  Flags, 
  Lower 
  

   Ludlow. 
  

  

  Family 
  2. 
  EcbtnocystidjE. 
  

  

  Diagnosi 
  s. 
  — 
  Cystocidaroida 
  with 
  a 
  spherical 
  body. 
  The 
  ambu- 
  

   lacral 
  plates 
  are 
  perforated 
  by 
  pores 
  ; 
  the 
  pore- 
  pairs 
  are 
  biserial, 
  and 
  

   some 
  demi-plates 
  are 
  present. 
  The 
  anus 
  is 
  excentric. 
  

  

  Genus 
  Echinocystis, 
  Wyv. 
  Thomson, 
  1861 
  [11] 
  (non 
  Hall, 
  1867). 
  * 
  

  

  S 
  y 
  n 
  o 
  n 
  y 
  m. 
  — 
  Cystocidaris, 
  Zittel, 
  1 
  879 
  . 
  

  

  Type-species. 
  — 
  Echinocystis 
  pomum, 
  Wyv. 
  Thomson, 
  1861. 
  

  

  Von 
  Zittel 
  includes 
  the 
  genus 
  Spatangopsis, 
  founded 
  by 
  Torell 
  

   [12, 
  p. 
  11] 
  on 
  a 
  species 
  named 
  Sp. 
  costata, 
  as 
  a 
  possible 
  cystocidarid. 
  

   Torell, 
  however, 
  describes 
  the 
  genus 
  as 
  having 
  five 
  sharp 
  radial 
  

   costae 
  ; 
  and 
  though 
  it 
  is 
  possibly 
  an 
  echinoderm, 
  the 
  narrowness 
  

   of 
  the 
  costse 
  would 
  exclude 
  it 
  from 
  the 
  Cystocidaroida, 
  whether 
  

   these 
  radial 
  structures 
  are 
  ambulacral 
  or 
  not. 
  

  

  IX. 
  Summary 
  of 
  Conclusions. 
  

  

  1. 
  The 
  structures 
  of 
  the 
  Silurian 
  genera 
  Pala-odiscus, 
  Salt., 
  and 
  

  

  Echinocystis, 
  Wyv. 
  Th., 
  are 
  redescribed. 
  

  

  2. 
  It 
  is 
  concluded 
  that 
  Echinocystis 
  is 
  an 
  echinid, 
  and 
  not 
  a 
  cyst 
  id 
  : 
  

  

  and 
  that 
  Palceodiscus 
  is 
  an 
  echinid, 
  and 
  not 
  an 
  asterid. 
  

  

  3. 
  The 
  name 
  Scolocystis 
  is 
  given 
  to 
  the 
  cystid 
  named 
  Echino- 
  

  

  cystis 
  by 
  Hall; 
  and 
  that 
  of 
  Discocgstis 
  to 
  the 
  form 
  named 
  

   Echinodiscus 
  by 
  Worthen 
  and 
  Miller. 
  

  

  4. 
  In 
  a 
  discussion 
  of 
  the 
  affinities 
  of 
  Echinocystis, 
  the 
  two 
  latest 
  

  

  1 
  Zittel 
  called 
  attention 
  to 
  the 
  fact 
  that 
  the 
  name 
  Echinocystis 
  has 
  been 
  used 
  

   by 
  Hall 
  for 
  cystids 
  as 
  well 
  as 
  by 
  Thomson 
  for 
  echinids. 
  By 
  a 
  slight 
  oversight 
  

   he 
  omitted 
  to 
  notice 
  that 
  Thomson 
  had 
  priority 
  by 
  six 
  years, 
  and 
  Zittel 
  therefore 
  

   kept 
  the 
  name 
  Echinocystis 
  for 
  cystids 
  and 
  renamed 
  Thomson's 
  genus 
  Cysto- 
  

   cidaris. 
  Although 
  this 
  has 
  been 
  more 
  than 
  once 
  pointed 
  out 
  (e.g. 
  Duncan, 
  2, 
  

   p. 
  20), 
  Hackel 
  retains 
  Echinocystis 
  for 
  the 
  cystid. 
  To 
  save 
  further 
  confusion, 
  

   I 
  propose 
  the 
  name 
  Scolocystis 
  for 
  this 
  cystid 
  (from 
  <tkoj\os, 
  a 
  thorn). 
  

   Acanthocystis, 
  also 
  accepted 
  by 
  Hackel, 
  has 
  long 
  been 
  preoccupied 
  among 
  

   Protozoa 
  by 
  Carter. 
  Hall's 
  date 
  is 
  sometimes 
  given 
  as 
  1864 
  ; 
  some 
  advance 
  

   copies 
  of 
  the 
  Report 
  were 
  issued 
  in 
  1865, 
  but 
  it 
  was 
  not 
  published 
  until 
  1867. 
  

  

  