﻿Vol. 
  53.] 
  ECHINOCYSTIS 
  AND 
  PAL^IODISCUS. 
  123 
  

  

  9. 
  On 
  Echinocystis 
  and 
  Pal^odiscus 
  — 
  two 
  Silurian 
  Genera 
  of 
  

   Echinoidea. 
  By 
  J. 
  W. 
  Gregory, 
  D.Sc, 
  E.G.S., 
  F.Z.S., 
  Assist- 
  

   ant 
  in 
  the 
  Geological 
  Department, 
  British 
  Museum. 
  (Read 
  

   December 
  2nd, 
  1896.) 
  

  

  [Plate 
  VII.] 
  

  

  Contents. 
  Page> 
  

  

  I. 
  Introduction 
  123 
  

  

  II. 
  History 
  of 
  the 
  Genera 
  124 
  

  

  III. 
  The 
  Structure 
  of 
  Echinocystis 
  124 
  

  

  IV. 
  The 
  Affinities 
  of 
  Echinocystis 
  ... 
  126 
  

  

  V. 
  The 
  Structure 
  of 
  Palceodiseus 
  129 
  

  

  VI. 
  The 
  Affinities 
  of 
  PalcBodisc 
  us 
  130 
  

  

  VII. 
  The 
  Homologies 
  of 
  the 
  Masticatory 
  Pyramids 
  and 
  

  

  Apical 
  Plates 
  of 
  Echinoidea 
  131 
  

  

  VIII. 
  Classification 
  of 
  the 
  Cystocidaroida 
  132 
  

  

  IX. 
  Summary 
  of 
  Conclusions 
  133 
  

  

  X. 
  Literature 
  134 
  

  

  I. 
  Introduction. 
  

  

  The 
  persistence 
  with 
  which 
  the 
  Crinoidea, 
  Echinoidea, 
  and 
  Stel- 
  

   leroidea 
  have 
  maintained 
  their 
  essential 
  characters, 
  from 
  their 
  first 
  

   appearance 
  until 
  the 
  present 
  time, 
  has 
  been 
  often 
  remarked 
  and 
  

   has 
  received 
  very 
  different 
  explanations. 
  Thus, 
  Prof. 
  Poulton 
  l 
  [8, 
  

   pp. 
  507-509], 
  in 
  his 
  recent 
  address 
  to 
  the 
  Zoological 
  Section 
  of 
  the 
  

   British 
  Association, 
  deduces 
  from 
  the 
  fact 
  the 
  vast 
  duration 
  of 
  that 
  

   part 
  of 
  the 
  pre-Cambrian 
  era 
  during 
  which 
  the 
  earth 
  was 
  inhabited. 
  

   When 
  the 
  palaBontological 
  record 
  begins 
  in 
  the 
  Cambrian 
  period, 
  

   the 
  phylum 
  Echinoderma 
  was 
  already 
  in 
  existence. 
  It 
  was 
  repre- 
  

   sented 
  by 
  the 
  crinids, 
  cystids, 
  and 
  stellerids, 
  which 
  were 
  joined 
  in 
  

   the 
  Ordovician 
  by 
  echinids. 
  But, 
  in 
  spite 
  of 
  their 
  early 
  appearance, 
  

   the 
  oldest 
  known 
  members 
  of 
  the 
  surviving 
  classes 
  may 
  be 
  assigned 
  

   to 
  their 
  respective 
  divisions 
  as 
  readily 
  as 
  if 
  they 
  were 
  perfectly- 
  

   preserved 
  recent 
  specimens. 
  That 
  the 
  different 
  classes 
  of 
  echino- 
  

   derms 
  have 
  descended 
  from 
  a 
  common 
  ancestor 
  is 
  now 
  unquestioned 
  ; 
  

   but 
  the 
  search 
  for 
  missing 
  links 
  between 
  those 
  that 
  still 
  exist 
  has 
  

   not 
  been 
  attended 
  with 
  any 
  success. 
  The 
  two 
  recent 
  classifications 
  

   of 
  the 
  Echinoderma 
  by 
  Bell 
  [1] 
  and 
  Hiickel 
  [3, 
  p. 
  164] 
  unite 
  the 
  

   Stelleroidea 
  and 
  Echinoidea 
  into 
  one 
  section. 
  But 
  the 
  oldest 
  known 
  

   members 
  of 
  these 
  two 
  classes 
  resemble 
  one 
  another 
  less 
  than 
  do 
  

   some 
  of 
  their 
  later 
  representatives 
  ; 
  and 
  we 
  do 
  not 
  yet 
  know 
  a 
  

   single 
  intermediate 
  form 
  between 
  them. 
  Similarly, 
  Tiar 
  echinus, 
  

   which 
  of 
  all 
  Echinoidea 
  most 
  resembles 
  the 
  Crinoidea, 
  came 
  far 
  too 
  

   late 
  to 
  give 
  any 
  evidence 
  as 
  to 
  the 
  ancestry 
  of 
  either 
  class. 
  The 
  

   only 
  fossil 
  echinids 
  which 
  give 
  much 
  help 
  in 
  linking 
  their 
  class 
  with 
  

   any 
  of 
  the 
  others 
  are 
  the 
  Silurian 
  Echinocystis 
  and 
  Palceodiseus, 
  of 
  

   which 
  the 
  structure 
  is 
  imperfectly 
  known. 
  

  

  I 
  prepared 
  some 
  notes 
  on 
  both 
  genera 
  in 
  1888, 
  which 
  the 
  late 
  

  

  1 
  The 
  numerals 
  in 
  thick 
  type 
  throughout 
  this 
  paper 
  refer 
  to 
  the 
  biblio- 
  

   graphical 
  list 
  on 
  p. 
  134. 
  

  

  