﻿Vol. 
  53.] 
  ECHINOCYSTIS 
  AND 
  PAL.EODISCUS. 
  131 
  

  

  We 
  must, 
  however, 
  compare 
  this 
  genus 
  with 
  the 
  Cystoidea, 
  for 
  

   there 
  is 
  a 
  considerable 
  agreement 
  in 
  some 
  respects 
  between 
  it 
  and 
  

   Agelacrinus. 
  Palceodiscus, 
  is 
  not 
  aborally 
  fixed 
  ; 
  it 
  has 
  no 
  pinnules, 
  

   and, 
  in 
  all 
  probability, 
  it 
  had 
  no 
  calycinal 
  plates. 
  Of 
  the 
  nine 
  

   characters 
  which 
  Hackel 
  assigns 
  to 
  the 
  Cystoidea, 
  Palceodiscus 
  has 
  

   only 
  such 
  as 
  are 
  common 
  to 
  the 
  Echinoidea. 
  

  

  The 
  only 
  reason 
  for 
  regarding 
  it 
  as 
  a 
  cystid 
  is 
  based 
  on 
  its 
  

   resemblance 
  to 
  Agelacrinus, 
  which, 
  however, 
  is 
  merely 
  superficial. 
  

   In 
  Agelacrinus 
  the 
  anus 
  is 
  on 
  the 
  oral 
  surface 
  ; 
  the 
  ambulacra 
  are 
  

   limited 
  to 
  part 
  of 
  the 
  oral 
  surface 
  alone 
  ; 
  there 
  is 
  no 
  jaw-apparatus 
  ; 
  

   the 
  ambulacra 
  are 
  sinuous, 
  and 
  there 
  is 
  a 
  peripheral 
  zone 
  of 
  plates 
  

   round 
  the 
  disc. 
  

  

  Hiickel 
  is 
  probably 
  correct 
  in 
  regarding 
  Agelacrinus 
  l 
  as 
  a 
  flattened 
  

   form 
  of 
  Hemicystis, 
  in 
  which 
  greater 
  extension 
  of 
  the 
  pinnule- 
  

   bearing 
  grooves 
  is 
  obtained 
  by 
  their 
  being 
  twisted. 
  Hemicystis 
  has 
  

   no 
  resemblance 
  to 
  a 
  stellerid 
  ; 
  and 
  to 
  accept 
  this 
  view 
  of 
  the 
  origin 
  

   of 
  Agelacrinus 
  means 
  the 
  abandonment 
  of 
  any 
  fundamental 
  affinity 
  

   between 
  it 
  and 
  Palceodiscus. 
  

  

  There 
  is, 
  however, 
  one 
  genus, 
  which, 
  if 
  correctly 
  described 
  by 
  

   Worthenand 
  Miller[13,p. 
  335], 
  maybe 
  a 
  Carboniferous 
  representative 
  

   of 
  either 
  Palceodiscus 
  or 
  Echinocystis. 
  According 
  to 
  the 
  description 
  

   of 
  its 
  authors, 
  this 
  remarkable 
  form 
  has 
  an 
  irregular, 
  sac-like 
  body 
  ; 
  

   above 
  the 
  mouth 
  there 
  is 
  a 
  series 
  of 
  plates 
  which 
  may 
  represent 
  

   jaws. 
  The 
  name 
  Echinodiscus 
  was 
  given 
  to 
  this 
  fossil 
  by 
  Worthen 
  

   and 
  Miller, 
  but 
  this 
  term 
  having 
  been 
  preoccupied 
  by 
  Agassiz 
  for 
  an 
  

   echinid, 
  for 
  which 
  it 
  is 
  still 
  in 
  use, 
  the 
  name 
  of 
  the 
  cystid 
  may 
  be 
  

   changed 
  to 
  Discocystis. 
  

  

  VII. 
  The 
  Homologies 
  of 
  the 
  Masticatory 
  Pyramids 
  and 
  

   Apical 
  Plates 
  of 
  Echinoidea. 
  

  

  From 
  the 
  considerations 
  stated 
  in 
  the 
  previous 
  section, 
  it 
  

   seems 
  advisable 
  to 
  regard 
  Palosodiscus 
  as 
  an 
  echinid, 
  allied 
  to 
  

   Echinocystis, 
  although 
  not 
  congeneric 
  as 
  Duncan 
  made 
  it. 
  In 
  

   this 
  case 
  it 
  is 
  certainly 
  the 
  most 
  primitive 
  of 
  Echinoidea, 
  and 
  

   represents 
  a 
  condition 
  when 
  the 
  ambulacral 
  plates 
  were 
  only 
  in 
  part 
  

   perforated 
  by 
  pores. 
  The 
  presence 
  of 
  a 
  masticatory 
  apparatus 
  is 
  of 
  

   interest, 
  especially 
  as 
  it 
  gives 
  us 
  some 
  suggestions 
  regarding 
  the 
  

   origin 
  of 
  that 
  structure. 
  The 
  figures 
  published 
  by 
  Loven 
  and 
  his 
  

   accompanying 
  remarks 
  [5, 
  pp. 
  8 
  & 
  9, 
  pi. 
  iii. 
  figs. 
  22 
  & 
  23], 
  show 
  that 
  

   while 
  the 
  teeth 
  are 
  interradial 
  in 
  development, 
  the 
  plates 
  that 
  form 
  

   the 
  half-pyramids 
  lie 
  above 
  the 
  ambulacra, 
  and 
  are 
  to 
  be 
  regarded 
  

   as 
  ambulacral 
  in 
  origin. 
  In 
  Echinocystis 
  the 
  pyramids 
  agree 
  with 
  

   those 
  of 
  Cidaris, 
  at 
  least 
  in 
  their 
  fundamental 
  characters 
  ; 
  but 
  in 
  

   Palceodiscus 
  the 
  structure 
  is 
  far 
  more 
  primitive. 
  Beside 
  the 
  oral 
  

   ends 
  of 
  the 
  ambulacra 
  lie 
  two 
  strong 
  plates, 
  which 
  are 
  fixed 
  together 
  

   aborally, 
  but 
  are 
  separated 
  by 
  a 
  deep 
  groove 
  near 
  the 
  mouth. 
  

  

  1 
  The 
  type-species 
  of 
  Ayelacrinits 
  is 
  A. 
  hamiUonensis, 
  Vanux. 
  (Geol. 
  Rep. 
  

   New 
  York, 
  1842, 
  pp. 
  158, 
  306), 
  nut 
  A. 
  voriicdlatus, 
  Hall, 
  as 
  stated 
  by 
  Hiickel. 
  

   The 
  genus 
  Agelacystis 
  founded 
  [Hackel, 
  3, 
  p. 
  114] 
  on. 
  A. 
  hamiUonensis, 
  is 
  there- 
  

   fore 
  a 
  synonym 
  of 
  Agelacrinus. 
  

  

  