﻿130 
  DR. 
  J. 
  W. 
  GREGORY 
  ON 
  [Feb. 
  I 
  81,7, 
  

  

  Interambulacra 
  of 
  angular, 
  irregular, 
  scale-like 
  plates 
  in 
  

   many 
  series 
  ; 
  at 
  the 
  ambitus 
  there 
  are 
  ten 
  in 
  width 
  in 
  each 
  inter- 
  

   radius. 
  

  

  The 
  interambulacral 
  plates 
  bear 
  numerous 
  short 
  spines. 
  

  

  Apical 
  plates 
  unknown, 
  probably 
  absent. 
  The 
  periproctal 
  area 
  

   is 
  large. 
  Madreporite 
  unknown. 
  

  

  Jaws. 
  — 
  According 
  to 
  Salter's 
  figure 
  [9, 
  pi. 
  ix. 
  fig. 
  6ft], 
  the 
  oral 
  

   armament 
  apparently 
  consists 
  of 
  an 
  oral 
  ring 
  of 
  the 
  ainbulacral 
  

   type, 
  that 
  is, 
  the 
  ambulacra! 
  constituents 
  form 
  

  

  the 
  prominent 
  projections. 
  Specimens 
  in 
  the 
  Fig. 
  5. 
  — 
  Pair 
  of 
  

   British 
  Museum, 
  however 
  (such 
  as 
  40,307), 
  half-pyramids 
  of 
  

   show 
  that 
  the 
  masticatory 
  apparatus 
  is 
  verj- 
  Paleeodiscus. 
  

   different 
  from 
  that 
  of 
  stellerids. 
  It 
  consists 
  

   (PL 
  VII. 
  fig. 
  5 
  b) 
  of 
  five 
  pairs 
  of 
  irregularly 
  

   triangular 
  plates 
  ; 
  those 
  of 
  each 
  pair 
  are 
  

   joined 
  together 
  proximally, 
  but 
  are 
  free 
  at 
  

   the 
  distal 
  (i. 
  e. 
  the 
  oral) 
  ends, 
  where 
  they 
  are 
  

   separated 
  by 
  a 
  deep 
  cleft. 
  The 
  distal 
  ends 
  are 
  

   expanded 
  (fig. 
  5). 
  According 
  to 
  Salter, 
  some 
  spines 
  are 
  situated 
  at 
  

   the 
  apex 
  of 
  each 
  half-pyramid. 
  

  

  VI. 
  The 
  Affinities 
  oe 
  Paleeodiscus. 
  

  

  The 
  previous 
  discussion 
  as 
  to 
  the 
  affinities 
  of 
  Echinocystis 
  has 
  

   cleared 
  the 
  way 
  for 
  the 
  consideration 
  of 
  those 
  of 
  Paleeodiscus, 
  which 
  

   in 
  many 
  respects 
  is 
  the 
  simpler 
  form. 
  It 
  differs 
  still 
  more 
  markedly 
  

   from 
  the 
  typical 
  echinids, 
  but 
  it 
  is 
  even 
  less 
  like 
  a 
  cystid, 
  and 
  the 
  

   genus 
  is 
  usually 
  assigned 
  to 
  the 
  Stelleroidea, 
  as 
  by 
  Neumayr, 
  

   Zittel, 
  and 
  Wright. 
  

  

  Paleeodiscus 
  resembles 
  the 
  stellerids 
  in 
  two 
  important 
  respects. 
  

   In 
  the 
  first 
  place, 
  the 
  ambulacra 
  appear 
  in 
  most 
  specimens 
  to 
  be 
  

   limited 
  to 
  the 
  oral 
  surface 
  ; 
  but 
  the 
  case 
  is 
  not 
  clear, 
  for 
  if 
  Wyville 
  

   Thomson's 
  figures 
  be 
  correct, 
  the 
  genus 
  is 
  truly 
  desmactinic. 
  

   Secondly, 
  the 
  ambulacral 
  plates 
  are 
  not 
  perforated 
  by 
  pores, 
  at 
  least 
  

   on 
  the 
  oral 
  surface 
  ; 
  but 
  in 
  the 
  echinia* 
  Asthenosoma 
  some 
  of 
  the 
  

   podia 
  pass 
  out 
  through 
  the 
  sutures 
  between 
  the 
  plates, 
  for 
  their 
  

   klasma-plates 
  (see 
  ante, 
  p. 
  115) 
  are 
  reduced 
  to 
  spicular 
  rings. 
  

  

  Fortunately, 
  however, 
  the 
  fundamental 
  difference 
  between 
  the 
  

   Echinoidea 
  and 
  Stelleroidea 
  can 
  generally 
  be 
  applied 
  : 
  in 
  the 
  former 
  

   the 
  ambulacral 
  plates 
  cover 
  the 
  radial 
  water-vascular 
  vessel, 
  which 
  

   is 
  within 
  the 
  test, 
  whereas 
  in 
  the 
  stellerids 
  this 
  vessel 
  lies 
  in 
  a 
  groove 
  

   on 
  the 
  external 
  side 
  of 
  the 
  ambulacral 
  plates. 
  In 
  Paleeodiscus 
  the 
  

   ambulacral 
  plates 
  are 
  flush 
  with 
  those 
  of 
  the 
  interradii, 
  and 
  in 
  all 
  

   probability 
  the 
  radial 
  water-vascular 
  vessel 
  was 
  within 
  the 
  test. 
  

   The 
  general 
  characters 
  of 
  the 
  skeleton 
  are 
  also 
  more 
  echinoid 
  than 
  

   asteroid 
  ; 
  the 
  absence 
  of 
  special 
  adambulacral 
  plates, 
  the 
  occurrence 
  

   of 
  small 
  articulating 
  spines, 
  the 
  resemblance 
  of 
  the 
  interradial 
  

   perisomatic 
  plates 
  to 
  those 
  of 
  Echinocystis, 
  and 
  the 
  character 
  of 
  the 
  

   masticatory 
  apparatus, 
  are 
  all 
  points 
  which 
  ally 
  Paleeodiscus 
  to 
  the 
  

   Echinoidea 
  and 
  separate 
  it 
  from 
  the 
  Stelleroidea. 
  

  

  