﻿120 
  DK. 
  J. 
  W. 
  GREGORY 
  ON 
  THE 
  [Feb. 
  1 
  897, 
  

  

  at 
  ambitus, 
  5 
  millim. 
  ; 
  width 
  of 
  interambulacral 
  plates 
  in 
  each 
  

   series, 
  7 
  millim. 
  

  

  Distribution. 
  — 
  Weisser 
  Jura, 
  Germany. 
  Type, 
  B. 
  M., 
  no. 
  34,724. 
  

  

  Affinities. 
  — 
  This 
  specimen 
  was 
  sent 
  to 
  the 
  Museum 
  labelled 
  Diplo- 
  

   cidaris. 
  Its 
  deep 
  branchial 
  slits 
  and 
  compound 
  ambulacra! 
  plates 
  

   show 
  that 
  it 
  cannot 
  belong 
  to 
  the 
  order 
  Cidarida. 
  There 
  can 
  be 
  no 
  

   doubt 
  that 
  it 
  is 
  a 
  member 
  of 
  the 
  order 
  Diademoida, 
  for 
  the 
  genus 
  is 
  

   regular 
  and 
  ectobranchiatc. 
  It 
  is 
  distinguished 
  from 
  the 
  suborder 
  

   Calycinae 
  (including 
  the 
  Saleniidae 
  and 
  Acrosaleniidae), 
  in 
  the 
  absence 
  

   of 
  knowledge 
  of 
  the 
  structure 
  of 
  the 
  apical 
  system, 
  by 
  the 
  com- 
  

   plexity 
  of 
  the 
  ambulacral 
  plates. 
  Prom 
  the 
  suborders 
  Arbacina 
  and 
  

   Echinina 
  it 
  differs 
  in 
  having 
  the 
  ambulacral 
  plates 
  diademoid, 
  

   instead 
  of 
  on 
  the 
  arbaeioid 
  or 
  echinoid 
  type. 
  The 
  genus 
  falls 
  easily 
  

   into 
  the 
  suborder 
  Diademina, 
  owing 
  to 
  the 
  character 
  of 
  the 
  ambu- 
  

   lacral 
  plates. 
  In 
  this 
  suborder 
  there 
  are 
  six 
  families 
  ; 
  of 
  which 
  

   the 
  Orthopsidae 
  have 
  simple 
  ambulacral 
  plates 
  ; 
  the 
  Diadematida? 
  

   and 
  the 
  Diplopedinidae 
  respectively 
  have 
  regularly 
  uniserial 
  and 
  

   biserial 
  pore-pairs 
  ; 
  the 
  Cyphosomatidae 
  have 
  the 
  pore-pairs 
  in 
  high 
  

   curved 
  arcs. 
  This 
  leaves 
  only 
  the 
  two 
  families 
  of 
  the 
  Pedinidae 
  and 
  

   Echinothuridae, 
  with 
  both 
  of 
  which 
  the 
  genus 
  agrees 
  in 
  some 
  

   respects. 
  Thus 
  the 
  ambulacral 
  plates 
  may 
  consist 
  of 
  a 
  central 
  

   primary 
  and 
  two 
  demi-plates, 
  one 
  or 
  both 
  of 
  which 
  may 
  be 
  further 
  

   reduced 
  to 
  klasma-plates. 
  The 
  plates 
  just 
  below 
  the 
  ambitus 
  of 
  

   Pediiwthuria 
  agree 
  exactly 
  with 
  those 
  of 
  Asthenosoma, 
  except 
  that 
  

   they 
  are 
  thicker 
  and 
  solidly 
  united 
  to 
  their 
  fellows. 
  They 
  are 
  more 
  

   of 
  the 
  echinothurid 
  type 
  than 
  of 
  that 
  of 
  Pedina. 
  Nevertheless, 
  

   owing 
  to 
  the 
  rigidity 
  of 
  the 
  test, 
  and 
  the 
  unituberculate, 
  cidaroid 
  

   character 
  of 
  the 
  interambulacral 
  plates, 
  it 
  appears 
  advisable 
  to 
  

   include 
  the 
  genus 
  in 
  the 
  family 
  Pedinidae. 
  

  

  The 
  exact 
  horizon 
  whence 
  the 
  fossil 
  came 
  is 
  unfortunately 
  

   unknown. 
  It 
  is 
  simply 
  labelled 
  ' 
  Weisser 
  Jura, 
  Germany.' 
  The 
  

   test 
  has 
  apparently 
  been 
  washed 
  in 
  weak 
  acid, 
  and 
  thus 
  it 
  may 
  not 
  be 
  

   safe 
  to 
  guess 
  its 
  horizon 
  from 
  its 
  appearance. 
  But, 
  as 
  far 
  as 
  I 
  can 
  

   judge, 
  it 
  probably 
  came 
  from 
  either 
  Western 
  Bavaria 
  or 
  Wiirttein- 
  

   berg, 
  and 
  from 
  the 
  horizon 
  y 
  of 
  the 
  Weisser 
  Jura. 
  In 
  that 
  case 
  it 
  

   would 
  be 
  older 
  than 
  the 
  earliest 
  echinothurid, 
  which 
  lived 
  in 
  the 
  

   succeeding 
  stage 
  of 
  the 
  Weisser 
  Jura 
  e. 
  

  

  As 
  the 
  main 
  difference 
  between 
  Pedinothuria 
  and 
  PelanecMnus 
  is 
  

   that 
  the 
  latter 
  had 
  a 
  somewhat 
  flexible 
  test, 
  it 
  is 
  advisable 
  to 
  

   consider 
  the 
  value 
  of 
  this 
  character. 
  The 
  evidence 
  of 
  Astropyya 
  

   and 
  of 
  the 
  deep-sea 
  spatangids, 
  such 
  as 
  Calymne 
  and 
  CystecMnus, 
  

   shows 
  that 
  flexibility 
  is 
  not 
  limited 
  to 
  a 
  single 
  family, 
  and 
  may 
  be 
  

   independently 
  acquired; 
  it 
  is 
  probably 
  due 
  to 
  changes 
  of 
  environ- 
  

   ment 
  leading 
  to 
  diminished 
  calcification 
  of 
  the 
  test. 
  We 
  must 
  

   therefore 
  be 
  prepared 
  to 
  meet 
  with 
  imbricating-plates 
  in 
  any 
  group 
  

   of 
  echinids. 
  

  

  In 
  various 
  Mesozoic 
  rocks 
  there 
  occur 
  isolated 
  plates, 
  with 
  

   truncated 
  margins, 
  which, 
  if 
  imbrication 
  and 
  flexibility 
  were 
  

   limited 
  to 
  the 
  Echinothuridae, 
  would 
  have 
  to 
  be 
  referred 
  to 
  that 
  

  

  