﻿310 
  P. 
  H. 
  CARPENTER 
  ON 
  THE 
  RELATIONS 
  OE 
  

  

  " 
  arms'*' 
  of 
  this 
  type 
  as 
  recurrent 
  ambulacra 
  similar 
  to 
  those 
  of 
  the 
  

   anterolateral 
  rays. 
  

  

  One 
  very 
  striking 
  feature 
  about 
  these 
  ambulacra 
  is 
  the 
  great 
  

   variation 
  in 
  their 
  length 
  and 
  in 
  the 
  relative 
  position 
  of 
  their 
  distal 
  

   ends. 
  Thus 
  in 
  Prof. 
  Wetherby's 
  specimen 
  the 
  anterior 
  ambulacrum 
  

   (figs. 
  8, 
  11) 
  passes 
  down 
  over 
  the 
  surface 
  of 
  its 
  radial, 
  and 
  is 
  con- 
  

   tinued 
  onwards 
  almost 
  as 
  far 
  as 
  the 
  attachment 
  of 
  the 
  stem, 
  exactly 
  

   occupying 
  the 
  position 
  of 
  the 
  suture 
  between 
  the 
  two 
  anterior 
  basal 
  

   plates. 
  But 
  in 
  both 
  the 
  other 
  specimens 
  (iigs. 
  14, 
  17, 
  20, 
  22) 
  it 
  

   ends 
  on 
  the 
  radial 
  without 
  reachiug 
  the 
  basals 
  at 
  all. 
  In 
  the 
  one 
  

   it 
  terminates 
  almost 
  exactly 
  above 
  the 
  anterior 
  basal 
  suture 
  (figs. 
  

  

  20, 
  22), 
  while 
  iu 
  the 
  other 
  (figs. 
  14, 
  17) 
  it 
  is 
  slightly 
  displaced 
  

   towards 
  the 
  left 
  side. 
  

  

  The 
  two 
  anterolateral 
  ambulacra 
  (figs. 
  9, 
  11, 
  12 
  ; 
  15, 
  17, 
  18 
  ; 
  

  

  21, 
  22, 
  23) 
  pass 
  down 
  onto 
  the 
  basals 
  in 
  all 
  the 
  specimens, 
  just 
  as 
  

   described 
  by 
  Prof. 
  Wetherby 
  ; 
  but 
  they 
  vary 
  considerably 
  in 
  other 
  

   respects. 
  That 
  on 
  the 
  right 
  side 
  occupies 
  nearly 
  the 
  same 
  relative 
  

   position 
  in 
  Wetherby's 
  specimen 
  (figs. 
  11, 
  12) 
  as 
  in 
  Wachsmuth's 
  

   smaller 
  one 
  (figs. 
  22, 
  23). 
  In 
  both 
  cases 
  the 
  ambulacrum 
  bends 
  

   slightly 
  backwards 
  as 
  it 
  passes 
  over 
  the 
  radial, 
  though 
  in 
  tho 
  

   former 
  it 
  ends 
  rather 
  further 
  behind 
  the 
  suture 
  of 
  the 
  right 
  and 
  

   right 
  anterior 
  basals 
  than 
  it 
  does 
  in 
  the 
  latter. 
  In 
  Wachsmuth's 
  

   larger 
  specimen, 
  however, 
  (figs. 
  17, 
  18) 
  the 
  backward 
  curvature 
  is 
  

   stronger 
  and 
  more 
  continuous, 
  very 
  much 
  as 
  in 
  Prof. 
  Wetherby's 
  

   fig. 
  la. 
  It 
  does 
  not 
  pass 
  into 
  a 
  forward 
  curve 
  again 
  ; 
  and 
  the 
  

   suture 
  between 
  the 
  subjacent 
  basals 
  is 
  quite 
  obscure, 
  so 
  that 
  the 
  

   base 
  appears 
  to 
  consist 
  of 
  four 
  unequal 
  plates. 
  

  

  In 
  the 
  characters 
  of 
  the 
  left 
  anterior 
  ambulacrum, 
  on 
  the 
  other 
  

   hand, 
  Prof. 
  Wetherby's 
  specimen 
  (figs. 
  9, 
  11) 
  agrees 
  with 
  Mr. 
  

   Wachsmuth's 
  larger 
  one 
  (iigs. 
  1 
  5, 
  17). 
  In 
  each 
  case 
  the 
  ambula- 
  

   crum 
  comes 
  down 
  onto 
  the 
  left 
  posterior 
  basal 
  ; 
  but 
  it 
  is 
  distinctly 
  

   longer 
  in 
  Wetherby's 
  specimen, 
  and 
  has 
  a 
  trace 
  of 
  a 
  forward 
  curve, 
  

   which 
  is 
  absent 
  in 
  the 
  other. 
  In 
  Wachsmuth's 
  smaller 
  specimen 
  

   (figs. 
  21, 
  22), 
  however, 
  this 
  ambulacrum 
  comes 
  right 
  down 
  onto 
  the 
  

   suture 
  between 
  the 
  two 
  left 
  basals, 
  so 
  as 
  to 
  conceal 
  all 
  but 
  a 
  trace 
  

   at 
  its 
  central 
  end. 
  

  

  Passing 
  now 
  to 
  the 
  right 
  posterior 
  ambulacrum, 
  we 
  find 
  another 
  

   resemblance 
  between 
  Wetherby's 
  specimen 
  (figs. 
  12, 
  13) 
  and 
  Wachs- 
  

   muth's 
  smaller 
  one 
  (figs. 
  23, 
  24) 
  in 
  the 
  ambulacrum 
  passing 
  from 
  

   the 
  surface 
  of 
  the 
  small 
  radial 
  onto 
  the 
  azygos 
  plate, 
  This 
  appears 
  

   to 
  me 
  to 
  support 
  the 
  view 
  that 
  this 
  plate 
  in 
  the 
  analogous 
  form 
  

   Hybocrinus 
  really 
  belongs 
  to 
  the 
  radial 
  series, 
  and 
  is 
  not 
  to 
  be 
  

   regarded 
  as 
  an 
  anal 
  plate 
  like 
  the 
  smaller 
  one 
  supported 
  upon 
  it. 
  

   The 
  ambulacrum 
  is 
  slightly 
  longer 
  in 
  Wetherby's 
  specimen 
  ; 
  but 
  

   in 
  both 
  cases 
  it 
  is 
  visible 
  in 
  a 
  view 
  of 
  the 
  calyx 
  from 
  below 
  

   (figs. 
  11, 
  22). 
  In 
  Wachsmuth's 
  larger 
  specimen, 
  however, 
  it 
  does 
  

   not 
  extend 
  onto 
  the 
  azygos 
  plate 
  at 
  all 
  (figs. 
  18, 
  19), 
  and 
  is 
  there- 
  

   fore 
  invisible 
  in 
  a 
  dorsal 
  view 
  (fig. 
  17). 
  

  

  Tho 
  left 
  posterior 
  ambulacrum 
  is 
  also 
  invisible 
  from 
  below, 
  

   though 
  it 
  does 
  extend 
  downwards 
  for 
  a 
  short 
  distance 
  onto 
  its 
  

  

  