﻿Anatomy 
  o/Talseechinus 
  {Scolder), 
  M 
  l 
  Goy. 
  199 
  

  

  are 
  long 
  and 
  narrow, 
  and 
  are 
  composed 
  of 
  a 
  vast 
  number 
  of 
  

   small, 
  low, 
  thick 
  plates. 
  The 
  vertical 
  rows 
  of 
  pairs 
  of 
  pores 
  

   are 
  in 
  slightly 
  depressed 
  poriferous 
  zones, 
  the 
  interpori- 
  

   ferous 
  areas 
  are 
  slightly 
  convex 
  and 
  broad, 
  the 
  sutures 
  

   between 
  the 
  plates 
  are 
  usually 
  distinct. 
  The 
  surface 
  of 
  a 
  

   plate 
  of 
  some 
  size 
  is 
  granular 
  around 
  the 
  pair 
  of 
  pores 
  and 
  

   has 
  three 
  horizontal 
  rows 
  of 
  very 
  small, 
  distant, 
  primary 
  tu- 
  

   bercles, 
  which 
  have 
  a 
  flat 
  circular 
  scrobicule 
  and 
  a 
  small 
  boss 
  ; 
  

   granules 
  are 
  also 
  present 
  ; 
  the 
  minute 
  ornamentation 
  contrasts 
  

   with 
  the 
  dimensions 
  of 
  the 
  test. 
  The 
  ambulacral 
  plates 
  are 
  of 
  

   two 
  kinds 
  (a 
  and 
  b), 
  each 
  being 
  perforated 
  by 
  a 
  pair 
  of 
  pores 
  

   near 
  its 
  outer 
  end. 
  One 
  kind 
  of 
  plate 
  (a) 
  is 
  low, 
  broad, 
  thick, 
  

   occupies 
  much 
  of 
  the 
  interporiferous 
  surface 
  of 
  the 
  ambu- 
  

   lacrum, 
  and 
  is 
  perforated 
  by 
  a 
  pair 
  of 
  pores 
  not 
  very 
  close 
  to 
  

   the 
  ambulacro-interradial 
  suture 
  ; 
  it 
  is 
  a 
  plate 
  of 
  the 
  inner 
  

   vertical 
  series 
  of 
  pairs 
  of 
  pores. 
  These 
  plates 
  may 
  be 
  perfect 
  

   or 
  imperfect 
  primaries. 
  In 
  the 
  first 
  instance 
  (fig. 
  I., 
  a) 
  a 
  very 
  

   low 
  and 
  often 
  almost 
  linear 
  part 
  is 
  seen 
  external 
  to 
  the 
  pair 
  

   of 
  pores 
  and 
  reaching 
  the 
  reentering 
  angle 
  of 
  the 
  ambulacro- 
  

   interradial 
  suture. 
  The 
  low 
  part 
  is 
  so 
  small 
  that 
  it 
  permits 
  

   the 
  large-ended 
  plates 
  of 
  the 
  outer 
  vertical 
  series 
  of 
  pairs 
  of 
  

   pores 
  (b) 
  to 
  come 
  nearly 
  close 
  together. 
  In 
  the 
  other 
  instance 
  

   the 
  outer 
  linear 
  projection 
  of 
  the 
  plate 
  has 
  been 
  lost 
  (fig. 
  I., 
  a'), 
  

   and 
  the 
  plate 
  ends 
  in 
  a 
  point, 
  which 
  does 
  not 
  reach 
  the 
  

   edge 
  of 
  the 
  ambulacro-interradial 
  suture 
  ; 
  the 
  linear 
  part 
  has 
  

   been 
  jammed 
  out 
  by 
  the 
  increasing 
  growth 
  of 
  the 
  plates 
  of 
  the 
  

   outer 
  series. 
  (Fig. 
  I., 
  a, 
  perfect 
  primary, 
  «', 
  a 
  primary 
  blocked 
  

   out 
  from 
  the 
  ambulacro-interradial 
  suture.) 
  Both 
  kinds 
  of 
  

   plates 
  form 
  geometrical 
  figures 
  at 
  the 
  median 
  suture 
  of 
  the 
  

   ambulacrum, 
  are 
  highest 
  there, 
  and 
  their 
  horizontal 
  or 
  trans- 
  

   verse 
  sutures 
  are 
  distinct 
  for 
  one 
  third 
  or 
  one 
  half 
  of 
  the 
  

   distance 
  from 
  the 
  median 
  line 
  outwards. 
  The 
  existence 
  of 
  the 
  

   second 
  kind 
  of 
  ambulacral 
  plate 
  (fig. 
  I.,b), 
  which 
  carries 
  

   a 
  pair 
  of 
  pores 
  of 
  the 
  outer 
  vertical 
  series, 
  prevents 
  the 
  

   transverse 
  sutural 
  line 
  being 
  continuous 
  to 
  the 
  ambulacro- 
  

   interradial 
  edge. 
  

  

  The 
  second 
  kind 
  of 
  ambulacral 
  plate 
  (fig. 
  I., 
  b) 
  is 
  perforated 
  

   by 
  an 
  outer 
  pair 
  of 
  pores, 
  and 
  its 
  outer 
  edge 
  forms 
  the 
  

   salient 
  angle 
  at 
  the 
  suture 
  already 
  noticed. 
  The 
  plates 
  of 
  

   this 
  kind, 
  although 
  large 
  externally, 
  are 
  smaller 
  than 
  the 
  

   others, 
  are 
  placed 
  alternately 
  with 
  them, 
  are 
  narrow 
  in- 
  

   ternally, 
  and 
  do 
  not 
  reach 
  the 
  ambulacral 
  median 
  line. 
  They 
  

   are 
  low, 
  inward-pointed 
  demi-plates 
  with 
  a 
  large 
  outer 
  end, 
  

   where 
  the 
  pores 
  are. 
  The 
  outer 
  and 
  large 
  ends 
  of 
  these 
  demi- 
  

   plates 
  are 
  often 
  so 
  close 
  vertically 
  that, 
  as 
  has 
  been 
  already 
  

   noticed, 
  they 
  occlude 
  the 
  outer 
  process 
  of 
  some 
  primary 
  plates 
  

  

  