﻿194 
  MISS 
  G. 
  L. 
  ELLES 
  ON 
  THE 
  SUBGENERA 
  [May 
  1 
  89 
  7, 
  

  

  series. 
  The 
  sicula 
  is 
  very 
  long* 
  relatively 
  to 
  the 
  length 
  of 
  the 
  thecae, 
  

   compared 
  with 
  P. 
  folium 
  (His.). 
  It 
  is 
  furnished 
  with 
  an 
  apertural 
  

   spine, 
  which 
  seems 
  to 
  have 
  been 
  particularly 
  stout 
  and 
  strong 
  in 
  the 
  

   forms 
  which 
  lived 
  in 
  the 
  Gala-Tarannon 
  times, 
  but 
  which 
  is 
  also 
  

   found 
  in 
  the 
  forms 
  of 
  other 
  horizons. 
  All 
  the 
  Gala-Tarannon 
  

   specimens 
  have 
  the 
  spine 
  exceedingly 
  well-preserved, 
  even 
  when 
  

   the 
  fossils 
  are 
  not 
  very 
  clear 
  in 
  other 
  respects. 
  But 
  in 
  the 
  case 
  of 
  

   forms 
  occurring 
  at 
  lower 
  horizons 
  the 
  spine 
  seems 
  to 
  have 
  been 
  broken 
  

   off 
  altogether 
  in 
  some 
  specimens, 
  while 
  in 
  others 
  only 
  a 
  small 
  

   portion 
  of 
  it 
  remains. 
  

  

  The 
  first 
  theca 
  of 
  the 
  primordial 
  series 
  arises 
  at 
  the 
  base 
  of 
  the 
  

   sicula 
  and 
  grows 
  outward 
  almost 
  at 
  once, 
  making 
  a 
  very 
  decided 
  

   concave 
  curve. 
  There 
  appears 
  to 
  be 
  no 
  downward 
  growth 
  in 
  this 
  

   case 
  below 
  the 
  base 
  of 
  the 
  sicula. 
  The 
  second 
  theca 
  (l 
  2 
  ) 
  arises 
  very 
  

   much 
  as 
  in 
  P. 
  folium 
  (His.) 
  except 
  that 
  the 
  curvature 
  is 
  greater 
  and 
  

   takes 
  place 
  sooner 
  than 
  in 
  that 
  species, 
  so 
  that 
  the 
  sicula 
  is 
  not 
  free 
  

   for 
  so 
  large 
  a 
  fraction 
  of 
  its 
  length 
  on 
  the 
  right 
  side. 
  Viewing 
  the 
  

   rhabdosoma 
  from 
  the 
  obverse 
  side, 
  the 
  sicula 
  is 
  free 
  for 
  rather 
  less 
  

   than 
  2j 
  of 
  its 
  length 
  ; 
  on 
  the 
  reverse 
  side 
  only 
  the 
  base 
  is 
  seen 
  to 
  the 
  

   left, 
  the 
  rest 
  being 
  concealed 
  by 
  the 
  connecting-canal, 
  the 
  initial 
  

   part 
  of 
  theca 
  l 
  2 
  , 
  and 
  subsequent 
  thecal 
  bases. 
  The 
  theca 
  l 
  2 
  here 
  

   follows 
  the 
  outline 
  of 
  the 
  sicula 
  for 
  a 
  short 
  distance 
  before 
  curving 
  

   outward, 
  so 
  that 
  less 
  of 
  the 
  sicula 
  is 
  seen 
  on 
  the 
  reverse 
  than 
  on 
  the 
  

   obverse 
  side. 
  The 
  two 
  earliest 
  thecae 
  are 
  rather 
  less 
  than 
  y 
  1 
  ^ 
  inch 
  

   in 
  length, 
  and 
  are 
  equally 
  curved, 
  giving 
  a 
  symmetrical 
  appearance 
  

   to 
  the 
  proximal 
  end. 
  Subsequent 
  thecae 
  are 
  not 
  so 
  much 
  curved 
  as- 
  

   the 
  earliest 
  ones. 
  The 
  angle 
  at 
  which 
  the 
  thecae 
  are 
  inclined 
  is 
  

   approximately 
  constant 
  in 
  the 
  narrower 
  forms 
  at 
  35°, 
  but 
  in 
  the 
  

   wider 
  forms 
  the 
  angle 
  becomes 
  steeper 
  towards 
  the 
  distal 
  end, 
  though 
  

   it 
  never 
  diminishes 
  below 
  20°. 
  Taken 
  as 
  a 
  whole, 
  the 
  proximal 
  end 
  

   may 
  be 
  said 
  to 
  be 
  short 
  and 
  blunt. 
  

  

  Character 
  of 
  Mature 
  Rhabdosoma. 
  — 
  The 
  mature 
  rhabdosoma 
  gene- 
  

   rally 
  attains 
  the 
  length 
  of 
  about 
  1 
  inch, 
  but 
  in 
  some 
  specimens 
  has 
  

   been 
  known 
  to 
  be 
  as 
  much 
  as 
  1J 
  inch 
  long. 
  The 
  maximum 
  width 
  

   is 
  attained 
  almost 
  at 
  once 
  ; 
  it 
  varies 
  between 
  -j- 
  1 
  ^- 
  and 
  ^ 
  inch. 
  The 
  

   sides 
  are 
  parallel. 
  This 
  is 
  noticeable 
  even 
  in 
  quite 
  young 
  forms,, 
  

   which 
  maintain 
  a 
  constant 
  width 
  right 
  up 
  to 
  the 
  distal 
  end. 
  That 
  

   end 
  may 
  be 
  truncate 
  or 
  slightly 
  rounded. 
  

  

  The 
  thecae 
  are 
  alternate 
  ; 
  they 
  consist 
  of 
  short 
  tubes 
  three 
  times 
  

   as 
  long 
  as 
  wide, 
  whose 
  outer 
  wall 
  is 
  free 
  for 
  a 
  small 
  fraction 
  of 
  its 
  

   length 
  near 
  the 
  distal 
  end, 
  but 
  at 
  the 
  proximal 
  end 
  the 
  tubes 
  are 
  often 
  

   in 
  contact 
  throughout 
  their 
  length. 
  The 
  thecae 
  are 
  adorned 
  with, 
  

   lines 
  of 
  growth 
  parallel 
  to 
  the 
  aperture. 
  The 
  apertures 
  are 
  slightly 
  

   concave, 
  and 
  are 
  perpendicular 
  to 
  the 
  long 
  axis 
  of 
  the 
  thecae 
  ; 
  they 
  

   overlap 
  somewhat, 
  at 
  the 
  proximal 
  end, 
  but 
  this 
  diminishes 
  distally. 
  

   There 
  are 
  32 
  thecal 
  apertures 
  in 
  the 
  space 
  of 
  an 
  inch. 
  

  

  There 
  appears 
  to 
  have 
  been 
  a 
  continuous 
  septum 
  through 
  the 
  

   rhabdosoma. 
  The 
  virgula 
  is, 
  as 
  a 
  rule, 
  very 
  conspicuous 
  in 
  all 
  

   specimens 
  ; 
  it 
  is 
  usually 
  prolonged 
  distally 
  and 
  is 
  often 
  split 
  at 
  

  

  