﻿V°l- 
  53'] 
  PETALOGRAPTUS 
  AND 
  CBPHALOGRAPTTTS. 
  191 
  

  

  primordial 
  series 
  is 
  ^ 
  inch. 
  Exceptionally 
  it 
  is 
  as 
  much 
  as 
  -\- 
  inch. 
  

   Below 
  that 
  point 
  the 
  rhabdosoma 
  tapers 
  somewhat 
  abruptly. 
  

  

  Character 
  of 
  Mature 
  Rhabdosoma. 
  — 
  The 
  greatest 
  length 
  attained 
  

   by 
  the 
  mature 
  rhabdosoma 
  is 
  1 
  inch. 
  The 
  width 
  increases 
  rapidly 
  

   up 
  to 
  the 
  4th 
  theca 
  (4 
  1 
  ), 
  but 
  afterwards 
  the 
  increase 
  is 
  so 
  slight 
  

   and 
  so 
  gradual 
  that 
  the 
  sides 
  appear 
  approximately 
  parallel 
  for 
  a 
  

   greater 
  or 
  shorter 
  distance, 
  according 
  to 
  the 
  length 
  of 
  the 
  rhabdo- 
  

   soma. 
  The 
  maximum 
  width 
  attained 
  is 
  about 
  | 
  inch. 
  Some 
  forms 
  

   barely 
  attain 
  -^ 
  inch, 
  while 
  one 
  form 
  which 
  I 
  saw 
  in 
  Dr. 
  Tornquist's 
  

   collection 
  at 
  Lund 
  attained 
  its 
  maximum 
  width 
  opposite 
  theca 
  4 
  1 
  , 
  

   and 
  then 
  again 
  diminished 
  near 
  the 
  distal 
  end 
  : 
  the 
  resulting 
  form 
  

   resembling 
  that 
  variety 
  of 
  P. 
  jpalmeus 
  (Barr.) 
  to 
  which 
  Kurck 
  has 
  

   given 
  the 
  name 
  ovato-elongatus. 
  

  

  The 
  thecae 
  of 
  the 
  middle 
  of 
  the 
  rhabdosoma 
  are 
  slightly 
  longer 
  

   than 
  the 
  two 
  earliest 
  thecae 
  ; 
  they 
  are 
  about 
  J 
  inch 
  long, 
  distally 
  

   they 
  are 
  less, 
  and 
  at 
  the 
  actual 
  extremity 
  are 
  quite 
  short. 
  The 
  

   relation 
  between 
  thecal 
  length 
  and 
  breadth 
  is 
  as 
  6 
  : 
  1. 
  

  

  The 
  thecae 
  are 
  alternate 
  ; 
  they 
  are 
  tubular, 
  and 
  appear 
  to 
  be 
  

   square 
  in 
  section. 
  The 
  apertures 
  are 
  slightly 
  concave 
  ; 
  they 
  are 
  

   perpendicular 
  to 
  the 
  long 
  axis 
  of 
  the 
  theca, 
  and 
  oblique 
  to 
  the 
  

   general 
  direction 
  of 
  the 
  rhabdosoma, 
  except 
  at 
  the 
  distal 
  end. 
  They 
  

   are 
  often 
  striated 
  parallel 
  to 
  the 
  aperture 
  ; 
  these 
  striae 
  are 
  growth- 
  

   lines. 
  There 
  are 
  twenty-five 
  apertures 
  in 
  the 
  space 
  of 
  1 
  inch. 
  The 
  

   outer 
  wall 
  of 
  each 
  theca 
  is 
  free 
  for 
  a 
  small 
  fraction 
  of 
  its 
  length, 
  

   except 
  at 
  the 
  distal 
  extremity. 
  The 
  angle 
  at 
  which 
  the 
  thecae 
  are 
  

   inclined 
  to 
  a 
  median 
  line 
  varies 
  ; 
  it 
  is 
  about 
  5° 
  at 
  the 
  proximal 
  end, 
  

   it 
  then 
  increases 
  to 
  about 
  20° 
  in 
  the 
  middle 
  of 
  the 
  rhabdosoma, 
  and 
  

   again 
  decreases 
  to 
  5° 
  near 
  the 
  distal 
  end. 
  The 
  thecae 
  are 
  all 
  con- 
  

   cavely 
  curved, 
  but 
  not 
  equally 
  so. 
  The 
  curve 
  is 
  greatest 
  in 
  the 
  

   earliest 
  thecae, 
  but 
  subsequently 
  diminishes, 
  giving 
  to 
  the 
  whole 
  

   rhabdosoma 
  the 
  characteristic 
  foliate 
  appearance 
  to 
  which 
  it 
  owes 
  

   its 
  name. 
  

  

  The 
  appearance 
  of 
  rounding 
  off 
  at 
  the 
  distal 
  end 
  is 
  due 
  to 
  decrease 
  

   in 
  curvature 
  and 
  diminution 
  in 
  length 
  of 
  the 
  thecae 
  in 
  that 
  direction. 
  

   In 
  no 
  specimen 
  have 
  I 
  seen 
  any 
  indications 
  of 
  the 
  presence 
  of 
  a 
  

   septum, 
  and 
  I 
  certainly 
  do 
  not 
  think 
  that 
  one 
  can 
  ever 
  have 
  been 
  

   present. 
  One 
  reason 
  in 
  support 
  of 
  this 
  view 
  is 
  the 
  quite 
  irregular 
  

   course 
  taken 
  by 
  the 
  virgula 
  from 
  its 
  point 
  of 
  origin 
  at 
  the 
  sicula- 
  

   apex 
  to 
  the 
  point 
  at 
  which 
  it 
  emerges 
  from 
  the 
  rhabdosoma 
  at 
  the 
  

   distal 
  end. 
  This 
  irregularity 
  would 
  seem 
  to 
  point 
  to 
  its 
  having 
  

   been 
  perfectly 
  free 
  in 
  the 
  rhabdosoma, 
  and 
  I 
  cannot 
  believe 
  that 
  

   the 
  appearance, 
  which 
  is 
  the 
  same 
  in 
  many 
  differently 
  preserved 
  

   specimens, 
  is 
  merely 
  a 
  result 
  of 
  the 
  conditions 
  of 
  preservation. 
  The 
  

   virgula 
  is 
  often 
  distally 
  prolonged 
  for 
  a 
  considerable 
  length, 
  and 
  not 
  

   infrequently 
  is 
  split 
  at 
  one 
  or 
  more 
  points 
  along 
  its 
  length. 
  It 
  

   must 
  have 
  been 
  very 
  near 
  the 
  obverse 
  surface 
  of 
  the 
  rhabdosoma 
  

   for 
  at 
  least 
  the 
  earliest 
  part 
  of 
  its 
  course. 
  

  

  The 
  following 
  table 
  records 
  the 
  measurements 
  of 
  a 
  few 
  typical 
  

   forms 
  : 
  — 
  

  

  