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  53'] 
  PETALOGRAPTUS 
  AND 
  CEPHALOGRAPTUS. 
  209 
  

  

  IV. 
  General 
  Conclusions. 
  

  

  From 
  the 
  descriptions 
  that 
  have 
  been 
  given, 
  there 
  can, 
  I 
  think, 
  be 
  

   no 
  doubt 
  that 
  the 
  forms 
  referable 
  to 
  the 
  subgenera 
  Petalograptus 
  

   and 
  Cephalograptus 
  differ 
  from 
  those 
  more 
  common 
  forms 
  of 
  Diplo- 
  

   graptus, 
  included 
  in 
  Lap 
  worth's 
  subgenera 
  Orthograptus 
  and 
  

   Glyptograptus, 
  which 
  we 
  are 
  accustomed 
  to 
  regard 
  as 
  typical 
  of 
  the 
  

   genus. 
  The 
  structure 
  of 
  the 
  proximal 
  end 
  of 
  such 
  forms 
  has 
  been 
  

   described 
  in 
  detail 
  by 
  Dr. 
  Wiman 
  (' 
  Ueber 
  Diplograptidse 
  '). 
  

  

  Orthograptus 
  foliaceus 
  (Murch.) 
  and 
  Orthograptus 
  truncatus 
  

   (Lapw.) 
  fall 
  into 
  such 
  a 
  class, 
  and 
  the 
  external 
  resemblance 
  of 
  

   these 
  species 
  to 
  some 
  of 
  the 
  Petalograpti 
  is 
  evident 
  at 
  a 
  glance. 
  

   In 
  these 
  forms 
  the 
  proximal 
  end 
  is 
  not 
  protracted, 
  and 
  the 
  sicula 
  is 
  

   not 
  completely 
  visible 
  in 
  either 
  aspect 
  of 
  the 
  rhabdosoma, 
  but 
  the 
  

   characters 
  of 
  the 
  thecse 
  are 
  strikingly 
  similar 
  to 
  those 
  of 
  Petalo- 
  

   graptus 
  palmeus 
  (restr.). 
  I 
  regard 
  this 
  as 
  significant 
  ; 
  the 
  differences 
  

   separating 
  the 
  forms 
  can 
  so 
  easily 
  be 
  brought 
  about 
  by 
  very 
  slight 
  

   alteration 
  in 
  the 
  mode 
  of 
  development 
  that 
  I 
  cannot 
  refrain 
  from 
  

   offering 
  the 
  suggestion 
  that 
  the 
  Petalograpti 
  have 
  been 
  derived 
  

   from 
  Orthograptus 
  foliaceus, 
  and 
  that 
  Orthograptus 
  truncatus 
  is 
  

   merely 
  a 
  step 
  on 
  the 
  way. 
  

  

  In 
  0. 
  truncatus 
  (Lapw.) 
  the 
  canal 
  connecting 
  l 
  1 
  with 
  l 
  2 
  is 
  

   almost 
  horizontal, 
  so 
  that 
  the 
  theca 
  l 
  2 
  arises 
  at 
  nearly 
  the 
  same 
  

   level 
  as 
  l. 
  1 
  If 
  the 
  connecting-canal 
  became 
  more 
  oblique, 
  and 
  the 
  

   thecse 
  more 
  concavely 
  curved, 
  a 
  form 
  identical 
  with 
  P. 
  palmeus 
  

   (restr.) 
  would 
  be 
  the 
  result. 
  If 
  the 
  theca 
  l 
  2 
  arises 
  still 
  later, 
  and 
  

   the 
  thecse 
  at 
  first 
  grow 
  parallel 
  with 
  the 
  sicula 
  and 
  then 
  curve, 
  the 
  

   resulting 
  form 
  is 
  identical 
  with 
  Petalograptus 
  minor 
  (sp. 
  nov.). 
  

   The 
  thecse 
  in 
  this 
  form 
  are 
  longer. 
  

  

  A 
  further 
  increase 
  in 
  thecal 
  length 
  would 
  give 
  the 
  form 
  P. 
  folium 
  

   (His.). 
  Now 
  we 
  have 
  reached 
  an 
  important 
  stage. 
  Should 
  the 
  

   theca 
  l 
  2 
  develop 
  so 
  late 
  from 
  the 
  first 
  that 
  the 
  first 
  has 
  already 
  

   grown 
  past 
  the 
  apex 
  of 
  the 
  sicula, 
  the 
  sicula 
  will 
  be 
  entirely 
  

   free 
  on 
  the 
  side 
  remote 
  from 
  that 
  on 
  which 
  the 
  first 
  theca 
  of 
  the 
  

   primordial 
  series 
  arises. 
  Such 
  a 
  form 
  we 
  have 
  in 
  Cephalograptus 
  

   petalum, 
  other 
  changes 
  being 
  a 
  diminution 
  in 
  the 
  angle 
  of 
  incli- 
  

   nation 
  and 
  further 
  increase 
  in 
  thecal 
  length. 
  The 
  extreme 
  type 
  is 
  

   reached 
  in 
  Cephalograptus 
  cometa, 
  in 
  which 
  the 
  theca 
  l 
  2 
  arises 
  still 
  

   later, 
  the 
  thecse 
  are 
  still 
  longer, 
  and 
  are 
  almost 
  parallel 
  to 
  the 
  long 
  

   axis 
  of 
  the 
  rhabdosoma. 
  The 
  other 
  forms 
  of 
  Petalograptus 
  may 
  be 
  

   derived 
  from 
  P. 
  palmeus 
  by 
  slight 
  alterations 
  in 
  the 
  amount 
  of 
  

   curvature. 
  

  

  That 
  the 
  Petalograpti 
  have 
  a 
  Phyllograptus 
  as 
  a 
  remote 
  ancestor 
  

   seems 
  very 
  likely, 
  but 
  the 
  evidence 
  for 
  this 
  is 
  at 
  present 
  incomplete. 
  

   Neither 
  am 
  I 
  able 
  to 
  state 
  whether 
  the 
  C. 
  cometa 
  (Gein.) 
  type 
  has 
  

   a 
  further 
  stage 
  in 
  a 
  form 
  of 
  Dimorphograptus 
  or 
  not. 
  I 
  know 
  no 
  

   Dimorphograptus 
  which 
  resembles 
  it, 
  and 
  it 
  seems 
  possible 
  that, 
  

   owing 
  to 
  the 
  excessively 
  fragile 
  nature 
  of 
  the 
  proximal 
  end, 
  this 
  

   form 
  was 
  not 
  that 
  most 
  fitted 
  to 
  survive 
  in 
  the 
  struggle 
  for 
  life. 
  

  

  