﻿FOURTH 
  REPORT 
  OF 
  THE 
  DIRECTOR 
  I907 
  6l 
  

  

  ing 
  the 
  climacteric 
  period 
  of 
  the 
  species. 
  In 
  a 
  further 
  chapter 
  the 
  

   possible 
  influence 
  of 
  the 
  presence 
  of 
  spines 
  on 
  the 
  development 
  of 
  

   a 
  retioloid 
  structure 
  in 
  the 
  periderm 
  is 
  investigated 
  and 
  the 
  inference 
  

   attained 
  that 
  even 
  the 
  spinose 
  forms 
  of 
  Diplograptus 
  possess 
  in 
  

   their 
  thick 
  periderm 
  a 
  layer 
  of 
  retioloid 
  meshes, 
  and 
  of 
  stronger 
  

   ledges, 
  and 
  that 
  the 
  development 
  of 
  this 
  layer 
  is 
  roughly 
  propor- 
  

   tional 
  to 
  that 
  of 
  the 
  spines. 
  This 
  cause 
  has 
  combined 
  with 
  the 
  

   tendency 
  of 
  the 
  rhabdosomes 
  to 
  become 
  lighter 
  after 
  the 
  floating 
  

   and 
  swimming 
  habit 
  had 
  been 
  adopted, 
  and 
  produced 
  the 
  order 
  

   Retiolitidae 
  with 
  a 
  reticulate 
  periderm. 
  

  

  The 
  dilatations, 
  " 
  disks," 
  wings 
  or 
  vesicles 
  of 
  the 
  nemacaulus 
  of 
  

   Diplograptus, 
  Climacograptus 
  and 
  Cryptograptus 
  are 
  separately 
  con- 
  

   sidered 
  and 
  evidence 
  brought 
  forward 
  to 
  show 
  that 
  they 
  were 
  

   inflations 
  of 
  the 
  outer 
  periderm 
  of 
  the 
  nemacaulus 
  through 
  which 
  

   the 
  virgnla 
  or 
  axis 
  passes 
  uninterrupted. 
  

  

  The 
  vtrification 
  by 
  a 
  recent 
  investigation 
  by 
  Schepotiefr", 
  of 
  a 
  

   former 
  observation 
  by 
  the 
  writer, 
  that 
  the 
  axis 
  of 
  the 
  sicula 
  (the 
  

   virgula) 
  passes 
  through 
  the 
  nemacaulus 
  and 
  into 
  the 
  rhabdosome, 
  

   is 
  discussed 
  in 
  a 
  further 
  chapter. 
  Other 
  chapters 
  on 
  the 
  morphol- 
  

   ogy 
  of 
  the 
  graptolites 
  are 
  devoted 
  to 
  the 
  asymmetric 
  section 
  of 
  the 
  

   rhabdosome 
  in 
  some 
  graptolites 
  (as 
  Climacograptus 
  typi- 
  

   cal 
  i 
  s), 
  to 
  the 
  axes 
  of 
  the 
  Dicranograptidae, 
  the 
  morphology 
  of 
  the 
  

   thecae 
  of 
  the 
  Dichograptidae 
  and 
  Dicranograptidae. 
  

  

  In 
  a 
  part 
  entitled 
  "Notes 
  on 
  Phylogeny 
  " 
  the 
  phylogenetic' 
  rela- 
  

   tions 
  of 
  the 
  Leptograptidae 
  and 
  Dicranograptidae 
  are 
  first 
  discussed 
  

   and 
  the 
  derivation 
  of 
  Dicranograptus 
  from 
  Dicellograptus 
  shown 
  

   It 
  is 
  argued 
  that 
  the 
  branches 
  of 
  the 
  Dicranograptidae 
  formed 
  

   together 
  always 
  a 
  more 
  or 
  less 
  slender 
  double 
  spiral 
  whereby 
  certain 
  

   advantages 
  of 
  the 
  arrangement 
  of 
  the 
  thecae 
  and 
  a 
  great 
  elasticity 
  

   of 
  the 
  suspended 
  rhabdosome 
  were 
  attained, 
  but 
  at 
  the 
  same 
  time 
  

   the 
  strain 
  at 
  the 
  further 
  (sicular) 
  end 
  where 
  the 
  two 
  branches 
  are 
  

   connected, 
  increased 
  ; 
  hence 
  the 
  formation 
  of 
  the 
  biserial 
  portion 
  in 
  

   Dicranograptus 
  to 
  strengthen 
  this 
  end. 
  

  

  A 
  synoptic 
  and 
  synonymic 
  list 
  of 
  graptolites 
  recorded 
  from 
  North 
  

   America 
  concludes 
  the 
  general 
  part 
  of 
  the 
  memoir. 
  

  

  A 
  Devonic 
  brittlestar 
  

  

  In 
  a 
  recent 
  publication 
  1 
  I 
  called 
  incidental 
  attention 
  to 
  the 
  dis- 
  

   covery 
  by 
  D. 
  D. 
  Luther 
  of 
  specimens 
  of 
  Helianthaster 
  in 
  the 
  Portage 
  

   (Cashaqua) 
  shales 
  at 
  Interlaken, 
  N. 
  Y. 
  a 
  village 
  lying 
  on 
  the 
  divide 
  

  

  1 
  Report 
  State 
  Paleontologist 
  1006, 
  p. 
  36. 
  

  

  