﻿Report 
  of 
  the 
  State 
  Geologist. 
  887 
  

  

  Figs. 
  13, 
  14, 
  15, 
  16, 
  17, 
  18 
  and 
  19 
  show 
  a 
  regular 
  gradation 
  from 
  

   a 
  typical 
  Polypora 
  to 
  a 
  typical 
  Fenestella. 
  Polypora 
  lilcea 
  of 
  

   the 
  Lower 
  Helderberg 
  group 
  has 
  from 
  two 
  to 
  four 
  ranges 
  1Q 
  

   of 
  cell 
  apertures 
  on 
  a 
  branch, 
  with 
  the 
  dissepiments 
  both 
  

   celluliferous 
  and 
  noncelluliferous. 
  

  

  F. 
  paxillata 
  of 
  the 
  Upper 
  Helderberg 
  group 
  (figs. 
  2, 
  

   1')), 
  for 
  some 
  distance 
  above 
  the 
  bifurcations 
  has 
  three 
  

   ranges 
  of 
  cell 
  apertures, 
  thence 
  to 
  the 
  next 
  bifurcation 
  

   four 
  ranges. 
  

  

  I>. 
  compacta 
  of 
  the 
  Lower 
  Helderberg 
  group 
  has 
  for 
  

   some 
  distance 
  above 
  the 
  bifurcations 
  two 
  ranges 
  of 
  cell 
  

   apertures, 
  with 
  rounded 
  interspace, 
  followed 
  by 
  three 
  ranges 
  to 
  

   the 
  next 
  bifurcation, 
  and 
  is, 
  therefore, 
  in 
  neither 
  a 
  typical 
  

   Polypora 
  or 
  Fenestella. 
  In 
  tracing 
  these 
  characters 
  through 
  

   several 
  species 
  we 
  find 
  the 
  space 
  occupied 
  by 
  three 
  ranges 
  of 
  

   cell 
  apertures 
  becoming 
  less 
  until 
  in 
  Fenestella 
  Fudora 
  of 
  the 
  

   Lower 
  Helderberg 
  group 
  there 
  are 
  only 
  two 
  ranges 
  of 
  cell 
  

   apertures 
  except 
  for 
  a 
  short 
  distance 
  below 
  the 
  bifurcation, 
  

   where 
  there 
  are 
  three. 
  The 
  two 
  ranges 
  are 
  separated 
  by 
  a 
  ridge 
  

   (carina). 
  The 
  three 
  ranges 
  have 
  no 
  separating 
  carina. 
  We 
  have, 
  

   therefore, 
  in 
  this 
  one 
  specimen 
  a 
  typical 
  Fenestella 
  as 
  limited 
  

   by 
  King 
  and 
  a 
  typical 
  Polypora 
  of 
  McCoy. 
  Later 
  on 
  I 
  

   will 
  discuss 
  more 
  fully 
  the 
  genus 
  Polypora 
  and 
  related 
  

   forms. 
  

  

  We 
  have 
  seen 
  how 
  intimately 
  related 
  are 
  the 
  forms 
  previously 
  

   considered. 
  

  

  If 
  we 
  take 
  typical 
  forms 
  of 
  Fenestella 
  and 
  Hemitrypa 
  the 
  

   difference 
  is 
  even 
  more 
  marked 
  than 
  in 
  the 
  previous 
  examples 
  ; 
  

   but 
  by 
  the 
  examination 
  of 
  a 
  large 
  number 
  of 
  species 
  we 
  will 
  see 
  

   that 
  the 
  change 
  from 
  one 
  form 
  to 
  another 
  is 
  just 
  as 
  gradual, 
  and 
  

   that 
  in 
  the 
  connecting 
  series 
  of 
  forms 
  no 
  link 
  is 
  missing, 
  so 
  that 
  it 
  

   is 
  very 
  difficult 
  to 
  fix 
  any 
  limit 
  at 
  which 
  Fenestella 
  ends 
  and 
  

   Hemitrypa 
  begins. 
  In 
  order 
  to 
  elucidate 
  this 
  statement 
  a 
  series 
  

   of 
  drawings 
  is 
  here 
  given, 
  illustrating 
  sections 
  of 
  branches, 
  show- 
  

   ing 
  the 
  gradual 
  development 
  of 
  the 
  parts 
  upon 
  which 
  the 
  genus 
  

   Hemitrypa 
  has 
  been 
  founded. 
  

  

  The 
  drawings 
  are 
  all 
  from 
  accurate 
  measurements 
  of 
  specimens 
  

   and 
  are 
  in 
  no 
  case 
  in 
  the 
  slightest 
  degree 
  exaggerated. 
  

  

  