﻿C. 
  E. 
  Beecher 
  — 
  Development 
  of 
  Bilobites. 
  

  

  55 
  

  

  These 
  features 
  are 
  maintained 
  until 
  the 
  nealogic 
  stage 
  repre- 
  

   sented 
  by 
  figure 
  15, 
  after 
  which 
  arrested 
  hinge 
  extension 
  and 
  

   increasing 
  areal 
  growth 
  in 
  the 
  ventral 
  valve 
  rapidly 
  obliterate 
  

   the 
  early 
  characters, 
  and 
  in 
  addition, 
  the 
  growing 
  lobation 
  of 
  

   the 
  valves 
  emphasizes 
  the 
  expression 
  of 
  Bilobites. 
  

  

  Figttke 
  2. 
  — 
  G-eoesis 
  of 
  Bilobites. 
  

  

  a, 
  nepionic 
  stage 
  ( 
  x 
  4). 
  Ordovician 
  type 
  like 
  Platystrophia 
  bijorata. 
  

  

  b, 
  nealogic 
  period 
  ( 
  x 
  4) 
  at 
  which 
  divergence 
  begins. 
  

  

  c, 
  Bilobites 
  bilobus 
  (x2). 
  Epacmic 
  form, 
  Niagara 
  horizon. 
  

  

  d, 
  Bilobites 
  Vearneuilianus 
  ( 
  x 
  2). 
  Acmic 
  form, 
  Niagara 
  horizon. 
  

  

  e, 
  Bilobites 
  various 
  ( 
  x 
  2). 
  Paracmic 
  form, 
  Lower 
  Helderberg 
  horizon. 
  

  

  The 
  genesis 
  of 
  the 
  species 
  is 
  represented 
  in 
  the 
  accompany- 
  

   ing 
  illustrations, 
  in 
  which 
  it 
  is 
  shown, 
  that 
  all 
  these 
  species 
  are 
  

   alike 
  in 
  their 
  development 
  up 
  to 
  an 
  early 
  nealogic 
  period, 
  

  

  