﻿52 
  C. 
  K 
  Beecher 
  — 
  Development 
  of 
  Bilobites. 
  

  

  in 
  the 
  outline, 
  figure 
  1. 
  This 
  form 
  was 
  recently 
  described 
  by 
  

   Kingueberg, 
  as 
  Orthis 
  acutildba* 
  

  

  The 
  Lower 
  Helderberg 
  species 
  known 
  as 
  B. 
  various 
  Conrad, 
  

   sp., 
  presents 
  an 
  amount 
  of 
  departure 
  from 
  typical 
  B. 
  bilobus, 
  

   as 
  would 
  be 
  anticipated 
  from 
  the 
  change 
  in 
  the 
  chronological 
  

   and 
  physical 
  conditions 
  of 
  the 
  species, 
  combined 
  with 
  its 
  ex- 
  

   tremely 
  prolific 
  development 
  at 
  this 
  time. 
  The 
  abundance 
  

   and 
  comparatively 
  large 
  size 
  of 
  individuals 
  clearly 
  indicate 
  

   most 
  favorable 
  conditions 
  for 
  their 
  existence 
  and 
  multiplica- 
  

   tion, 
  and, 
  also, 
  for 
  the 
  assumption 
  and 
  transmission 
  of 
  any 
  

   varietal 
  forms 
  in 
  harmony 
  with 
  the 
  environment. 
  

  

  Mature 
  individuals 
  from 
  Dudley, 
  England, 
  and 
  Gotland, 
  

   Sweden, 
  represented 
  by 
  figure 
  28, 
  correspond 
  in 
  all 
  characters 
  

   with 
  specimens 
  of 
  B. 
  various 
  which 
  are 
  about 
  half 
  or 
  two- 
  

   thirds 
  grown. 
  After 
  reaching 
  the 
  adult 
  bilobus 
  stage, 
  B. 
  

   various 
  continues 
  its 
  growth, 
  but 
  this 
  subsequent 
  increment 
  is 
  

   geratologic 
  in 
  its 
  nature, 
  although 
  such 
  senile 
  features 
  are 
  here 
  

   the 
  conditions 
  of 
  simple 
  maturity 
  or 
  the 
  completed 
  ephebolic 
  

   stage. 
  Evidences 
  of 
  this 
  are 
  seen 
  in 
  the 
  gradual 
  obsolescence 
  

   of 
  the 
  pronounced 
  lobation 
  of 
  the 
  shell 
  and 
  the 
  cessation 
  of 
  

   a 
  real 
  growth 
  in 
  the 
  nealogic 
  period. 
  The 
  form 
  known 
  as 
  B. 
  

   bilobus, 
  var. 
  Verneuilianus, 
  Lm., 
  from 
  Gotland, 
  shows 
  a 
  tend- 
  

   ency 
  to 
  develop 
  in 
  the 
  opposite 
  direction, 
  as 
  the 
  lobation 
  be- 
  

   comes 
  more 
  and 
  more 
  pronounced 
  with 
  growth, 
  and 
  the 
  shell 
  

   exceeds 
  in 
  size 
  the 
  normal 
  species. 
  The 
  decrease 
  in 
  the 
  loba- 
  

   tion 
  of 
  B. 
  various 
  is 
  a 
  degeneration 
  towards 
  an 
  embryonic 
  

   character, 
  while 
  the 
  arrested 
  areal 
  development 
  produces 
  a 
  

   condition 
  of 
  partial 
  isomorphism 
  resembling 
  one 
  of 
  the 
  higher 
  

   groups 
  of 
  Orthis, 
  such 
  as 
  Khipidomella 
  [R. 
  Michelini 
  Lev). 
  

  

  From 
  what 
  has 
  been 
  stated, 
  it 
  seems 
  evident, 
  that 
  the 
  form 
  

   typified 
  by 
  B. 
  bilobus 
  from 
  the 
  Niagara 
  was, 
  at 
  that 
  time, 
  not 
  

   -a 
  very 
  plastic 
  type, 
  and 
  capable 
  of 
  only 
  slight 
  degrees 
  of 
  varia- 
  

   tion 
  or 
  departure 
  from 
  the 
  normal 
  form. 
  Naturally, 
  all 
  the 
  

   modifications 
  which 
  occur 
  containing 
  a 
  differentiation 
  of 
  the 
  

   essential 
  idea 
  of 
  the 
  genus 
  appear 
  in 
  the 
  early 
  history 
  of 
  the 
  

   groupj 
  and 
  are 
  found 
  previous 
  to 
  the 
  Lower 
  Helderberg 
  form. 
  

   The 
  latter 
  species 
  while 
  losing, 
  in 
  a 
  manner, 
  its 
  bilobus 
  expres- 
  

   sion 
  at 
  maturity, 
  degenerates 
  into 
  forms 
  resembling 
  ancestral 
  

   and 
  other 
  groups. 
  

  

  The 
  material 
  for 
  the 
  basis 
  of 
  this 
  paper 
  was 
  collected 
  by 
  

   the 
  writer 
  from 
  the 
  lower 
  members 
  of 
  the 
  Shaly 
  Limestone 
  of 
  

   the 
  Lower 
  Helderberg 
  group, 
  along 
  the 
  top 
  of 
  the 
  main 
  

   escarpment 
  of 
  the 
  Helderberg 
  Mountains, 
  between 
  Clarksville 
  

   and 
  the 
  Indian 
  Ladder, 
  Albany 
  County, 
  New 
  York. 
  Half- 
  

  

  * 
  Proceedings 
  of 
  the 
  Academy 
  of 
  Natural 
  Sciences, 
  Philadelphia, 
  p. 
  134, 
  1888. 
  

  

  