﻿54 
  C. 
  E. 
  Beecher 
  — 
  Development 
  of 
  Bilobites. 
  

  

  that 
  upon 
  reaching 
  maturity 
  a 
  teratologic 
  tendency 
  to 
  oblite- 
  

   rate 
  the 
  marginal 
  sinus 
  is 
  initiated 
  ; 
  thus 
  degenerating 
  to 
  an 
  

   embryonal 
  condition 
  of 
  lobation 
  similar 
  to 
  figure 
  14. 
  

  

  The 
  length 
  of 
  the 
  hinge 
  line 
  from 
  an 
  initial 
  dimension 
  equal 
  

   to 
  the 
  greatest 
  width 
  of 
  the 
  shell 
  becomes 
  equal 
  to 
  but 
  one- 
  

   half 
  the 
  width 
  of 
  the 
  shell 
  in 
  a 
  specimen 
  3-5 
  mm 
  wide 
  ; 
  and 
  in 
  

   a 
  full 
  grown 
  individual, 
  as 
  represented 
  by 
  figure 
  25, 
  the 
  hinge 
  

   is 
  not 
  more 
  than 
  one-quarter 
  the 
  width 
  of 
  the 
  shell. 
  From 
  

   having 
  subequal 
  areas, 
  the 
  change 
  is 
  rapid, 
  so 
  that 
  in 
  a 
  very 
  

   early 
  stage, 
  but 
  two 
  or 
  three 
  removes 
  from 
  the 
  initial 
  one 
  of 
  

   the 
  series, 
  the 
  ventral 
  area 
  is 
  the 
  larger 
  and 
  the 
  fissure 
  higher. 
  

   This 
  ratio 
  progressively 
  increases, 
  and 
  after 
  the 
  shell 
  reaches 
  a 
  

   length 
  of 
  l'5 
  mm 
  , 
  the 
  dorsal 
  area 
  ceases 
  to 
  be 
  a 
  conspicuous 
  

   feature. 
  All 
  areal 
  growth 
  and 
  hinge 
  extension 
  end 
  in 
  the 
  

   middle 
  nealogic 
  period, 
  and 
  to 
  this 
  cause 
  is 
  due 
  the 
  great 
  dis- 
  

   parity 
  between 
  the 
  length 
  of 
  the 
  hinge 
  and 
  the 
  width 
  of 
  the 
  

   shell 
  in 
  ephebolic 
  individuals. 
  The 
  nepionic 
  shells 
  show 
  some 
  

   extension 
  of 
  the 
  cardinal 
  angles, 
  but 
  the 
  auriculation 
  does 
  not 
  

   become 
  apparent 
  until 
  the 
  lobation 
  of 
  the 
  valves 
  is 
  initiated. 
  

  

  On 
  account 
  of 
  the 
  greater 
  length 
  of 
  the 
  incipient 
  dorsal 
  

   valve 
  and 
  consequent 
  obliquity 
  of 
  the 
  area, 
  the 
  fissure 
  and 
  area 
  

   of 
  that 
  valve 
  may 
  be 
  seen 
  when 
  the 
  shell 
  is 
  viewed 
  from 
  the 
  

   ventral 
  side, 
  as 
  in 
  figure 
  10, 
  and, 
  consequently, 
  the 
  ventral 
  area 
  

   is 
  concealed 
  from 
  the 
  dorsal 
  aspect, 
  as 
  shown 
  in 
  figures 
  3-9, 
  

   and 
  11-15. 
  This 
  is 
  a 
  remarkable 
  reversion 
  of 
  characters, 
  and 
  

   one 
  which 
  appears 
  to 
  be 
  of 
  considerable 
  significance 
  from 
  a 
  

   phylogenetic 
  standpoint. 
  

  

  The 
  radiating 
  strise 
  first 
  appear 
  on 
  the 
  lower 
  half 
  of 
  the 
  

   initial 
  shell 
  of 
  the 
  series, 
  indicating 
  that 
  in 
  an 
  earlier 
  condition, 
  

   the 
  shell 
  was 
  smooth. 
  The 
  strise 
  appear 
  in 
  pairs. 
  The 
  first 
  

   two 
  striae 
  extend 
  to 
  the 
  antero-lateral 
  borders. 
  An 
  additional 
  

   intercalated 
  pair 
  is 
  next 
  introduced, 
  together 
  with 
  a 
  single 
  

   one 
  on 
  each 
  side 
  between 
  the 
  primary 
  radii 
  and 
  the 
  cardinal 
  

   border. 
  The 
  number 
  after 
  this 
  stage 
  is 
  more 
  rapidly 
  increased 
  

   by 
  increment 
  in 
  the 
  cardinal 
  lateral 
  areas 
  than 
  in 
  the 
  median 
  

   region. 
  

  

  Observations. 
  — 
  As 
  shown 
  in 
  the 
  ontogeny 
  of 
  B. 
  various, 
  the 
  

   generic 
  stock 
  was 
  derived 
  from 
  a 
  radicle 
  having, 
  in 
  many 
  re- 
  

   spects, 
  the 
  characters 
  of 
  the 
  group 
  represented 
  by 
  JPlaty- 
  

   strophia 
  biforata. 
  The 
  general 
  proportions 
  of 
  the 
  nepionic 
  

   shell 
  in 
  B. 
  various 
  resemble 
  it 
  very 
  closely. 
  The 
  length 
  of 
  

   the 
  hinge 
  at 
  this 
  period, 
  the 
  high 
  hinge 
  areas 
  in 
  both 
  valves, 
  

   with 
  subequal 
  triangular 
  fissures, 
  and 
  the 
  extent 
  of 
  the 
  dorsal 
  

   and 
  ventral 
  beaks, 
  are 
  characters 
  very 
  much 
  the 
  same 
  as 
  in 
  

   Platystrophia 
  biforata. 
  

  

  