﻿104 
  Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  manites 
  or 
  Pro'etus, 
  with 
  their 
  prominent 
  eyes 
  and 
  glabella 
  

   distinctly 
  terminated 
  and 
  rounded 
  in 
  front, 
  have 
  characters 
  

   which 
  do 
  not 
  appear 
  in 
  the 
  larval 
  stages 
  of 
  ancient 
  genera, 
  

   but 
  which 
  may 
  come 
  in 
  their 
  adult 
  stages. 
  Evidently 
  such 
  

   modifications 
  have 
  been 
  acquired 
  by 
  the 
  action 
  of 
  the 
  law 
  of 
  

   earlier 
  inheritance, 
  or 
  tachygenesis. 
  

  

  In 
  a 
  classification 
  of 
  trilobites, 
  for 
  the 
  purpose 
  of 
  illustrat- 
  

   ing 
  the 
  principles 
  here 
  enunciated, 
  the 
  ontogenies 
  of 
  Sao 
  and 
  

   Dalmanites, 
  Plate 
  III, 
  figs. 
  1-8, 
  are 
  selected. 
  Sao 
  belongs 
  to 
  

   the 
  ancient 
  family 
  Olenidse 
  of 
  the 
  order 
  Opisthoparia, 
  and 
  

   naturally 
  may 
  be 
  expected 
  to 
  furnish 
  very 
  clear 
  evidence 
  as 
  to 
  

   the 
  relations 
  of 
  many 
  lower 
  and 
  older 
  genera. 
  Dalmanites, 
  

   also, 
  with 
  its 
  simple 
  head 
  structure, 
  will 
  give 
  similar 
  data 
  

   regarding 
  the 
  Proparia. 
  

  

  The 
  early 
  protaspis 
  stage 
  of 
  Sao, 
  Plate 
  III, 
  fig. 
  1, 
  has 
  no 
  

   dorsal 
  development 
  of 
  the 
  free 
  cheeks, 
  and 
  with 
  the 
  elongate 
  

   form 
  of 
  the 
  cephalic 
  portion 
  may 
  be 
  compared 
  with 
  the 
  

   cephala 
  of 
  Agnostics 
  and 
  Microdiscus, 
  and 
  therefore 
  correlates 
  

   with 
  the 
  Hypoparia. 
  The 
  cephalon, 
  at 
  a 
  later 
  period 
  of 
  devel- 
  

   opment, 
  when 
  the 
  animal 
  has 
  two 
  free 
  thoracic 
  segments, 
  Plate 
  

   III, 
  fig, 
  2, 
  shows 
  the 
  narrow 
  marginal 
  free 
  cheeks 
  and 
  distinct 
  

   eye 
  lines. 
  Here 
  the 
  resemblance 
  to 
  the 
  cephala 
  of 
  Atops 
  and 
  

   Conocoryphe, 
  Plate 
  III, 
  figs. 
  14, 
  15, 
  is 
  very 
  marked, 
  and 
  indi- 
  

   cates 
  that 
  the 
  Conocoryphidse 
  is 
  genetically 
  the 
  first 
  family 
  of 
  the 
  

   Opisthoparia. 
  When 
  Sao 
  has 
  eight 
  thoracic 
  segments, 
  Plate 
  III, 
  

   fig. 
  3, 
  the 
  characters 
  of 
  the 
  cephalon 
  accord 
  closely 
  with 
  Ptycho- 
  

   paria 
  and 
  Olenus, 
  showing 
  that 
  these 
  genera 
  should 
  precede 
  it 
  

   in 
  arranging 
  the 
  genera 
  of 
  the 
  family 
  Olenidae. 
  Evidence 
  is 
  

   thus 
  furnished 
  for 
  the 
  proper 
  position 
  of 
  the 
  first 
  two 
  families 
  

   of 
  the 
  order. 
  Now, 
  if 
  the 
  relative 
  values 
  of 
  the 
  differentia- 
  

   tion 
  of 
  the 
  glabella, 
  the 
  position 
  of 
  the 
  eyes, 
  and 
  the 
  size 
  of 
  

   the 
  free 
  cheeks 
  are 
  considered 
  in 
  the 
  light 
  of 
  the 
  preceding 
  

   analyses 
  of 
  these 
  features, 
  the 
  remaining 
  families 
  of 
  the 
  order, 
  

   as 
  represented 
  in 
  their 
  typical 
  genera, 
  naturally 
  arrange 
  them- 
  

   selves 
  as 
  indicated 
  in 
  Plate 
  III, 
  figs. 
  18-23. 
  There 
  results 
  (1) 
  

   the 
  Conocoryphidae 
  (represented 
  by 
  Atops 
  and 
  Conocoryphe, 
  

   figs. 
  14, 
  15); 
  (2) 
  the 
  Olenidae 
  {Ptychoparia 
  and 
  Olenus, 
  figs. 
  

   16, 
  17) 
  ; 
  (3) 
  the 
  Asaphidse 
  {Asaphus 
  and 
  lllamits, 
  figs. 
  18, 
  19) 
  ; 
  

   (4) 
  the 
  Proetidse 
  (Proetus, 
  fig. 
  20) 
  ; 
  (5) 
  the 
  Bronteidse 
  (Bron- 
  

   teus, 
  fig. 
  21); 
  (6) 
  the 
  Lichadidse 
  (Lichas, 
  fig. 
  22); 
  and 
  (7) 
  the 
  

   Acidaspidse 
  (Acidaspis, 
  fig. 
  23). 
  

  

  For 
  the 
  Proparia, 
  similar 
  results 
  are 
  brought 
  out 
  by 
  the 
  

   study 
  of 
  the 
  ontogeny 
  of 
  Dalmanites, 
  and 
  by 
  comparisons 
  

   with 
  the 
  characters 
  governing 
  the 
  sequence 
  of 
  families 
  in 
  the 
  

   Opisthoparia. 
  The 
  narrow 
  marginal 
  free 
  cheeks 
  place 
  the 
  

   Encrinuridse 
  and 
  Calymenidse 
  as 
  primitive. 
  The 
  small 
  or 
  

   obsolete 
  eyes 
  and 
  the 
  larval 
  form 
  of 
  the 
  glabella 
  in 
  the 
  former 
  

  

  