﻿102 
  Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  in 
  the 
  youngest 
  larva 
  of 
  Ptychoparia, 
  Solenopleura, 
  and 
  Lios- 
  

   tracus. 
  The 
  eye 
  line 
  is 
  present 
  in 
  the 
  later 
  larval 
  and 
  adoles- 
  

   cent 
  stages 
  of 
  these 
  genera, 
  and 
  persists 
  to 
  the 
  adult 
  condition. 
  

   In 
  Sao, 
  it 
  has 
  been 
  pushed 
  forward 
  to 
  the 
  earliest 
  protaspis, 
  

   and 
  is 
  also 
  found 
  in 
  the 
  two 
  known 
  larval 
  stages 
  of 
  Triarthrus. 
  

   Sao 
  retains 
  the 
  eye 
  line 
  throughout 
  life, 
  but 
  in 
  Triarthrus 
  the 
  

   adult 
  has 
  no 
  traces 
  of 
  it. 
  A 
  study 
  of 
  the 
  genera 
  of 
  trilobites 
  

   shows 
  that 
  this 
  is 
  a 
  very 
  archaic 
  feature, 
  chiefly 
  characteristic 
  

   of 
  Cambrian 
  genera, 
  and 
  only 
  appearing 
  in 
  the 
  primitive 
  

   genera 
  of 
  higher 
  and 
  later 
  groups 
  or 
  as 
  an 
  evidence 
  of 
  degen- 
  

   eration. 
  It 
  first 
  develops 
  in 
  the 
  later 
  larval 
  stages 
  of 
  certain 
  

   genera 
  (Ptychoparia, 
  etc.) 
  ; 
  next 
  in 
  the 
  early 
  larval 
  stages 
  

   (Sao) 
  ; 
  then 
  disappears 
  from 
  the 
  adult 
  stages 
  (Triarthrus) 
  ; 
  and 
  

   finally 
  is 
  pushed 
  out 
  of 
  the 
  ontogeny 
  (Dalrnanites). 
  

  

  In 
  Ptychoparia, 
  Solenopleura, 
  Liostracus, 
  Sao, 
  and 
  Triar- 
  

   thrus, 
  the 
  eyes 
  are 
  first 
  visible 
  on 
  the 
  margin 
  of 
  the 
  dorsal 
  

   shield 
  after 
  the 
  protaspis 
  stages 
  have 
  been 
  passed 
  through, 
  and 
  

   later 
  than 
  the 
  appearance 
  of 
  the 
  eye 
  lines 
  ; 
  but 
  in 
  Proetus, 
  

   Acidaspis, 
  Arges, 
  and 
  Dalrnanites, 
  through 
  acceleration, 
  they 
  

   are 
  present 
  in 
  all 
  the 
  protaspis 
  stages, 
  and 
  persist 
  to 
  the 
  mature 
  

   or 
  ephebic 
  condition, 
  moving 
  in 
  from 
  the 
  margin 
  to 
  near 
  the 
  

   sides 
  of 
  the 
  glabella. 
  Progression 
  in 
  these 
  characters 
  may 
  be 
  

   expressed, 
  and 
  in 
  so 
  far 
  taken 
  for 
  general 
  application 
  among 
  

   adult 
  forms 
  to 
  indicate 
  rank, 
  as 
  follows: 
  (1) 
  absence 
  of 
  eyes; 
  

   (2) 
  eye 
  lines 
  ; 
  (3) 
  eye 
  lines 
  and 
  marginal 
  eyes 
  ; 
  (4) 
  marginal 
  

   eyes 
  ; 
  (5) 
  submarginal 
  eyes 
  ; 
  (6) 
  eyes 
  near 
  the 
  pleura 
  of 
  the 
  

   neck 
  segment. 
  

  

  The 
  changes 
  in 
  the 
  glabella 
  are 
  equally 
  important 
  and 
  inter- 
  

   esting. 
  Throughout 
  the 
  larval 
  stages, 
  the 
  axis 
  of 
  the 
  cranid- 
  

   ium 
  shows 
  distinctly 
  by 
  the 
  annulations 
  that 
  it 
  is 
  composed 
  of 
  

   five 
  fused 
  segments, 
  indicating 
  the 
  presence 
  of 
  as 
  many 
  paired 
  

   appendages 
  on 
  the 
  ventral 
  side. 
  In 
  its 
  simplest 
  and 
  most 
  

   primitive 
  state, 
  it 
  expands 
  in 
  front, 
  joining 
  and 
  forming 
  the 
  

   anterior 
  margin 
  of 
  the 
  head 
  (larval 
  Ptychojiaria 
  and 
  Sao). 
  

   During 
  later 
  growth, 
  it 
  becomes 
  rounded 
  in 
  front, 
  and 
  termi- 
  

   nates 
  within 
  the 
  margin. 
  In 
  higher 
  genera, 
  through 
  accelera- 
  

   tion, 
  it 
  is 
  rounded 
  and 
  well-defined 
  in 
  front, 
  even 
  in 
  the 
  earliest 
  

   larval 
  stages, 
  and 
  often 
  ends 
  within 
  the 
  margin 
  (larval 
  Triar- 
  

   thrus 
  and 
  Acidaspis). 
  From 
  these 
  simple 
  types 
  of 
  simple 
  

   pentamerous 
  glabellse, 
  all 
  the 
  diverse 
  forms 
  among 
  species 
  of 
  

   various 
  genera 
  have 
  been 
  derived, 
  through 
  changes 
  affecting 
  

   any 
  or 
  all 
  the 
  lobes. 
  The 
  modifications 
  usually 
  consist 
  in 
  the 
  

   progressive 
  obsolescence 
  of 
  the 
  anterior 
  annulations, 
  finally 
  pro- 
  

   ducing 
  a 
  smooth 
  glabella, 
  as 
  in 
  Illamus 
  and 
  Niobe. 
  The 
  

   neck 
  segment 
  is 
  the 
  most 
  persistent 
  of 
  all, 
  and 
  is 
  rarely 
  ob- 
  

   scured. 
  The 
  third, 
  or 
  mandibular, 
  segment 
  is 
  frequently 
  

   marked 
  by 
  two 
  entirely 
  separate 
  lateral 
  lobes, 
  as 
  in 
  Aeidaspis, 
  

  

  