﻿Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  99 
  

  

  Application 
  of 
  Principles 
  for 
  Ordinal 
  Divisions. 
  

  

  In 
  other 
  classes 
  of 
  animals 
  above 
  the 
  lower 
  coelenterates, 
  the 
  

   phylembryonic 
  stage 
  is 
  the 
  starting 
  point 
  from 
  which 
  correla- 
  

   tions 
  are 
  made, 
  and 
  out 
  of 
  which 
  all 
  the 
  higher 
  groups 
  are 
  

   developed 
  by 
  a 
  series 
  of 
  changes 
  along 
  certain 
  lines. 
  The 
  

   protoconch 
  represents 
  this 
  period 
  in 
  the 
  cephalopods 
  and 
  gas- 
  

   tropods 
  ; 
  the 
  prodissoconch, 
  in 
  the 
  pelecypods 
  ; 
  the 
  proteg- 
  

   ulum, 
  in 
  the 
  brachiopods, 
  and 
  the 
  protechinus, 
  in 
  the 
  

   echinoids. 
  In 
  the 
  trilobites, 
  the 
  protaspis, 
  as 
  already 
  stated, 
  

   has 
  the 
  value 
  of 
  the 
  phylembryo, 
  and 
  in 
  its 
  geological 
  history 
  

   and 
  the 
  metamorphoses 
  it 
  undergoes 
  to 
  produce 
  the 
  perfect 
  

   trilobite, 
  accurate 
  information 
  can 
  be 
  gained 
  as 
  to 
  what 
  the 
  

   primitive 
  characters 
  are, 
  and 
  the 
  relative 
  values 
  of 
  other 
  

   features 
  acquired 
  during 
  the 
  long 
  existence 
  of 
  the 
  class. 
  

  

  The 
  simple 
  characters 
  possessed 
  by 
  the 
  protaspis 
  are 
  the 
  fol- 
  

   lowing, 
  as 
  drawn 
  from 
  the 
  study 
  of 
  this 
  stage 
  in 
  all 
  the 
  princi- 
  

   pal 
  groups 
  of 
  trilobites: 
  Dorsal 
  shield 
  minute, 
  not 
  more 
  than 
  

   *4 
  to 
  l 
  mm 
  in 
  length 
  ; 
  circular 
  or 
  ovate 
  in 
  form 
  ; 
  axis 
  distinct, 
  

   more 
  or 
  less 
  strongly 
  annulated, 
  limited 
  by 
  longitudinal 
  

   grooves 
  ; 
  head 
  portion 
  predominating 
  ; 
  axis 
  of 
  cranidium 
  with 
  

   five 
  annulations 
  ; 
  abdominal 
  portion 
  usually 
  less 
  than 
  one-third 
  

   the 
  length 
  of 
  the 
  shield 
  ; 
  axis 
  with 
  from 
  one 
  to 
  several 
  annu- 
  

   lations 
  ; 
  pleural 
  portion 
  smooth 
  or 
  grooved 
  ; 
  eyes, 
  when 
  pres- 
  

   ent, 
  anterior, 
  marginal, 
  or 
  submarginal 
  ; 
  free 
  cheeks, 
  when 
  

   visible, 
  narrow 
  and 
  marginal. 
  Examples, 
  Plate 
  III, 
  figs. 
  1, 
  5. 
  

  

  During 
  this 
  stage, 
  several 
  moults 
  took 
  place 
  before 
  the 
  com- 
  

   plete 
  separation 
  of 
  the 
  pygidium 
  or 
  the 
  introduction 
  of 
  thoracic 
  

   segments. 
  These 
  brought 
  about 
  various 
  changes, 
  as 
  the 
  

   stronger 
  ambulation 
  of 
  the 
  axis, 
  the 
  appearance 
  of 
  the 
  free 
  

   cheeks 
  on 
  the 
  dorsal 
  side, 
  and 
  the 
  growth 
  of 
  the 
  pygidium 
  by 
  

   the 
  introduction 
  of 
  new 
  appendages 
  and 
  segments, 
  as 
  indicated 
  

   by 
  the 
  additional 
  grooves 
  on 
  the 
  axis 
  and 
  limb. 
  A 
  full 
  repre- 
  

   sentation 
  of 
  the 
  variety 
  and 
  succession 
  of 
  these 
  early 
  protaspis 
  

   stages 
  is 
  presented 
  in 
  the 
  writer's 
  paper 
  on 
  the 
  "Larval 
  Stages 
  

   of 
  Trilobites." 
  5 
  Some 
  of 
  the 
  conclusions 
  and 
  discussions 
  in 
  

   that 
  paper 
  are 
  made 
  use 
  of 
  here. 
  

  

  In 
  the 
  earliest 
  or 
  Cambrian 
  genera, 
  the 
  protaspis 
  stage 
  is 
  by 
  

   far 
  the 
  simplest 
  expression 
  of 
  this 
  period 
  to 
  be 
  found. 
  In 
  the 
  

   higher 
  and 
  later 
  genera, 
  the 
  process 
  of 
  acceleration 
  or 
  earlier 
  

   inheritance 
  has 
  pushed 
  forward 
  certain 
  characters 
  until 
  they 
  

   appear 
  in 
  the 
  protaspis, 
  thus 
  making 
  it 
  more 
  and 
  more 
  complex. 
  

  

  Taking 
  the 
  early 
  protaspis 
  stages 
  in 
  Solenopleara, 
  Liostra- 
  

   cus, 
  or 
  Ptychoparia, 
  it 
  is 
  found 
  that 
  they 
  agree 
  exactly 
  with 
  

   the 
  foregoing 
  diagnosis 
  in 
  its 
  most 
  elementary 
  sense. 
  Since 
  

   they 
  are 
  the 
  characters 
  shared 
  in 
  common 
  by 
  all 
  the 
  larvae 
  at 
  

   this 
  stage, 
  they 
  are 
  taken 
  as 
  primitive 
  and 
  accorded 
  that 
  value 
  

  

  