﻿98 
  Beecher— 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  derived 
  form 
  may 
  be 
  so 
  involved 
  with 
  larval 
  adaptations 
  and 
  

   accelerated 
  characters 
  as 
  to 
  be 
  misleading. 
  

  

  The 
  trilobites 
  lend 
  themselves 
  to 
  this 
  treatment 
  in 
  fulfilling 
  

   most 
  of 
  the 
  necessary 
  conditions. 
  They 
  have 
  a 
  known 
  geo- 
  

   logical 
  history 
  stretching 
  through 
  the 
  entire 
  paleozoic, 
  from 
  

   the 
  beginning 
  of 
  the 
  Cambrian 
  to 
  the 
  Permian. 
  Their 
  struc- 
  

   ture 
  is 
  generalized 
  and 
  quite 
  uniform, 
  and 
  no 
  sessile, 
  attached, 
  

   parasitic, 
  land 
  or 
  fresh 
  water 
  species 
  are 
  known. 
  The 
  ontog- 
  

   eny 
  of 
  all 
  the 
  principal 
  groups 
  has 
  been 
  studied, 
  including 
  

   Cambrian, 
  Ordovician, 
  Silurian, 
  and 
  Devonian 
  types. 
  

  

  The 
  trilobites 
  necessarily 
  furnish 
  little 
  information 
  of 
  the 
  

   stages 
  of 
  growth 
  which 
  may 
  be 
  classed 
  as 
  embryonic. 
  The 
  

   early 
  embryonic 
  stages 
  are 
  not 
  preserved 
  as 
  fossils, 
  and 
  there- 
  

   fore 
  may 
  be 
  omitted. 
  In 
  this 
  category 
  are 
  the 
  protembryo, 
  or 
  

   the 
  ovum 
  in 
  its 
  unsegmented 
  and 
  segmented 
  stages 
  (the 
  so- 
  

   called 
  " 
  eggs 
  of 
  trilobites" 
  may 
  of 
  course 
  represent 
  any 
  stage 
  

   of 
  embryonic 
  development 
  before 
  the 
  escape 
  of 
  the 
  young) 
  ; 
  

   the 
  mesembryo, 
  or 
  blastosphere 
  ; 
  the 
  metembryo, 
  or 
  gastrula 
  ; 
  

   the 
  neoembryo, 
  or 
  planula-like 
  stage 
  ; 
  and 
  the 
  lypembryo, 
  when 
  

   the 
  first 
  distinctive 
  features 
  make 
  their 
  appearance. 
  The 
  first 
  

   embryonic 
  stage 
  recognized 
  in 
  the 
  trilobites 
  can 
  be 
  referred 
  to 
  

   the 
  phylembryo 
  as 
  defined 
  by 
  Jackson, 
  22 
  when 
  the 
  animal 
  may 
  

   be 
  clearly 
  referred 
  to 
  its 
  proper 
  class. 
  Since 
  this 
  period 
  is 
  

   distinctive 
  for 
  each 
  class 
  of 
  animals 
  and 
  usually 
  bears 
  a 
  sepa- 
  

   rate 
  name, 
  it 
  has 
  been 
  termed 
  by 
  the 
  writer 
  6 
  the 
  protaspis 
  

   stage 
  of 
  trilobites. 
  It 
  closely 
  approximates 
  the 
  protonauplius 
  

   form, 
  or 
  the 
  theoretical, 
  primitive, 
  ancestral 
  larval 
  form 
  of 
  the 
  

   Crustacea. 
  Like 
  the 
  homologous 
  nauplius 
  of 
  modern 
  higher 
  

   Crustacea, 
  it 
  is 
  the 
  characteristic 
  larval 
  type 
  common 
  to 
  the 
  

   class. 
  The 
  nauplius 
  is 
  therefore 
  considered 
  as 
  a 
  derived 
  larva 
  

   modified 
  by 
  adaptation. 
  

  

  The 
  post-embryonic 
  stages 
  of 
  ontogeny 
  have 
  received 
  the 
  

   names 
  nepionic, 
  for 
  the 
  infantile 
  or 
  young 
  ; 
  neanic, 
  for 
  the 
  

   immature 
  or 
  adolescent; 
  ephebic, 
  for 
  the 
  mature 
  or 
  adult; 
  

   and 
  gerontic, 
  for 
  the 
  senile 
  or 
  old. 
  When 
  especially 
  applied 
  

   to 
  trilobites, 
  the 
  nepionic 
  stages 
  may 
  include 
  the 
  animal 
  when 
  

   the 
  cephalon 
  and 
  pygidium 
  are 
  distinct 
  and 
  the 
  thorax 
  incom- 
  

   plete. 
  There 
  would 
  thus 
  be 
  as 
  many 
  nepionic 
  stages 
  as 
  the 
  

   number 
  of 
  thoracic 
  segments. 
  The 
  neanic 
  stages 
  would 
  be 
  

   represented 
  by 
  the 
  animal 
  with 
  all 
  parts 
  complete, 
  but 
  with 
  

   the 
  average 
  growth 
  incomplete. 
  Final 
  progressive 
  growth 
  

   and 
  development 
  of 
  the 
  individual 
  would 
  fall 
  under 
  the 
  

   ephebic 
  stage. 
  Lastly, 
  general 
  evidences 
  of 
  senility 
  would 
  be 
  

   interpreted 
  as 
  belonging 
  to 
  the 
  gerontic 
  stage. 
  

  

  