﻿100 
  Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  in 
  dealing 
  with 
  adult 
  forms 
  possessing 
  homologous 
  features. 
  

   Therefore, 
  any 
  trilobite 
  with 
  a 
  large 
  elongate 
  cephalon, 
  eyes 
  

   rudimentary 
  or 
  absent, 
  free 
  cheeks 
  ventral 
  or 
  marginal, 
  and 
  

   glabella 
  long, 
  cylindrical, 
  and 
  with 
  five 
  annulations, 
  would 
  

   naturally 
  be 
  placed 
  near 
  the 
  beginning 
  of 
  any 
  genetic 
  series 
  or 
  

   as 
  belonging 
  to 
  a 
  very 
  primitive 
  stock. 
  

  

  Next 
  must 
  be 
  considered 
  the 
  progressive 
  addition 
  of 
  char- 
  

   acters 
  during 
  the 
  geological 
  history 
  of 
  the 
  protaspis, 
  and 
  in 
  

   the 
  ontogeny 
  of 
  the 
  individual 
  during 
  its 
  growth 
  from 
  the 
  

   larval 
  to 
  the 
  mature 
  condition. 
  It 
  was 
  shown 
  in 
  the 
  paper 
  

   already 
  referred 
  to, 
  that 
  there 
  was 
  an 
  exact 
  correlation 
  to 
  be 
  

   made 
  between 
  the 
  geological 
  and 
  zoological 
  succession 
  of 
  first 
  

   larval 
  stages 
  and 
  adult 
  forms, 
  and 
  therefore 
  both 
  may 
  be 
  

   reviewed 
  together. 
  

  

  The 
  first 
  important 
  structures 
  not 
  especially 
  noticeable 
  in 
  all 
  

   stages 
  of 
  the 
  protaspis 
  are 
  the 
  free 
  cheeks, 
  which 
  usually 
  man- 
  

   ifest 
  themselves 
  in 
  the 
  meta- 
  or 
  para-protaspis 
  stages, 
  though 
  

   sometimes 
  even 
  later. 
  Since 
  they 
  bear 
  the 
  visual 
  areas 
  of 
  the 
  

   eyes, 
  when 
  they 
  are 
  present, 
  their 
  appearance 
  on 
  the 
  dorsal 
  

   shield 
  is 
  practically 
  simultaneous 
  with 
  these 
  organs, 
  and 
  before 
  

   the 
  eyes 
  have 
  travelled 
  over 
  the 
  margin, 
  the 
  free 
  cheeks 
  must 
  

   be 
  wholly 
  ventral 
  in 
  position. 
  When 
  first 
  discernible, 
  they 
  

   are 
  very 
  narrow, 
  and 
  in 
  Ptydwparia 
  and 
  Sao 
  include 
  the 
  

   genal 
  angles. 
  In 
  JDalmanites 
  and 
  CAeirurus, 
  however, 
  the 
  

   genal 
  angles 
  are 
  borne 
  on 
  the 
  fixed 
  cheeks. 
  If, 
  as 
  Bernard 
  7 
  

   concludes, 
  the 
  crustacean 
  head 
  has 
  been 
  formed 
  by 
  the 
  bending 
  

   under, 
  to 
  the 
  ventral 
  side, 
  of 
  the 
  anterior 
  segments 
  of 
  an 
  

   ancestral 
  carnivorous 
  annelid, 
  this 
  furnishes 
  a 
  means 
  of 
  further 
  

   determining 
  and 
  also 
  of 
  satisfactorily 
  correlating 
  the 
  prime 
  

   significance 
  and 
  importance 
  of 
  the 
  free 
  cheeks. 
  

  

  Since 
  the 
  free 
  cheeks 
  are 
  ventral 
  in 
  the 
  earliest 
  larval 
  stages 
  

   of 
  all 
  but 
  the 
  highest 
  trilobites, 
  and 
  as 
  this 
  is 
  an 
  adult 
  feature 
  

   among 
  a 
  number 
  of 
  genera 
  which 
  on 
  other 
  grounds 
  are 
  very 
  

   primitive, 
  this 
  is 
  taken 
  as 
  generally 
  indicative 
  of 
  a 
  very 
  low 
  

   rank. 
  It 
  seems 
  to 
  mean 
  that 
  the 
  second 
  segment 
  remains 
  

   where 
  it 
  was 
  mechanically 
  placed, 
  and 
  retains 
  its 
  full 
  somitic 
  

   nature, 
  though 
  from 
  the 
  necessities 
  of 
  such 
  a 
  condition, 
  true 
  

   ventral 
  segments 
  must 
  soon 
  disappear 
  through 
  modification 
  

   into 
  other 
  structures 
  or 
  through 
  disuse 
  as 
  segments. 
  The 
  

   genera 
  Harpes, 
  Agnoslus, 
  Trinucleus, 
  and 
  their 
  allies 
  agree 
  in 
  

   having 
  well-developed, 
  continuous, 
  ventral 
  free 
  cheeks, 
  and 
  

   constitute 
  a 
  natural 
  group. 
  As 
  they 
  possess 
  one 
  expression 
  or 
  

   type 
  of 
  the 
  genesis 
  of 
  an 
  important 
  common 
  character, 
  based 
  

   upon 
  facts 
  of 
  development, 
  it 
  should 
  stand 
  as 
  an 
  ordinal 
  char- 
  

   acter, 
  and 
  as 
  such 
  it 
  is 
  here 
  taken. 
  For 
  this 
  group, 
  the 
  name 
  

   Hypoparia 
  is 
  proposed. 
  It 
  is 
  fully 
  defined, 
  and 
  its 
  limitations 
  

   established 
  in 
  the 
  proper 
  place 
  in 
  the 
  classification. 
  

  

  