﻿Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  101 
  

  

  The 
  remaining 
  genera 
  of 
  trilobites 
  present 
  two 
  distinct 
  types 
  

   of 
  head 
  structure, 
  dependent 
  upon 
  the 
  extent 
  and 
  character 
  of 
  

   the 
  free 
  cheeks. 
  In 
  both, 
  the 
  free 
  cheeks 
  make 
  up 
  an 
  essential 
  

   part 
  of 
  the 
  dorsal 
  crust 
  of 
  the 
  cephalon, 
  being 
  continued 
  on 
  

   the 
  ventral 
  side 
  only 
  as 
  a 
  doublure 
  or 
  infolding 
  of 
  the 
  edge, 
  

   'similar 
  to 
  that 
  of 
  the 
  free 
  edge 
  of 
  the 
  cranidium, 
  the 
  ends 
  of 
  

   the 
  thoracic 
  pleura, 
  and 
  the 
  margin 
  of 
  the 
  pygidium. 
  They 
  

   may 
  be 
  separated 
  only 
  by 
  the 
  cranidium, 
  as 
  in 
  Ptychoparia, 
  or 
  

   by 
  the 
  cranidium 
  and 
  epistoma, 
  as 
  in 
  Illwnus 
  and 
  Homalono- 
  

   tus, 
  or 
  they 
  may 
  be 
  united 
  and 
  continuous 
  in 
  front, 
  as 
  in 
  

   yEglina 
  and 
  Dalmanites. 
  One 
  type 
  of 
  structure 
  is 
  distin- 
  

   guished 
  by 
  having 
  the 
  free 
  cheeks 
  include 
  the 
  genal 
  angles, 
  

   thus 
  cutting 
  off 
  more 
  or 
  less 
  of 
  the 
  pleura 
  of 
  the 
  occipital 
  

   segment. 
  The 
  genera 
  belonging 
  to 
  this 
  group 
  constitute 
  the 
  

   second 
  order, 
  the 
  Opisthoparia. 
  

  

  The 
  third 
  and 
  last 
  type 
  of 
  structure 
  includes 
  forms 
  in 
  which 
  

   the 
  pleura 
  of 
  the 
  occipital 
  segment 
  extend 
  the 
  full 
  width 
  of 
  

   the 
  base 
  of 
  the 
  cephalon, 
  embracing 
  the 
  genal 
  angles. 
  The 
  

   free 
  cheeks 
  are 
  therefore 
  separated 
  from 
  the 
  cranidium 
  by 
  

   sutures 
  cutting 
  the 
  lateral 
  margins 
  of 
  the 
  cephalon 
  in 
  front 
  of 
  

   the 
  genal 
  angles. 
  Genera 
  having 
  this 
  structure 
  are 
  here 
  placed 
  

   in 
  the 
  order 
  Proparia. 
  

  

  Several 
  genera, 
  as 
  Calymene 
  and 
  Triarthrus, 
  have 
  been 
  

   described 
  as 
  having 
  the 
  facial 
  sutures 
  beginning 
  at 
  or 
  cutting 
  

   the 
  apex 
  of 
  the 
  genal 
  angle, 
  thus 
  making 
  it 
  indeterminate 
  

   whether 
  they 
  should 
  be 
  classed 
  with 
  the 
  Opisthoparia 
  or 
  Pro- 
  

   paria. 
  It 
  will 
  be 
  found, 
  however, 
  that 
  some 
  species 
  of 
  these 
  

   genera 
  leave 
  no 
  doubt 
  as 
  to 
  the 
  anterior 
  or 
  posterior 
  position 
  

   of 
  the 
  suture. 
  The 
  small 
  genal 
  spines 
  of 
  Calymene 
  calli- 
  

   eephala 
  Green 
  are 
  situated 
  on 
  the 
  ends 
  of 
  the 
  fixed 
  cheeks, 
  while 
  

   similar 
  but 
  larger 
  spines 
  in 
  Triarthrus 
  spinosus 
  Billings 
  are 
  

   on 
  the 
  free 
  cheeks, 
  making 
  the 
  former 
  belong 
  to 
  the 
  Proparia 
  

   and 
  the 
  latter 
  to 
  the 
  Opisthoparia. 
  

  

  Application 
  of 
  Principles 
  for 
  Arrangement 
  of 
  

   Families 
  and 
  Genera. 
  

  

  The 
  remaining 
  characters 
  to 
  be 
  noticed 
  have 
  chiefly 
  family 
  

   and 
  generic 
  values, 
  and 
  naturally 
  follow 
  the 
  preceding 
  discus- 
  

   sions. 
  They 
  are 
  of 
  great 
  assistance, 
  both 
  in 
  determining 
  the 
  

   place 
  of 
  a 
  family 
  in 
  an 
  order, 
  and 
  the 
  rank 
  and 
  genetic 
  position 
  

   of 
  a 
  genus 
  in 
  a 
  family. 
  

  

  There 
  is 
  very 
  satisfactory 
  evidence 
  that 
  the 
  eyes 
  have 
  

   migrated 
  from 
  the 
  ventral 
  side, 
  first 
  forward 
  to 
  the 
  margin 
  

   and 
  then 
  backward 
  over 
  the 
  cephalon 
  to 
  their 
  adult 
  position. 
  

   The 
  most 
  primitive 
  larvge 
  should 
  therefore 
  present 
  no 
  evidence 
  

   of 
  eyes 
  on 
  the 
  dorsal 
  shield. 
  Just 
  such 
  conditions 
  are 
  fulfilled 
  

  

  