﻿194 
  Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  the 
  very 
  tumid 
  form 
  of 
  the 
  large 
  cephalon 
  and 
  the 
  obscure 
  or 
  

   obsolete 
  boundaries 
  of 
  the 
  glabella 
  and 
  occipital 
  lobe. 
  The 
  

   pygidium 
  often 
  closely 
  resembles 
  the 
  cephalon 
  in 
  size 
  and 
  

   form, 
  and 
  the 
  axis 
  is 
  frequently 
  scarcely 
  defined. 
  

  

  Considerable 
  variation 
  is 
  shown 
  in 
  the 
  size, 
  position, 
  and 
  

   direction 
  of 
  the 
  visual 
  surfaces. 
  There 
  is 
  also 
  a 
  ratio 
  between 
  

   the 
  size 
  of 
  the 
  fixed 
  cheeks 
  and 
  the 
  eyes. 
  In 
  proportion 
  as 
  

   the 
  fixed 
  cheeks 
  are 
  large, 
  the 
  eyes 
  are 
  small, 
  and 
  as 
  the 
  area 
  

   of 
  the 
  fixed 
  cheeks 
  diminishes 
  from 
  a 
  widening 
  of 
  the 
  axis 
  of 
  

   the 
  animal, 
  the 
  eyes 
  become 
  larger. 
  Thus, 
  in 
  Holocephalina, 
  

   with 
  extremely 
  large 
  fixed 
  cheeks 
  and 
  narrow 
  axis, 
  the 
  eyes 
  

   are 
  quite 
  small. 
  In 
  Illamopsis, 
  Dysplanus, 
  Panderia, 
  and 
  

   Octillcenus, 
  they 
  are 
  progressively 
  larger, 
  and 
  in 
  Illcenus, 
  

   Bumastus, 
  and 
  JVileus, 
  where 
  the 
  axis 
  is 
  wide 
  and 
  the 
  fixed 
  

   cheeks 
  reduced, 
  the 
  eyes 
  are 
  relatively 
  large. 
  This 
  variation 
  

   reaches 
  its 
  limit 
  in 
  the 
  species 
  of 
  JEglina, 
  where 
  the 
  axis 
  is 
  

   very 
  w 
  T 
  ide 
  and 
  the 
  fixed 
  cheeks 
  are 
  reduced 
  to 
  almost 
  nothing, 
  

   so 
  that 
  the 
  glabella 
  and 
  eyes 
  make 
  up 
  the 
  entire 
  dorsal 
  surface 
  

   of 
  the 
  cephalon. 
  In 
  JEglina 
  pri?iceps 
  Barrande, 
  the 
  eyes 
  

   extend 
  about 
  half 
  the 
  length 
  of 
  the 
  cephalon. 
  The 
  eyes 
  of 
  

   JE. 
  rediviva 
  Barr. 
  bound 
  the 
  whole 
  length 
  of 
  the 
  sides 
  of 
  the 
  

   head, 
  and 
  in 
  JE. 
  armata 
  Barr., 
  the 
  coalesced 
  free 
  cheek 
  pieces 
  

   are 
  almost 
  wholly 
  converted 
  into 
  a 
  visual 
  area, 
  so 
  that 
  there 
  is 
  

   a 
  continuous 
  eye 
  around 
  the 
  sides 
  and 
  front 
  of 
  the 
  head. 
  

  

  Variations 
  in 
  the 
  position 
  of 
  the 
  eyes 
  are 
  to 
  be 
  noted 
  in 
  

   nearly 
  all 
  the 
  genera. 
  In 
  Ectillamus 
  and 
  Psilocephalus, 
  they 
  

   are 
  in 
  front 
  of 
  the 
  middle 
  of 
  the 
  length 
  of 
  the 
  cephalon, 
  and 
  

   in 
  Pysplanus, 
  Illcenopsis, 
  and 
  Holocephalina, 
  they 
  are 
  near 
  

   the 
  posterior 
  angles 
  of 
  the 
  cranidium. 
  Panderia 
  has 
  the 
  eyes 
  

   directed 
  obliquely 
  backward, 
  and 
  in 
  Tkaleops, 
  they 
  are 
  carried 
  

   on 
  conical 
  extensions 
  pointing 
  outwards. 
  

  

  Family 
  VII. 
  Proetid^e 
  Barrande. 
  

  

  Cephalon 
  about 
  one-third 
  of 
  the 
  whole 
  animal 
  : 
  genal 
  angles 
  

   generally 
  produced 
  into 
  spines 
  ; 
  glabella 
  tumid, 
  with 
  two 
  

   lateral 
  basal 
  lobes 
  defined 
  by 
  oblique 
  furrows 
  in 
  front 
  of 
  the 
  

   neck 
  segment. 
  Free 
  cheeks 
  large, 
  separate. 
  Sutures 
  extend- 
  

   ing 
  from 
  the 
  posterior 
  margin 
  inward 
  to 
  the 
  eyes, 
  and 
  then 
  

   forward, 
  cutting 
  the 
  anterior 
  margins 
  separately. 
  Eyes 
  prom- 
  

   inent, 
  often 
  large. 
  Thorax 
  of 
  from 
  eight 
  to 
  twenty-two 
  free 
  

   segments, 
  with 
  grooved 
  pleura. 
  Pygidium 
  usually 
  of 
  many 
  

   segments 
  ; 
  pleural 
  and 
  axial 
  portions 
  strongly 
  grooved 
  ; 
  mar- 
  

   gin 
  entire 
  or 
  dentate. 
  Ordovician 
  to 
  Permian. 
  

  

  Including 
  the 
  genera 
  and 
  subgenera 
  Proehcs 
  Steininger, 
  

  

  