﻿188 
  JBeecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  consider 
  the 
  advanced 
  specialized 
  genera 
  of 
  some 
  families 
  as 
  

   representing 
  their 
  normal 
  facies, 
  for 
  each 
  one 
  has 
  undergone 
  

   an 
  independent 
  evolution, 
  and 
  some 
  characters 
  have 
  reached 
  

   as 
  great 
  a 
  degree 
  of 
  differentiation 
  as 
  will 
  be 
  found 
  in 
  much 
  

   higher 
  families. 
  

  

  It 
  has 
  been 
  recognized 
  that 
  variations 
  in 
  the 
  positions 
  of 
  the 
  

   eyes, 
  the 
  relative 
  size 
  of 
  the 
  free 
  and 
  fixed 
  cheeks, 
  and 
  the 
  

   degree 
  of 
  specialization 
  of 
  the 
  glabella 
  have 
  a 
  definite 
  order 
  in 
  

   the 
  ontogeny 
  of 
  any 
  trilobite, 
  and 
  also 
  that 
  these 
  characters 
  

   have 
  a 
  greater 
  taxonomic 
  value 
  than 
  many 
  others. 
  Applying 
  

   these 
  principles 
  in 
  arranging 
  the 
  families 
  which 
  come 
  under 
  

   the 
  Opisthoparia, 
  we 
  have 
  the 
  sequence 
  as 
  indicated 
  above, 
  

   beginning 
  with 
  the 
  Conocoryphidse 
  and 
  followed 
  by 
  the 
  

   Olenidse, 
  Asaphidse, 
  Proetidse, 
  Bronteidse, 
  Lichadidse, 
  and 
  

   Acidaspidse, 
  in 
  regular 
  progression. 
  See 
  Plate 
  III, 
  figs. 
  14-23. 
  

  

  Family 
  IV. 
  Conocorypiiid^e 
  Angelin. 
  

  

  Free 
  cheeks 
  very 
  narrow, 
  forming 
  the 
  lateral 
  margins 
  of 
  the 
  

   cephalon, 
  and 
  bearing 
  the 
  genal 
  spines. 
  Fixed 
  cheeks 
  large, 
  

   usually 
  traversed 
  by 
  an 
  eye 
  line 
  extending 
  from 
  near 
  the 
  ante- 
  

   rior 
  end 
  of 
  the 
  glabella. 
  Facial 
  sutures 
  running 
  from 
  just 
  

   within 
  the 
  genal 
  angles, 
  curving 
  forward, 
  and 
  cutting 
  the 
  ante- 
  

   rior 
  lateral 
  margins 
  of 
  the 
  cephalon. 
  Eyes 
  rudimentary 
  or 
  

   absent. 
  Thorax 
  with 
  from 
  fourteen 
  to 
  seventeen 
  segments. 
  

   Pygidium 
  small 
  and 
  of 
  few 
  segments. 
  Cambrian. 
  

  

  Including 
  the 
  genera 
  and 
  subgenera 
  Conocoryjphe 
  Corda 
  

   (= 
  Conocephalites 
  Barrande), 
  Aneucanthus 
  Angelin, 
  Atops 
  

   Emmons, 
  Avalonia 
  Walcott, 
  Bailiella 
  Matthew 
  (=Salteria 
  

   Walcott 
  and 
  Erinnys 
  Salter), 
  Bathynotus 
  Hall, 
  Carausia 
  

   Hicks, 
  Carmon 
  Barrande, 
  Ctenocephalus 
  Corda, 
  Dictyocephal- 
  

   ites 
  Bergeron, 
  Eryx 
  Angelin, 
  Ilarttia 
  Walcott, 
  and 
  Toxotis 
  

   Wallerius. 
  

  

  The 
  genera 
  coming 
  under 
  this 
  family 
  present 
  a 
  number 
  of 
  

   very 
  primitive 
  characters 
  such 
  as 
  are 
  shown 
  only 
  in 
  the 
  larval 
  

   stages 
  of 
  higher 
  forms. 
  The 
  free 
  cheeks 
  are 
  narrow 
  and 
  mar- 
  

   ginal, 
  and 
  can 
  be 
  compared 
  with 
  those 
  in 
  the 
  nepionic 
  stages 
  

   of 
  Sao 
  and 
  Ptychoparia. 
  The 
  eyes 
  have 
  not 
  been 
  detected, 
  

   but 
  the 
  presence 
  of 
  an 
  eye 
  line 
  suggests 
  their 
  possible 
  existence. 
  

   The 
  variations 
  of 
  the 
  glabella 
  are 
  very 
  marked, 
  and 
  are 
  as 
  great 
  as 
  

   those 
  which 
  in 
  higher 
  forms 
  attain 
  some 
  importance 
  as 
  family 
  

   characteristics. 
  In 
  Toxotis, 
  Carausia, 
  and 
  Aneucanthus, 
  the 
  

   glabella 
  expands 
  in 
  front, 
  joining 
  and 
  forming 
  part 
  of 
  the 
  

   anterior 
  margin, 
  as 
  in 
  the 
  glabella 
  of 
  the 
  larval 
  stages 
  of 
  Solen- 
  

  

  

  