﻿Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  201 
  

  

  Principally 
  Ordovician 
  and 
  Silurian, 
  but 
  extending 
  into 
  the 
  

   Devonian. 
  

  

  Including 
  the 
  genera 
  and 
  subgenera 
  Cheirurus 
  Beyrich, 
  

   Aetinopeltis 
  Corda, 
  Amphion 
  Pander, 
  Anacheirurus 
  Reed, 
  

   Ceraurus 
  Green, 
  Crotalocephalus 
  Salter, 
  Cyrtometopus 
  Ange- 
  

   lin, 
  Deipho?i 
  Barrande, 
  Diaphanometopus 
  Schmidt, 
  Eccopto- 
  

   cheile 
  Corda, 
  Hemisphazrocoryphe 
  Reed, 
  Nieszkowskia 
  Schmidt, 
  

   Onycopyge 
  Woodward, 
  Pseudosphcerexochus 
  Schmidt, 
  Sphce- 
  

   rexochus 
  Beyrich, 
  Sphmrocoryphe 
  Angelin, 
  Staurocephalus 
  

   Barrande, 
  and 
  Youngia 
  Lindstrom. 
  

  

  As 
  in 
  other 
  families, 
  the 
  most 
  primitive 
  genera 
  are 
  those 
  in 
  

   which 
  the 
  regular 
  pentamerous 
  lobation 
  of 
  the 
  glabella 
  is 
  

   retained, 
  the 
  eyes 
  are 
  well 
  forward, 
  the 
  free 
  cheeks 
  narrow, 
  

   and 
  the 
  fixed 
  cheeks 
  ample. 
  Diaphanometopus, 
  Anacheiru- 
  

   rus, 
  Eccoptocheile, 
  and 
  Cyrtometopus 
  agree 
  in 
  these 
  respects, 
  

   and 
  therefore 
  belong 
  at 
  the 
  beginning 
  of 
  a 
  phylogenetic 
  list. 
  

   Ceraurus 
  and 
  Nieszkowskia 
  appear 
  to 
  branch 
  off 
  here, 
  being 
  

   characterized 
  by 
  the 
  narrow 
  transverse 
  form 
  of 
  the 
  cephalon 
  

   and 
  the 
  great 
  development 
  of 
  the 
  two 
  anterior 
  pygidial 
  pleura 
  

   into 
  hollow 
  spines 
  directed 
  outward 
  and 
  backward. 
  These 
  

   features 
  are 
  simulated 
  in 
  Deiphon, 
  in 
  which, 
  however, 
  the 
  

   prominent 
  glabella 
  is 
  without 
  distinct 
  lobes, 
  and 
  the 
  large 
  

   pleural 
  extensions 
  of 
  the 
  pygidium 
  do 
  not 
  belong 
  to 
  the 
  

   anterior 
  segment. 
  Its 
  natural 
  place 
  is 
  at 
  the 
  end 
  of 
  the 
  series. 
  

   F. 
  Cowper 
  Peed 
  31 
  has 
  shown, 
  in 
  his 
  memoir 
  on 
  the 
  evolution 
  

   of 
  Cheirurus 
  and 
  its 
  subgenera, 
  not 
  including 
  the 
  other 
  gen- 
  

   era 
  of 
  the 
  family, 
  that 
  the 
  direct 
  line 
  from 
  Cyrtometopus 
  

   passes 
  through 
  Cheirurus 
  to 
  Crotalocephalus. 
  The 
  genera 
  

   Pseudosphmrexochus 
  and 
  Amphion 
  also 
  have 
  relations 
  with 
  

   these 
  genera 
  and 
  should 
  be 
  placed 
  here. 
  There 
  is 
  next 
  a 
  

   group 
  of 
  forms, 
  with 
  prominent 
  globular 
  glabellse, 
  leading 
  

   from 
  Cheirurus 
  to 
  Sphmrocoryphe, 
  and 
  including 
  Aetinopeltis, 
  

   Youngia, 
  and 
  Hemisphwrocoryphe. 
  Staurocephcdus 
  should 
  

   immediately 
  follow 
  these. 
  Sphairexochus 
  seems 
  to 
  be 
  related 
  

   to 
  Cheirurus 
  and 
  Aetinopeltis. 
  Like 
  them, 
  it 
  has 
  two 
  side 
  

   lobes 
  at 
  the 
  base 
  of 
  the 
  glabella, 
  and 
  the 
  anterior 
  furrows 
  are 
  

   obsolescent, 
  as 
  in 
  Aetinopeltis 
  and 
  Youngia. 
  Lastly 
  come 
  

   Onycopyge 
  and 
  Deiphon, 
  with 
  their 
  globular 
  glabellse 
  without 
  

   furrows, 
  the 
  spiniform 
  fixed 
  cheeks, 
  and 
  thoracic 
  and 
  pygydial 
  

   pleura, 
  and 
  the 
  free 
  cheeks 
  reduced 
  to 
  almost 
  nothing, 
  form- 
  

   ing 
  a 
  small 
  part 
  of 
  the 
  doublure 
  of 
  the 
  cephalon. 
  The 
  former 
  

   genus 
  has 
  four 
  spiniform 
  pygidial 
  pleura, 
  two 
  on 
  each 
  side, 
  

   but 
  in 
  the 
  latter, 
  two 
  are 
  reduced 
  and 
  the 
  remaining 
  pair 
  is 
  

   greatly 
  enlarged. 
  

  

  