﻿96 
  Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  are 
  to 
  be 
  accounted 
  for, 
  the 
  only 
  satisfactory 
  correlation 
  is 
  to 
  

   consider 
  them 
  as 
  such. 
  Furthermore, 
  the 
  free 
  cheeks 
  are 
  dis- 
  

   tinctly 
  separated 
  from 
  the 
  cranidium 
  by 
  an 
  open 
  suture, 
  and 
  

   may 
  be 
  wholly 
  converted 
  into 
  eyes, 
  as 
  in 
  JEglina 
  armata 
  Barr., 
  

   or 
  the 
  unfacetted 
  portion 
  may 
  be 
  reduced 
  to 
  almost 
  nothing, 
  

   as 
  in 
  Deiphon. 
  In 
  such 
  cases, 
  the 
  parallelism 
  is 
  exact 
  with 
  

   true 
  movable 
  eyes. 
  Bernard 
  7 
  concludes 
  from 
  his 
  studies 
  of 
  

   Apus 
  that 
  the 
  hypostoma 
  is 
  homologous 
  with 
  the 
  annelid 
  pro- 
  

   stomium. 
  This 
  would 
  make 
  the 
  hypostoma 
  represent 
  the 
  first, 
  

   and 
  the 
  free 
  cheeks 
  the 
  second 
  of 
  the 
  obsolete 
  segments. 
  

   Thus 
  the 
  trilobite 
  cephalon 
  would 
  fulfil 
  the 
  demand 
  for 
  addi- 
  

   tional 
  evidences 
  of 
  primitive 
  head 
  segments, 
  and 
  account 
  for 
  

   the 
  development 
  of 
  eyes 
  separate 
  from 
  the 
  cephalothorax 
  as 
  

   commonly 
  restricted. 
  

  

  Supposed 
  evidences 
  of 
  free 
  cheeks 
  or 
  of 
  facial 
  sutures 
  have 
  

   been 
  recognized 
  in 
  Limulus, 
  Hemiaspis, 
  and 
  Bunodes, 
  but 
  

   these 
  seem 
  really 
  to 
  correspond 
  to 
  the 
  lines 
  on 
  the 
  dorsal 
  sur- 
  

   face 
  of 
  the 
  cephalon 
  of 
  Harpes 
  and 
  some 
  Trinucleus, 
  running 
  

   from 
  the 
  glabella 
  to 
  the 
  eye 
  spots 
  and 
  to 
  the 
  margin, 
  and 
  are 
  

   not 
  the 
  sutures 
  marking 
  the 
  limits 
  of 
  the 
  free 
  cephalic 
  ele- 
  

   ments, 
  as 
  in 
  Asaphus 
  and 
  Proetus. 
  Zimulus, 
  however, 
  has 
  a 
  

   suture 
  comparable 
  to 
  that 
  in 
  Harpes 
  and 
  Trinucleus, 
  extending 
  

   around 
  the 
  ventral 
  border 
  of 
  the 
  cephalothorax 
  nearly 
  to 
  the 
  

   posterior 
  angles, 
  and 
  partly 
  separating 
  the 
  ventral 
  plate. 
  In 
  

   the 
  process 
  of 
  moulting, 
  this 
  suture 
  opens 
  and 
  enables 
  the 
  ani- 
  

   mal 
  to 
  free 
  itself 
  from 
  its 
  former 
  test. 
  

  

  These 
  interpretations 
  may 
  be 
  employed 
  to 
  some 
  advantage 
  

   in 
  correlating 
  the 
  segmentation 
  of 
  the 
  trilobite 
  cephalon. 
  As 
  

   previously 
  stated, 
  the 
  recognized 
  plan 
  in 
  the 
  nervous 
  system 
  of 
  

   a 
  generalized 
  crustacean 
  requires 
  that 
  there 
  should 
  be 
  a 
  brain 
  

   or 
  supra-oesophageal 
  ganglion 
  enervating 
  (a) 
  the 
  unpaired 
  eye, 
  

   (b) 
  the 
  frontal 
  sensory 
  organs 
  and 
  stalked 
  eyes, 
  and 
  (c) 
  the 
  

   anterior 
  antennae 
  ; 
  then 
  a 
  ventral 
  nervous 
  chord 
  consisting 
  of 
  

   a 
  succession 
  of 
  double 
  ganglia 
  enervating, 
  respectively, 
  the 
  

   second 
  pair 
  of 
  antennae, 
  the 
  mandibles, 
  the 
  first 
  pair 
  of 
  max- 
  

   illae, 
  the 
  second 
  pair 
  of 
  maxillae, 
  and 
  lastly 
  each 
  of 
  the 
  paired 
  

   thoracic 
  and 
  abdominal 
  appendages. 
  Altogether, 
  there 
  are 
  

   seven 
  neuromeres 
  pertaining 
  to 
  the 
  head, 
  and 
  on 
  the 
  basis 
  that 
  

   each 
  neuromere 
  corresponds 
  to 
  an 
  original 
  segment, 
  as 
  on 
  the 
  

   post-cephalic 
  region, 
  there 
  would 
  need 
  to 
  be 
  this 
  number 
  

   accounted 
  for. 
  The 
  anterior 
  segment, 
  or 
  number 
  one 
  in 
  the 
  

   trilobites, 
  would 
  be 
  represented 
  by 
  the 
  hypostoma 
  ; 
  the 
  second 
  

   segment, 
  by 
  the 
  paired 
  eyes, 
  free 
  cheeks, 
  and 
  epistoma; 
  the 
  

   third, 
  by 
  the 
  anterior 
  lobe 
  of 
  the 
  glabella 
  and 
  the 
  first 
  antennae; 
  

   the 
  fourth, 
  by 
  the 
  second 
  lobe 
  of 
  the 
  glabella 
  and 
  the 
  second 
  

   pair 
  of 
  antennae 
  ; 
  the 
  fifth, 
  by 
  the 
  third 
  lobe 
  of 
  the 
  glabella 
  

   and 
  the 
  mandibles 
  ; 
  the 
  sixth, 
  by 
  the 
  fourth 
  lobe 
  of 
  the 
  glabella 
  

  

  