﻿Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  95 
  

  

  It 
  may 
  be 
  questioned 
  by 
  some 
  whether 
  the 
  present 
  state 
  of 
  

   knowledge 
  of 
  the 
  ventral 
  structure 
  of 
  trilobites 
  warrants 
  such 
  

   general 
  assertions 
  as 
  to 
  details 
  of 
  organization. 
  In 
  the 
  first 
  

   place, 
  it 
  must 
  be 
  granted 
  that 
  there 
  is 
  a 
  remarkable 
  uniformity 
  

   in 
  the 
  features 
  of 
  the 
  dorsal 
  crust, 
  which 
  naturally 
  reflects 
  to 
  

   a 
  degree 
  the 
  differentiation 
  and 
  variation 
  of 
  the 
  organs 
  and 
  

   appendages 
  of 
  the 
  ventral 
  side. 
  Furthermore, 
  the 
  actual 
  ap- 
  

   pendages 
  have 
  been 
  observed 
  in 
  such 
  diverse 
  and 
  characteristic 
  

   genera 
  as 
  Trinucleus, 
  Triarthrus, 
  Asaphus, 
  Ceraurus, 
  and 
  

   Calymene, 
  and 
  found 
  to 
  conform 
  closely 
  to 
  a 
  single 
  type, 
  so 
  

   that 
  it 
  seems 
  safe 
  to 
  assume 
  a 
  like 
  agreement 
  throughout. 
  

  

  Morphology 
  of 
  the 
  Cephalon. 
  

  

  The 
  structure 
  of 
  the 
  trilobite 
  head 
  suggests 
  homologies 
  

   which 
  should 
  be 
  noticed 
  here, 
  and 
  if 
  these 
  correlations 
  are 
  

   based 
  upon 
  true 
  structural 
  likenesses, 
  they 
  serve 
  not 
  only 
  to 
  

   emphasize 
  the 
  primitive 
  character 
  of 
  the 
  trilobite, 
  but 
  also 
  aid 
  

   in 
  interpreting 
  certain 
  organs 
  and 
  structures 
  in 
  the 
  higher 
  

   Crustacea. 
  

  

  The 
  five 
  fused 
  somites 
  of 
  the 
  crustacean 
  head 
  are 
  generally 
  

   believed 
  to 
  correspond 
  to 
  the 
  third, 
  fourth, 
  fifth, 
  sixth, 
  and 
  

   seventh 
  neuromeres, 
  leaving 
  the 
  first 
  and 
  second 
  unrepresented 
  

   either 
  by 
  distinct 
  segments 
  or 
  appendages. 
  These 
  two 
  neuro- 
  

   meres 
  commonly 
  constitute 
  most 
  of 
  the 
  cerebral 
  mass 
  above 
  

   the 
  oesophagus, 
  and 
  enervate 
  the 
  ocelli 
  and 
  paired 
  eyes. 
  In 
  

   some, 
  the 
  antennae 
  are 
  enervated 
  from 
  supra-oesophageal 
  gan- 
  

   glia, 
  while 
  in 
  other 
  forms 
  their 
  ganglia 
  are 
  infra-cesophageal. 
  

   It 
  was 
  formerly 
  supposed 
  that 
  the 
  stalked 
  eyes 
  of 
  the 
  higher 
  

   Crustacea 
  represented 
  appendages 
  of 
  a 
  head 
  segment, 
  but 
  this 
  

   belief 
  has 
  been 
  abandoned 
  on 
  account 
  of 
  the 
  derivation 
  of 
  

   stalked 
  out 
  of 
  sessile 
  organs, 
  as 
  in 
  Peneus, 
  and 
  also 
  because 
  the 
  

   eyes 
  do 
  not 
  always 
  have 
  a 
  relatively 
  fixed 
  position, 
  but 
  may 
  

   pertain 
  to 
  the 
  first, 
  second, 
  or 
  third 
  head 
  segments. 
  Their 
  

   structural 
  position 
  in 
  the 
  trilobites, 
  however, 
  is 
  invariable, 
  and 
  

   it 
  seems 
  probable 
  that 
  in 
  some 
  families 
  of 
  higher 
  Crustacea, 
  the 
  

   eyes 
  are 
  in 
  exact 
  correlation, 
  and 
  may 
  be 
  similarly 
  interpreted. 
  

  

  The 
  writer 
  5 
  has 
  previously 
  discussed 
  this 
  question, 
  and 
  ad- 
  

   duced 
  reasons 
  for 
  considering 
  the 
  free 
  cheeks 
  in 
  trilobites 
  as 
  

   -"the 
  pleura 
  of 
  an 
  occuliferous 
  head 
  segment." 
  In 
  many 
  

   species 
  (Dalmanites, 
  ^Eglina, 
  etc.), 
  the 
  free 
  cheeks 
  are 
  con- 
  

   tinuous, 
  forming 
  one 
  piece 
  extending 
  around 
  the 
  front 
  of 
  the 
  

   head, 
  between 
  the 
  cranidium 
  and 
  the 
  hypostoma, 
  while 
  in 
  

   others 
  there 
  is 
  a 
  separate 
  piece, 
  the 
  epistoma, 
  between 
  the 
  

   proximal 
  ends 
  of 
  the 
  cheek 
  pieces 
  holding 
  a 
  like 
  position. 
  

   These 
  structures 
  occupy 
  the 
  exact 
  position 
  of 
  a 
  true 
  segment, 
  

   and 
  since, 
  upon 
  theoretical 
  grounds, 
  additional 
  head 
  segments 
  

  

  