﻿Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  103 
  

  

  Conolichas, 
  Chasmops, 
  etc. 
  Likewise, 
  the 
  fourth 
  annulation 
  

   carrying 
  the 
  first 
  pair 
  of 
  inaxillse 
  is 
  often 
  similarly 
  modified 
  in 
  

   the 
  same 
  genera, 
  also 
  in 
  all 
  the 
  Proetidse, 
  and 
  in 
  Cheirurus, 
  

   Crotalocephalus, 
  Sphcerexochus, 
  Ampyx, 
  Harpes, 
  etc. 
  Here 
  

   again, 
  among 
  adult 
  forms, 
  the 
  stages 
  of 
  progressive 
  differentia- 
  

   tion 
  may 
  be 
  taken 
  as 
  indicating 
  the 
  relative 
  rank 
  of 
  the 
  genera. 
  

  

  The 
  comparative 
  areal 
  growth 
  of 
  the 
  free 
  cheeks 
  is 
  expressed 
  

   by 
  the 
  gradual 
  moving 
  of 
  the 
  facial 
  suture 
  toward 
  the 
  axis. 
  

   As 
  the 
  free 
  cheeks 
  become 
  larger, 
  the 
  fixed 
  cheeks 
  become 
  

   smaller. 
  In 
  the 
  most 
  primitive 
  protaspis 
  stages, 
  and 
  in 
  Agnos- 
  

   tics, 
  llarpes, 
  and 
  Trinucleus, 
  the 
  dorsal 
  surface 
  of 
  the 
  cepha- 
  

   lon 
  is 
  wholly 
  occupied 
  by 
  the 
  axis 
  and 
  the 
  fixed 
  cheeks, 
  while 
  

   in 
  the 
  higher 
  genera, 
  the 
  area 
  of 
  the 
  fixed 
  cheeks 
  becomes 
  

   reduced 
  until, 
  as 
  in 
  Stygina 
  and 
  Pliillipsia, 
  they 
  form 
  a 
  mere 
  

   border 
  to 
  the 
  glabella. 
  Therefore, 
  the 
  ratio 
  between 
  the 
  fixed 
  

   and 
  free 
  cheeks 
  furnishes 
  another 
  means 
  of 
  assisting 
  in 
  the 
  

   determination 
  of 
  rank. 
  

  

  The 
  pleura 
  from 
  the 
  segments 
  of 
  the 
  glabella 
  are 
  occasion- 
  

   ally 
  visible, 
  as 
  in 
  the 
  young 
  of 
  Olenellus, 
  but 
  usually 
  the 
  

   pleura 
  of 
  the 
  neck 
  segments 
  are 
  the 
  first 
  and 
  only 
  ones 
  to 
  be 
  

   distinguished 
  on 
  the 
  cephalon, 
  the 
  others 
  being 
  so 
  completely 
  

   coalesced 
  as 
  to 
  lose 
  all 
  traces 
  of 
  their 
  individuality. 
  The 
  

   pleura 
  of 
  the 
  pygidium 
  appear 
  soon 
  after 
  the 
  earliest 
  protaspis 
  

   stage, 
  and 
  in 
  some 
  genera 
  (Sao, 
  Dalmanites) 
  are 
  even 
  more 
  

   strongly 
  marked 
  than 
  in 
  the 
  adult 
  state 
  and 
  much 
  resemble 
  

   separate 
  segments. 
  The 
  growth 
  of 
  the 
  pygidium 
  is 
  very 
  con- 
  

   siderable 
  through 
  the 
  protaspis 
  stage. 
  At 
  first, 
  it 
  is 
  less 
  than 
  

   one-third 
  the 
  length 
  of 
  the 
  dorsal 
  shield, 
  but 
  by 
  successive 
  

   addition 
  of 
  segments, 
  it 
  soon 
  becomes 
  nearly 
  one-half 
  as 
  long. 
  

   In 
  some 
  genera, 
  it 
  is 
  completed 
  before 
  the 
  appearance 
  of 
  the 
  

   free 
  thoracic 
  segments, 
  all 
  of 
  which 
  are 
  added 
  during 
  the 
  

   nepionic 
  stages. 
  An 
  interpretation 
  of 
  these 
  facts, 
  to 
  apply 
  in 
  

   valuing 
  adult 
  characters, 
  would 
  indicate 
  that 
  a 
  very 
  few 
  seg- 
  

   ments, 
  both 
  in 
  thorax 
  and 
  pygidium, 
  may 
  be 
  evidence 
  of 
  

   arrested 
  development 
  or 
  degeneration. 
  On 
  the 
  other 
  hand, 
  

   the 
  apparently 
  unlimited 
  multiplication 
  of 
  thoracic 
  and 
  espe- 
  

   cially 
  of 
  abdominal 
  segments 
  in 
  some 
  genera 
  is 
  also 
  to 
  be 
  con- 
  

   sidered 
  as 
  a 
  primitive 
  character 
  expressive 
  of 
  an 
  annelidan 
  

   style 
  of 
  growth. 
  Genera 
  like 
  Asaphus, 
  Phacops, 
  etc., 
  having 
  

   a 
  constant 
  number 
  of 
  thoracic 
  segments 
  accompanied 
  by 
  other 
  

   characters 
  of 
  a 
  high 
  order, 
  undoubtedly 
  represent 
  the 
  normal 
  

   trilobite 
  type. 
  

  

  These 
  analyses 
  and 
  correlations 
  clearly 
  show 
  that 
  there 
  are 
  

   characters 
  appearing 
  in 
  the 
  adults 
  of 
  later 
  and 
  higher 
  genera, 
  

   which 
  successively 
  make 
  their 
  appearance 
  in 
  the 
  protaspis 
  

   stage, 
  sometimes 
  to 
  the 
  exclusion 
  or 
  modification 
  of 
  structures 
  

   present 
  in 
  the 
  most 
  primitive 
  larvae. 
  Thus, 
  the 
  larvse 
  of 
  Dal- 
  

  

  