﻿Beecher 
  — 
  Structure 
  and 
  Appendages 
  of 
  Trinucleus. 
  309 
  

  

  fixed 
  cheeks, 
  while 
  in 
  all 
  other 
  trilobites 
  with 
  distinct 
  eyes, 
  

   the 
  free 
  cheeks 
  carry 
  the 
  visual 
  areas. 
  This 
  type 
  of 
  eye 
  is 
  

   thus 
  quite 
  different 
  in 
  its 
  relations 
  to 
  the 
  parts 
  of 
  the 
  cephalon 
  

   from 
  that 
  of 
  Phacops 
  or 
  Asaphus, 
  and 
  more 
  nearly 
  resembles 
  

   the 
  eyes 
  of 
  some 
  of 
  the 
  Merostomata 
  (Bellinurus), 
  as 
  do 
  also 
  

   the 
  triangular 
  areas 
  in 
  the 
  young 
  Trinucleus, 
  so 
  distinctly 
  

   marked 
  off 
  from 
  the 
  fixed 
  cheeks 
  on 
  each 
  side 
  of 
  the 
  glabella 
  

   behind 
  the 
  eye-line. 
  Adult 
  Trinucleus 
  and 
  Ilarpes 
  have 
  

   these 
  areas 
  much 
  reduced, 
  and 
  often 
  obsolescent. 
  A 
  spot 
  or 
  node 
  

   in 
  the 
  median 
  line 
  on 
  the 
  glabella 
  has 
  been 
  noticed 
  by 
  many 
  

   observers, 
  and 
  although 
  its 
  nature 
  has 
  not 
  been 
  demonstrated, 
  

   it 
  has 
  generally 
  been 
  called 
  an 
  ocellus. 
  It 
  is 
  more 
  clearly 
  

   preserved 
  in 
  adult 
  specimens, 
  though 
  it 
  can 
  be 
  detected 
  in 
  

   young 
  examples, 
  as 
  indicated 
  in 
  figures 
  1, 
  2, 
  Plate 
  III. 
  

  

  An 
  eye-line 
  occurs 
  in 
  many 
  early 
  trilobite 
  genera, 
  and 
  is 
  

   well 
  marked 
  in 
  Conocoryp>he, 
  Olenus, 
  Ptychoparia, 
  and 
  Are- 
  

   thusina. 
  At 
  least 
  four-fifths 
  of 
  the 
  Cambrian 
  forms 
  preserve 
  

   this 
  feature, 
  which 
  is 
  almost 
  entirely 
  eliminated 
  before 
  Devo- 
  

   nian 
  time. 
  It 
  differs 
  in 
  extent, 
  but 
  not 
  necessarily 
  in 
  nature, 
  

   from 
  the 
  eye-line 
  of 
  Trinucleus 
  and 
  Harpes 
  in 
  running 
  

   entirely 
  across 
  the 
  fixed 
  cheeks 
  to 
  the 
  free 
  cheeks, 
  ending 
  in 
  

   the 
  palpebral 
  lobe 
  in 
  eyed 
  forms. 
  It 
  is 
  evidently 
  a 
  larval 
  

   character 
  in 
  the 
  trilobites, 
  as 
  shown 
  from 
  its 
  geological 
  history 
  

   and 
  the 
  ontogeny 
  of 
  Trinucleus. 
  From 
  the 
  direction 
  of 
  the 
  

   optic 
  nerve 
  in 
  Limulus, 
  and 
  its 
  relations 
  to 
  the 
  surface 
  features 
  

   of 
  the 
  cephalothorax, 
  the 
  eye-line 
  probably 
  represents 
  the 
  

   course 
  of 
  that 
  nerve, 
  and 
  is 
  of 
  much 
  less 
  morphological 
  im- 
  

   portance 
  than 
  the 
  different 
  types 
  and 
  arrangement 
  of 
  visual 
  

   organs. 
  

  

  The 
  pygidium 
  of 
  young 
  T. 
  concentricus 
  (Plate 
  III, 
  figure 
  3) 
  

   is 
  remarkable 
  for 
  the 
  lack 
  of 
  definition 
  between 
  the 
  axis 
  and 
  

   pleura. 
  In 
  later 
  and 
  adult 
  stages 
  the 
  number 
  of 
  ridges 
  on 
  the 
  

   pleura 
  and 
  axis 
  do 
  not 
  correspond, 
  and 
  from 
  figures 
  4, 
  5, 
  and 
  6, 
  

   it 
  is 
  evident 
  that 
  in 
  this 
  genus 
  the 
  number 
  of 
  pleura 
  is 
  no 
  indi- 
  

   cation 
  of 
  the 
  number 
  of 
  pygidial 
  segments 
  or 
  pairs 
  of 
  append- 
  

   ages, 
  which, 
  however, 
  may 
  be 
  shown, 
  as 
  in 
  this 
  case, 
  by 
  the 
  

   annulations 
  of 
  the 
  axis. 
  In 
  this 
  respect, 
  the 
  pygidia 
  in 
  Encri- 
  

   nurus, 
  Gybele, 
  and 
  Bindymene, 
  are 
  of 
  the 
  same 
  nature. 
  

   Figure 
  6 
  also 
  shows 
  a 
  narrow, 
  striated 
  doublure, 
  a 
  character 
  

   generally 
  overlooked 
  in 
  descriptions 
  of 
  Trinucleus. 
  

  

  Appendages. 
  

  

  Three 
  specimens 
  have 
  thus 
  far 
  been 
  observed 
  which 
  show 
  

   the 
  nature 
  of 
  the 
  appendages 
  in 
  Trinucleus. 
  Two 
  of 
  these 
  

   are 
  illustrated 
  in 
  figures 
  4, 
  5, 
  and 
  6, 
  of 
  Plate 
  III. 
  Figure 
  4 
  

   represents 
  the 
  thorax 
  and 
  pygidium 
  viewed 
  from 
  the 
  dorsal 
  

  

  