﻿P. 
  E. 
  Raymond 
  — 
  Beecher's 
  Classification 
  of 
  Trilobites. 
  203 
  

  

  seems 
  homologous 
  with 
  the 
  ventral 
  suture 
  of 
  Agnostics, 
  and 
  that, 
  

   not 
  being 
  at 
  an 
  edge, 
  can 
  not 
  well 
  be 
  explained 
  as 
  a 
  secondary 
  

   splitting. 
  

  

  There 
  remains 
  to 
  be 
  discussed 
  the 
  question 
  of 
  Orometopus. 
  

   Why 
  should 
  it 
  be 
  included 
  in 
  the 
  Trinucleidse 
  % 
  It 
  is 
  so 
  

   unlike 
  any 
  of 
  the 
  Trinucleids 
  that 
  the 
  burden 
  of 
  proof 
  that 
  it 
  

   belongs 
  to 
  that 
  family 
  should 
  be 
  upon 
  those 
  who 
  have 
  placed 
  

   it 
  there, 
  but 
  its 
  present 
  placing 
  has 
  been 
  accepted 
  with 
  such 
  

   avidity 
  that 
  it 
  is 
  necessary 
  to 
  make 
  some 
  comments. 
  Lake 
  

   says 
  of 
  Orometopus* 
  : 
  — 
  " 
  The 
  clavate 
  glabella, 
  the 
  horizontal 
  

   grooved 
  pleurae, 
  and 
  the 
  broad 
  triangular 
  tail, 
  are 
  characters 
  

   which 
  it 
  shares 
  with 
  those 
  genera 
  [Trinucleus 
  and 
  Ampyx], 
  

   and 
  which 
  differentiate 
  it 
  from 
  other 
  families." 
  

  

  The 
  characteristics 
  which 
  ally 
  Orometopus 
  with 
  the 
  Trinu- 
  

   cleidae 
  and 
  Raphiophoridse 
  are 
  all 
  to 
  be 
  found 
  in 
  the 
  thorax 
  

   and 
  pygidium, 
  and 
  the 
  result 
  of 
  the 
  experience 
  of 
  all 
  students 
  

   of 
  trilobites 
  has 
  been 
  to 
  learn 
  that 
  the 
  thorax 
  and 
  pygidinm 
  

   seldom 
  show 
  characteristics 
  of 
  family 
  rank. 
  Within 
  the 
  same 
  

   family 
  the 
  thorax 
  and 
  pygidinm 
  are 
  often 
  highly 
  variable, 
  and 
  

   the 
  th 
  or 
  aces 
  and 
  pygidia 
  of 
  some 
  species 
  in 
  totally 
  unrelated 
  

   families 
  are 
  quite 
  similar. 
  The 
  broadly 
  rounded 
  form, 
  flat 
  

   segments 
  and 
  wide 
  short 
  pygidium 
  of 
  Orometopus 
  make 
  it 
  

   look 
  like 
  a 
  trinucleid 
  or 
  ampycid, 
  but 
  those 
  families 
  share 
  

   with 
  Harpes 
  and 
  many 
  of 
  the 
  Olenidee 
  the 
  form 
  of 
  the 
  thoracic 
  

   segments, 
  and 
  the 
  pygidium 
  is 
  olenid 
  in 
  outline. 
  

  

  Orometopus 
  differs 
  from 
  the 
  Trinucleidse 
  in 
  the 
  possession 
  

   of 
  compound 
  e} 
  7 
  es 
  and 
  large 
  free 
  cheeks 
  on 
  the 
  dorsal 
  side, 
  in 
  

   lacking 
  a 
  specialized 
  glabella 
  which 
  enlarges 
  forward, 
  in 
  pos- 
  

   sessing 
  more 
  than 
  6 
  thoracic 
  segments, 
  and 
  especially 
  in 
  the 
  

   form 
  of 
  the 
  hypostoma, 
  which 
  is 
  nearly 
  square 
  in 
  Orometopus 
  

   with 
  a 
  straight 
  posterior 
  border 
  and 
  nearly 
  circular 
  with 
  a 
  

   median 
  tongue-shaped 
  posterior 
  projection 
  in 
  " 
  Trinucleus." 
  

   The 
  similarities 
  consist 
  merely 
  in 
  the 
  form 
  of 
  the 
  thoracic 
  

   segments 
  and 
  pygidium, 
  and 
  are 
  of 
  too 
  little 
  importance 
  to 
  

   outweigh 
  the 
  evidence 
  of 
  the 
  other 
  characteristics 
  of 
  the 
  

   animals. 
  

  

  Orometopus 
  can, 
  I 
  think, 
  be 
  quite 
  definitely 
  eliminated 
  as 
  

   a 
  possible 
  ancestor 
  of 
  the 
  Trinucleidae. 
  From 
  Barrande's 
  

   work 
  many 
  stages 
  in 
  the 
  development 
  of 
  " 
  Trinucleits" 
  are 
  

   known. 
  Although 
  the 
  protaspis 
  has 
  not 
  been 
  seen, 
  specimens 
  

   without 
  thoracic 
  segments 
  and 
  only 
  l 
  mm 
  long 
  have 
  been 
  figured. 
  

   The 
  striking 
  thing 
  about 
  the 
  young 
  u 
  Trinucltus" 
  is 
  that 
  the 
  

   pygidium 
  is 
  nearly 
  or 
  quite 
  as 
  large 
  as 
  the 
  cephalon. 
  This 
  is 
  

   very 
  unusual 
  in 
  the 
  young 
  of 
  trilobites, 
  and 
  if 
  we 
  can 
  trust 
  

   the 
  young 
  as 
  pointing 
  toward 
  the 
  ancestral 
  line, 
  some 
  isopygous 
  

   group 
  like 
  the 
  Agnostidse 
  or 
  Eodiscidse 
  is 
  indicated, 
  rather 
  

  

  *Op. 
  cit., 
  p. 
  44. 
  

  

  