﻿208 
  P. 
  E. 
  Raymond 
  — 
  Beecher^s 
  Classification 
  of 
  Trilobites. 
  

  

  shield, 
  and 
  then 
  by 
  secondary 
  growth 
  of 
  interstitial 
  tissue 
  at 
  

   the 
  back, 
  the 
  pleurae 
  were 
  again 
  forced 
  around 
  into 
  nearly 
  

   their 
  original 
  position, 
  but 
  as 
  the 
  pleurae 
  of 
  the 
  third, 
  fourth, 
  

   and 
  fifth 
  segments 
  had 
  been 
  largely 
  obliterated 
  in 
  the 
  turning 
  

   backward, 
  the 
  new 
  cheeks 
  were 
  composed 
  principally 
  of 
  the 
  

   pleurae 
  of 
  the 
  first 
  and 
  second 
  segments. 
  Unless 
  one 
  believes 
  

   with 
  Lindstrom 
  that 
  the 
  Mesonacidae 
  were 
  blind, 
  it 
  follows 
  

   that 
  even 
  in 
  these 
  young 
  forms 
  the 
  inner 
  edges 
  of 
  the 
  pleurae 
  

   of 
  the 
  first 
  (dorsal) 
  segment 
  carried 
  the 
  visual 
  surface 
  of 
  the 
  

   eye, 
  and 
  the 
  pleurae 
  of 
  the 
  second 
  segment 
  formed 
  the 
  palpe- 
  

   bral 
  lobes. 
  

  

  In 
  order 
  to 
  facilitate 
  moulting 
  there 
  should 
  have 
  been 
  some 
  

   sort 
  of 
  a 
  suture 
  in 
  the 
  Mesonacidae, 
  and 
  seemingly 
  there 
  was 
  

   a 
  ventral 
  suture, 
  at 
  least 
  in 
  the 
  young, 
  very 
  similar 
  to 
  the 
  

   suture 
  in 
  the 
  Agnostidae. 
  Walcott 
  has 
  figured 
  several 
  speci- 
  

   mens 
  in 
  which 
  the 
  hypostoma 
  is 
  attached 
  to 
  a 
  narrow 
  " 
  dou- 
  

   blure", 
  and 
  one 
  remarkable 
  specimen 
  from 
  Pennsylvania 
  in 
  

   which 
  the 
  hypostoma 
  attached 
  to 
  a 
  narrow 
  " 
  doublure 
  " 
  has 
  

   swung 
  back 
  so 
  that 
  it 
  presents 
  its 
  ventral 
  face 
  to 
  the 
  observer 
  

   on 
  the 
  same 
  block 
  with 
  and 
  still 
  attached 
  to 
  the 
  head 
  shield. 
  

   This 
  doublure 
  is 
  analogous 
  to 
  the 
  ventral 
  plate 
  of 
  Agnostics, 
  

   and 
  that 
  the 
  suture 
  along 
  which 
  it 
  separated 
  was 
  not 
  marginal 
  

   is 
  shown 
  by 
  the 
  fact 
  that 
  the 
  plate 
  is 
  attached 
  at 
  the 
  intergenal 
  

   and 
  not 
  at 
  the 
  genal 
  spines. 
  If 
  this 
  plate 
  is 
  not 
  accidentally 
  

   separated, 
  then 
  it 
  is 
  an 
  epistoma, 
  and 
  constitutes 
  the 
  seventh 
  

   and 
  anterior 
  segment 
  which 
  made 
  up 
  the 
  head 
  of 
  the 
  trilobite, 
  

   the 
  hypostoma 
  being 
  its 
  appendage. 
  

  

  It 
  would 
  be 
  easy 
  to 
  conceive 
  that, 
  to 
  facilitate 
  the 
  removal 
  

   of 
  the 
  covering 
  of 
  the 
  eye 
  in 
  moulting, 
  dorsal 
  facial 
  sutures 
  

   should 
  be 
  secondarily 
  developed. 
  Beginning 
  at 
  the 
  front 
  of 
  

   the 
  eye 
  and 
  running 
  back 
  to 
  the 
  posterior 
  margin 
  of 
  the 
  head, 
  

   there 
  was 
  a 
  natural 
  line 
  of 
  weakness, 
  because 
  it 
  was 
  the 
  line 
  of 
  

   division 
  between 
  the 
  original 
  first 
  and 
  second 
  segments 
  of 
  the 
  

   dorsal 
  side. 
  And 
  it 
  is 
  known 
  that 
  the 
  posterior 
  portion 
  of 
  the 
  

   suture 
  does 
  follow 
  this 
  line, 
  as, 
  when 
  present, 
  it 
  comes 
  out 
  at 
  

   the 
  intergenal 
  spine. 
  Forward 
  from 
  the 
  front 
  of 
  the 
  eye, 
  the 
  

   course 
  of 
  the 
  facial 
  suture 
  had 
  no 
  natural 
  line 
  to 
  follow, 
  but 
  in 
  

   most 
  cases 
  seems 
  to 
  be 
  guided 
  by 
  the 
  outline 
  of 
  the 
  hypostoma, 
  

   hence 
  the 
  outward 
  curve. 
  Following 
  this 
  line 
  of 
  reasoning 
  it 
  

   would 
  follow 
  that 
  the 
  lines 
  which 
  are 
  seen 
  on 
  the 
  heads 
  of 
  

   Mesonacids 
  are 
  rudimentary 
  sutures, 
  and 
  that 
  Swinnerton 
  was 
  

   right 
  in 
  regarding 
  the 
  ancestral 
  trilobite 
  as 
  being 
  without 
  free 
  

   cheeks. 
  And, 
  further, 
  it 
  is 
  conceivable 
  that 
  in 
  the 
  process 
  of 
  

   evolution, 
  the 
  free 
  cheeks 
  crowded 
  upon 
  the 
  epistoma 
  so 
  as 
  to 
  

   greatly 
  reduce 
  its 
  size, 
  or 
  even 
  force 
  it 
  out 
  entirely. 
  

  

  In 
  this 
  way 
  of 
  thinking 
  of 
  the 
  trilobite 
  head, 
  the 
  anterior 
  seg- 
  

   ment 
  was 
  not 
  oculiferous, 
  the 
  eyes 
  would 
  never 
  have 
  been 
  ven- 
  

  

  