﻿P. 
  E. 
  Raymond 
  — 
  Beecher's 
  Classification 
  of 
  Trilobites. 
  209 
  

  

  tral 
  in 
  position, 
  and 
  there 
  is 
  no 
  reason 
  why 
  in 
  development 
  

   they 
  should 
  travel 
  backward. 
  This 
  seems 
  the 
  greatest 
  argu- 
  

   ment 
  against 
  Swinnerton's 
  view, 
  for 
  in 
  practically 
  all 
  trilobites 
  

   except 
  the 
  Meson 
  aeidse, 
  the 
  eyes 
  appear 
  first 
  at 
  the 
  margin 
  and 
  

   do 
  travel 
  backward. 
  Beecher's 
  theory 
  seems 
  to 
  explain 
  all 
  

   trilobites 
  except 
  the 
  Mesonacidse, 
  while 
  Professor 
  Swinnerton's 
  

   applies 
  to 
  the 
  Mesonacidae 
  principally, 
  and 
  unless 
  it 
  can 
  be 
  

   shown 
  that 
  the 
  Trilobita 
  are 
  polyphyletic, 
  it 
  seems 
  better 
  to 
  

   adhere 
  to 
  Beecher's 
  idea, 
  and 
  await 
  an 
  explanation 
  of 
  the 
  

   Mesonacidae 
  which 
  is 
  in 
  accordance 
  with 
  our 
  notions 
  of 
  other 
  

   trilobites.* 
  

  

  As 
  a 
  matter 
  of 
  fact, 
  if 
  Swinnerton's 
  suggestion 
  of 
  the 
  Pro- 
  

   toparia 
  were 
  logically 
  carried 
  out, 
  it 
  would 
  affect 
  the 
  present 
  

   classification 
  but 
  slightly. 
  He 
  would 
  probably 
  remove 
  the 
  

   Raphiophoridae 
  to 
  the 
  Opisthoparia, 
  but 
  the 
  remaining 
  Hypo- 
  

   paria 
  would 
  become 
  Protoparia 
  and 
  the 
  Mesonacidee 
  would 
  

   replace 
  the 
  Raphiophoridse 
  as 
  the 
  highest 
  of 
  the 
  Protoparia. 
  

   It 
  is 
  not 
  at 
  all 
  a 
  consequence 
  of 
  his 
  definition 
  of 
  the 
  Protopa- 
  

   ria 
  that 
  the 
  Agnostidse 
  and 
  Trinucleidse 
  should 
  be 
  considered 
  

   degenerate 
  animals, 
  On 
  the 
  contrary, 
  as 
  they 
  possess 
  a 
  large 
  

   ventral 
  plate 
  (epistoma) 
  and 
  no 
  visible 
  free 
  cheeks, 
  the 
  traces 
  of 
  

   eyes 
  which 
  they 
  possess 
  may 
  well 
  be 
  considered 
  rudimentary 
  

   instead 
  of 
  vestigial, 
  and 
  when, 
  as 
  in 
  Pagetia 
  among 
  the 
  

   Eodiscidse, 
  eyes 
  become 
  fully 
  developed, 
  facial 
  sutures 
  and 
  free 
  

   cheeks 
  are 
  formed. 
  At 
  first 
  sight, 
  this 
  method 
  of 
  the 
  forma- 
  

   tion 
  of 
  the 
  free 
  cheeks 
  would 
  seem 
  to 
  apply 
  only 
  to 
  the 
  Opis- 
  

   thoparia, 
  but 
  in 
  the 
  secondary 
  forward 
  movement 
  of 
  the 
  

   pleural 
  portions 
  of 
  the 
  head, 
  as 
  evidenced 
  by 
  the 
  forward 
  

   migration 
  of 
  the 
  spines, 
  is 
  seen 
  a 
  modus 
  by 
  which 
  the 
  posterior 
  

   portion 
  of 
  the 
  facial 
  suture 
  may 
  be 
  brought 
  to 
  terminate 
  in 
  

   front 
  of 
  the 
  genal 
  angle, 
  and 
  so 
  produce 
  the 
  Proparia.f 
  

  

  I 
  have 
  thus 
  developed 
  Swinnerton's 
  scheme 
  at 
  greater 
  length 
  

   than 
  did 
  its 
  proposer 
  in 
  order 
  to 
  show 
  how 
  attractive 
  it 
  is, 
  and 
  

   how 
  well 
  it 
  explains 
  conditions 
  which 
  obtain 
  in 
  the 
  Mesona- 
  

   cidse, 
  and 
  at 
  the 
  same 
  time, 
  how 
  little 
  change 
  its 
  adoption 
  

   would 
  make 
  in 
  our 
  present 
  classification. 
  Little 
  is 
  known 
  of 
  

   the 
  morphology 
  of 
  the 
  head 
  of 
  the 
  trilobite, 
  and 
  it 
  is 
  entirely 
  

   possible 
  that 
  the 
  free 
  cheeks 
  may 
  be 
  a 
  part 
  of 
  the 
  epistomal 
  

   segment 
  in 
  one 
  group 
  of 
  them, 
  and 
  a 
  part 
  of 
  the 
  first 
  dorsal 
  

   segment 
  in 
  others. 
  The 
  problem 
  is 
  well 
  worth 
  keeping 
  in 
  

   mind 
  in 
  future 
  investigations. 
  Until 
  more 
  is 
  known, 
  however, 
  

   it 
  seems 
  as 
  though 
  the 
  Mesonacidse 
  were 
  in 
  as 
  secure 
  a 
  place 
  

  

  *Beecher 
  pointed 
  out 
  that 
  the 
  actual 
  protaspis 
  of 
  Elliptocephala 
  is 
  not 
  yet 
  

   known, 
  and 
  when 
  discovered 
  it 
  may 
  prove 
  to 
  be 
  eyeless, 
  and 
  in 
  conformity 
  

   with 
  other 
  young 
  of 
  Cambrian 
  trilobites. 
  

  

  f 
  See 
  in 
  this 
  connection 
  Dr. 
  Clarke's 
  figures 
  of 
  Proboloides 
  cuspidatus 
  from 
  

   the 
  Devonian 
  at 
  Ponta 
  Grossa, 
  Brazil. 
  Monographias 
  do 
  Servico 
  Geologico 
  

   e 
  Mineralogico 
  do 
  Brazil, 
  1, 
  1913, 
  pi. 
  7. 
  

  

  