﻿Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  187 
  

  

  Both 
  Salteria 
  W. 
  Thompson 
  and 
  Endymionia 
  Billings 
  have 
  

   been 
  described 
  as 
  subgenera 
  of 
  Dionide 
  Barrande, 
  though 
  there 
  

   is 
  little 
  positive 
  evidence 
  for 
  this 
  disposition 
  of 
  them. 
  Until 
  

   more 
  perfect 
  material 
  representing 
  these 
  forms 
  has 
  been 
  

   described, 
  it 
  will 
  not 
  be 
  possible 
  to 
  decide 
  satisfactorily 
  upon 
  

   their 
  relationships 
  or 
  place 
  in 
  a 
  classification. 
  Therefore, 
  they 
  

   are 
  left 
  with 
  doubt 
  in 
  the 
  present 
  family. 
  

  

  Order 
  B. 
  OPISTHOPARIA, 
  n. 
  ord. 
  

  

  (o77to-^e 
  behind, 
  and 
  7rapetd 
  cheek 
  piece.) 
  

  

  Free 
  cheeks 
  generally 
  separate, 
  always 
  bearing 
  the 
  genal 
  

   angles. 
  Facial 
  sutures 
  extending 
  forwards 
  from 
  the 
  posterior 
  

   part 
  of 
  the 
  cephalon 
  within 
  the 
  genal 
  angles, 
  and 
  cutting 
  the 
  

   anterior 
  margin 
  separately, 
  or 
  rarely 
  uniting 
  in 
  front 
  of 
  the 
  

   glabella. 
  Compound 
  paired 
  holochroal 
  eyes 
  on 
  free 
  cheeks, 
  

   and 
  well 
  developed 
  in 
  all 
  but 
  the 
  most 
  primitive 
  family. 
  

  

  Including 
  the 
  families 
  Conocoryphidse, 
  Olenidse, 
  Asaphidse, 
  

   Proetidse, 
  Bronteidse, 
  Lichadidse, 
  and 
  Acidaspidse. 
  

  

  This 
  order 
  is 
  nearly 
  equivalent 
  to 
  group 
  B, 
  or 
  the 
  Asaphini 
  

   of 
  Salter, 
  which 
  included, 
  also 
  the 
  families 
  Calymenidse 
  and 
  

   Harpedidse, 
  which 
  belong 
  elsewhere. 
  

  

  The 
  families 
  which 
  are 
  here 
  placed 
  under 
  this 
  order 
  lend 
  

   themselves 
  quite 
  readily 
  to 
  an 
  arrangement 
  based 
  upon 
  the 
  

   characters 
  successively 
  appearing 
  in 
  the 
  ontogeny 
  of 
  any 
  of 
  the 
  

   higher 
  forms. 
  Thus 
  Sao, 
  Ptychoparia, 
  and 
  other 
  genera 
  of 
  

   the 
  Olenidse 
  have 
  first 
  a 
  protaspis 
  stage 
  only 
  comparable 
  in 
  the 
  

   structure 
  of 
  the 
  cephalon 
  with 
  the 
  genera 
  of 
  the 
  preceding 
  

   order,, 
  the 
  Hypoparia. 
  Therefore 
  this 
  stage 
  does 
  not 
  enter 
  

   into 
  consideration 
  in 
  an 
  arrangement 
  of 
  the 
  families 
  of 
  the 
  

   Opisthoparia. 
  In 
  the 
  later 
  stages, 
  however, 
  there 
  is 
  a 
  direct 
  

   agreement 
  of 
  structure 
  with 
  the 
  lower 
  genera 
  of 
  this 
  order. 
  

   The 
  nepionic 
  Sao, 
  with 
  two 
  thoracic 
  segments 
  (Plate 
  III, 
  fig. 
  2), 
  

   has 
  a 
  head 
  structure 
  agreeing 
  in 
  essential 
  features 
  with 
  that 
  in 
  

   Atops 
  or 
  Conocoryphe 
  (Plate 
  III, 
  figs. 
  14, 
  15). 
  A 
  later 
  nepionic 
  

   stage, 
  with 
  eight 
  thoracic 
  segments 
  (Plate 
  III, 
  fig. 
  3), 
  agrees 
  

   closely 
  with 
  adult 
  Ptychoparia 
  or 
  Olenus 
  (figs. 
  16, 
  17). 
  These 
  

   facts 
  clearly 
  indicate 
  that 
  the 
  family 
  Conocoryphidse 
  should 
  be 
  

   put 
  at 
  the 
  base 
  of 
  this 
  extensive 
  order. 
  Then, 
  as 
  Ptychoparia 
  

   and 
  Olenus 
  are 
  more 
  primitive 
  and 
  simple 
  genera 
  than 
  Sao, 
  

   they, 
  as 
  typifying 
  the 
  family 
  Olenidse, 
  should 
  govern 
  its 
  posi- 
  

   tion, 
  which 
  accordingly 
  would 
  be 
  next 
  after 
  the 
  Conocoryphidae. 
  

   In 
  each 
  case, 
  a 
  family 
  is 
  considered 
  as 
  represented 
  by 
  its 
  typi- 
  

   cal 
  and 
  most 
  characteristic 
  forms. 
  It 
  would 
  be 
  impossible 
  to 
  

  

  