﻿198 
  Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  pleura. 
  The 
  Lichades 
  are 
  generally 
  much 
  larger 
  and 
  flatter, 
  

   but 
  the 
  smaller 
  and 
  highly 
  spinose 
  forms 
  of 
  Arges, 
  Ceratolichas, 
  

   and 
  Iloplolichas 
  approach 
  quite 
  near 
  some 
  of 
  the 
  Acidaspidse. 
  

   It 
  has 
  been 
  customary 
  of 
  late 
  years 
  to 
  regard 
  all 
  the 
  species 
  

   of 
  this 
  family 
  as 
  belonging 
  to 
  the 
  single 
  genus 
  Acidaspis, 
  and 
  

   to 
  consider 
  the 
  various 
  subdivisions 
  bearing 
  separate 
  names 
  as 
  

   of 
  the 
  value 
  of 
  subgenera. 
  Clarke 
  14 
  has 
  shown 
  that, 
  on 
  the 
  

   basis 
  of 
  priority, 
  Ceratocephala 
  is 
  the 
  first 
  distinctive 
  name 
  ever 
  

   applied 
  to 
  the 
  group, 
  and 
  is 
  therefore 
  entitled 
  to 
  full 
  generic 
  

   recognition. 
  He 
  further 
  recognizes 
  Odontopleura, 
  Acidaspis, 
  

   Dicranurus, 
  Selenopeltis, 
  and 
  Ancyropyge, 
  in 
  the 
  subordinate 
  

   position 
  of 
  subgenera 
  under 
  Ceratocephala. 
  

  

  Order 
  0. 
  PROPARIA, 
  n. 
  ord. 
  

   (77-pd 
  before, 
  and 
  irapua 
  cheek 
  piece.) 
  

  

  Free 
  cheeks 
  not 
  bearing 
  the 
  genal 
  angles. 
  Facial 
  sutures 
  

   extending 
  from 
  the 
  lateral 
  margins 
  of 
  the 
  cephalon 
  in 
  front 
  of 
  

   the 
  genal 
  angles, 
  inward 
  and 
  forward, 
  cutting 
  the 
  anterior 
  

   margin 
  separately 
  or 
  uniting 
  in 
  front 
  of 
  the 
  glabella. 
  Com- 
  

   pound 
  paired 
  eyes 
  scarcely 
  developed 
  or 
  sometimes 
  absent 
  in 
  

   the 
  most 
  primitive 
  family, 
  well-developed 
  and 
  schizochroal 
  in 
  

   last 
  family. 
  

  

  Including 
  the 
  families 
  Encrinuridse, 
  Calymenidse, 
  Cheiru- 
  

   ridae, 
  and 
  Phacopidse. 
  

  

  Salter's 
  first 
  division, 
  Phacopini, 
  included 
  the 
  two 
  families 
  

   Phacopidse 
  and 
  Cheiruridse. 
  The 
  Calymenidse 
  were 
  placed 
  in 
  

   his 
  second 
  division, 
  the 
  Asaphini. 
  

  

  This 
  is 
  the 
  only 
  order 
  of 
  trilobites 
  which 
  apparently 
  begins 
  

   within 
  the 
  known 
  Paleozoic, 
  and 
  unlike 
  the 
  other 
  orders, 
  it 
  had 
  

   no 
  pre- 
  Cambrian 
  existence. 
  The 
  earliest 
  forms 
  of 
  the 
  Proparia 
  

   came 
  at 
  the 
  close 
  of 
  the 
  Cambrian, 
  in 
  the 
  lower 
  Ordovician. 
  

   Its 
  greatest 
  generic 
  differentiation 
  was 
  attained 
  early. 
  There 
  

   is 
  a 
  rapid 
  decline 
  in 
  the 
  Silurian 
  and 
  Devonian, 
  and 
  only 
  one 
  or 
  

   two 
  genera 
  extend 
  to 
  the 
  beginning 
  of 
  the 
  Carboniferous. 
  

  

  In 
  the 
  Opisthoparia, 
  it 
  was 
  demonstrated 
  that 
  the 
  Conocoryph- 
  

   idse 
  formed 
  the 
  natural 
  base 
  or 
  most 
  primitive 
  family 
  in 
  the 
  

   order, 
  and 
  was 
  distinguished 
  by 
  the 
  narrow 
  marginal 
  free 
  

   cheeks 
  and 
  the 
  absence 
  of 
  well-developed 
  eyes. 
  It 
  is 
  of 
  much 
  

   interest 
  and 
  importance 
  to 
  be 
  able 
  to 
  recognize, 
  in 
  the 
  Proparia, 
  

   a 
  similar 
  primitive 
  family 
  having 
  characters 
  in 
  common 
  with 
  

   the 
  former, 
  but 
  still 
  clearly 
  belonging 
  to 
  the 
  higher 
  order. 
  

   Placoparia, 
  Areia, 
  and 
  Dindymene, 
  of 
  the 
  Encrinuridse, 
  con- 
  

   stitute 
  a 
  group 
  of 
  apparently 
  blind 
  trilobites, 
  with 
  narrow 
  mar- 
  

  

  