﻿Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  91 
  

  

  based 
  on 
  whether 
  the 
  abdomen 
  was 
  composed 
  of 
  fused 
  or 
  free 
  

   segments, 
  and 
  the 
  minor 
  divisions 
  depended 
  chiefly 
  on 
  the 
  

   structure 
  of 
  the 
  eyes 
  and 
  the 
  facial 
  suture. 
  (9) 
  Corda, 
  15 
  in 
  

   1847, 
  placed 
  all 
  trilobites 
  in 
  two 
  groups, 
  one 
  having 
  an 
  entire 
  

   pygidial 
  margin, 
  and 
  the 
  other 
  with 
  the 
  pygidium 
  lobed 
  or 
  

   denticulate. 
  (10) 
  McCoy, 
  25 
  in 
  18-19, 
  took 
  the 
  presence 
  or 
  

   absence 
  of 
  a 
  facet 
  on 
  the 
  pleura 
  for 
  a 
  divisional 
  character. 
  As 
  

   this 
  is 
  an 
  indication 
  of 
  the 
  power 
  or 
  the 
  inability 
  of 
  enrollment, 
  it 
  

   does 
  not 
  differ 
  materially 
  from 
  the 
  schemes 
  of 
  Milne-Edwards 
  

   and 
  Burmeister. 
  

  

  Zittel, 
  35 
  in 
  a 
  historial 
  review 
  brought 
  down 
  to 
  1885, 
  includes 
  

   in 
  addition 
  the 
  schemes 
  of 
  Barrande 
  and 
  Salter, 
  and 
  remarks 
  

   that 
  the 
  basis 
  of 
  Barrande's 
  general 
  grouping, 
  namely, 
  the 
  

   structure 
  of 
  the 
  pleura, 
  has 
  neither 
  a 
  high 
  physiological 
  nor 
  

   morphological 
  meaning. 
  Both 
  Barrande 
  and 
  Salter 
  recognize 
  

   nearly 
  the 
  same 
  families, 
  with 
  slight 
  differences, 
  and 
  the 
  latter 
  

   adopts 
  a 
  division 
  into 
  two 
  lines, 
  based 
  on 
  the 
  number 
  of 
  body 
  

   rings 
  and 
  the 
  size 
  of 
  the 
  pygidium. 
  These 
  include 
  and 
  are 
  them- 
  

   selves 
  included 
  in 
  four 
  groups, 
  founded 
  on 
  the 
  presence 
  and 
  

   form 
  of 
  the 
  facial 
  suture 
  and 
  the 
  structure 
  of 
  the 
  eyes. 
  

  

  Haeckel* 
  has 
  recently 
  given 
  the 
  trilobites 
  their 
  full 
  value 
  in 
  

   a 
  classification 
  of 
  the 
  articulates. 
  Although 
  he 
  has 
  not 
  

   advanced 
  a 
  detailed 
  classification, 
  still 
  it 
  is 
  desirable 
  to 
  review 
  

   the 
  ordinal 
  groups 
  which 
  he 
  proposes. 
  He 
  considers 
  the 
  Trilo- 
  

   bita 
  as 
  a 
  legion 
  under 
  the 
  first 
  class, 
  Aspidonia, 
  of 
  the 
  Crus- 
  

   tacea, 
  which 
  is 
  characterized 
  as 
  being 
  without 
  a 
  nauplius 
  larval 
  

   form 
  and 
  as 
  having 
  a 
  pair 
  of 
  preoral 
  antennae. 
  In 
  this 
  class 
  is 
  

   also 
  included 
  the 
  legion 
  Merostomata, 
  the 
  Trilobita 
  being 
  

   especially 
  distinguished 
  by 
  the 
  number 
  and 
  character 
  of 
  the 
  

   legs. 
  The 
  writer 
  5 
  believes 
  that 
  it 
  is 
  now 
  satisfactorily 
  demon- 
  

   strated 
  that 
  the 
  protaspis, 
  or 
  early 
  larval 
  form 
  of 
  the 
  trilobite, 
  

   is 
  a 
  protonauplius, 
  and 
  homologous 
  with 
  the 
  nauplius 
  of 
  higher 
  

   Crustacea, 
  Therefore, 
  the 
  Trilobita 
  cannot 
  remain 
  in 
  the 
  

   Aspidonia 
  as 
  here 
  defined. 
  

  

  Haeckel 
  further 
  divides 
  the 
  Trilobita 
  into 
  two 
  orders, 
  the 
  

   first, 
  the 
  Archiaspides 
  (or 
  Protrilobita\ 
  and 
  the 
  second, 
  the 
  

   Eutrildbita 
  (or 
  Pygidiatd). 
  The 
  Archiaspides 
  is 
  represented 
  

   by 
  the 
  families 
  Olenida 
  and 
  Triarthrida, 
  and 
  is 
  distinguished 
  

   by 
  the 
  absence 
  of 
  a 
  real 
  pygidium, 
  and 
  by 
  the 
  complete 
  homon- 
  

   omy 
  of 
  the 
  numerous 
  body 
  segments 
  and 
  their 
  phyllopodi- 
  

   form 
  appendages. 
  The 
  families 
  are 
  themselves 
  distinguished 
  

   by 
  the 
  semicircular 
  or 
  crescent-shaped 
  cephalon 
  and 
  by 
  the 
  

   presence 
  or 
  absence 
  of 
  genal 
  spines. 
  The 
  Eutrilobita 
  is 
  rep- 
  

   resented 
  by 
  the 
  families 
  Asaphida 
  and 
  Calymenida, 
  and 
  is 
  

   marked 
  by 
  the 
  heteronomy 
  of 
  the 
  body 
  segments 
  as 
  expressed 
  

   in 
  the 
  functional 
  pygidium. 
  

  

  * 
  Systematische 
  Phylogenie 
  der 
  wirbellosen 
  Thiere 
  (Invertebrata). 
  Zweiter 
  

   Theil. 
  1896. 
  

  

  