﻿92 
  Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  Salter, 
  Burmeister, 
  and 
  Emmrich 
  have, 
  as 
  previously 
  noticed, 
  

   attempted 
  to 
  use 
  the 
  comparative 
  size 
  and 
  development 
  of 
  the 
  

   pygidium 
  for 
  dividing 
  the 
  trilobites 
  into 
  groups 
  larger 
  than 
  

   families, 
  and 
  it 
  seems 
  evident 
  from 
  the 
  present 
  state 
  of 
  knowl- 
  

   edge 
  that 
  it 
  is 
  impossible 
  to 
  make 
  this 
  character 
  of 
  more 
  than 
  

   family 
  or 
  even 
  generic 
  value. 
  Many 
  of 
  the 
  genera 
  which 
  

   must 
  naturally 
  be 
  included 
  in 
  the 
  Archiaspida 
  have 
  pygidia 
  

   that 
  cannot 
  be 
  said 
  to 
  be 
  rudimentary, 
  obsolete, 
  or 
  wanting 
  

   in 
  function. 
  Even 
  those 
  genera 
  having 
  pygidia 
  with 
  few 
  seg- 
  

   ments, 
  as 
  Meso?iacis, 
  Holmia, 
  Paradoxides, 
  Selenopeltis, 
  

   Dicranurus, 
  Bronteus, 
  Harpes, 
  etc., 
  show 
  in 
  many 
  other 
  

   more 
  important 
  characters 
  that 
  they 
  are 
  highly 
  differentiated 
  

   and 
  specialized 
  forms 
  and 
  that 
  this 
  feature 
  is 
  one 
  expression 
  of 
  

   such 
  development. 
  The 
  futility 
  of 
  the 
  scheme 
  is 
  at 
  once 
  evi- 
  

   dent 
  when 
  a 
  comparison 
  is 
  made 
  between 
  allied 
  genera 
  which 
  

   present 
  marked 
  differences 
  in 
  the 
  size 
  and 
  segmentation 
  of 
  the 
  

   pygidium, 
  as 
  Phacops 
  and 
  Dalmanites, 
  Ceraurus 
  and 
  Encri- 
  

   nurus, 
  Calymene 
  and 
  Homolanotus, 
  Harpes 
  and 
  Trinucleus, 
  

   Mesonacis 
  and 
  Zecanthoides, 
  Paradoxides 
  and 
  Dikelocephalus. 
  

  

  The 
  last 
  classification 
  to 
  be 
  noticed 
  is 
  that 
  of 
  E. 
  J. 
  Chapman, 
  13 
  

   in 
  1889, 
  in 
  which 
  four 
  suborders 
  or 
  primary 
  groups 
  are 
  pro- 
  

   posed, 
  differing 
  considerably 
  from 
  any 
  previous 
  arrangement, 
  

   and 
  based 
  upon 
  arbitrary 
  features 
  of 
  general 
  structure 
  and 
  con- 
  

   figuration, 
  especially 
  the 
  form 
  of 
  the 
  glabella, 
  whether 
  wide, 
  

   conical, 
  or 
  enlarged. 
  Twenty-seven 
  families 
  are 
  recognized. 
  

   In 
  this 
  scheme, 
  Trinucleus, 
  Ampyx, 
  and 
  JEglina 
  form 
  one 
  

   section 
  ; 
  Paradoxides 
  and 
  Acidaspis, 
  together 
  with 
  Phacops 
  

   and 
  Encrinurus, 
  another 
  ; 
  all 
  under 
  one 
  suborder. 
  Omitting 
  

   the 
  Agnostidse, 
  there 
  are 
  here 
  considered 
  in 
  a 
  single 
  suborder 
  

   the 
  most 
  characteristic 
  representatives 
  of 
  nearly 
  all 
  the 
  types 
  

   of 
  trilobite 
  structure. 
  Proetus, 
  Oyphaspis, 
  and 
  Arethusina 
  

   fall 
  into 
  three 
  sections, 
  under 
  two 
  suborders, 
  although 
  these 
  

   genera, 
  on 
  account 
  of 
  their 
  great 
  similarity 
  in 
  essential 
  points, 
  

   are 
  placed 
  in 
  a 
  single 
  family 
  by 
  most 
  authors. 
  A 
  further 
  

   analysis 
  of 
  this 
  classification 
  in 
  its 
  broader 
  lines 
  would 
  be 
  

   unprofitable. 
  It 
  is 
  sufficient 
  to 
  state 
  that 
  the 
  facts 
  obtained 
  

   from 
  the 
  study 
  of 
  the 
  ontogeny 
  of 
  any 
  species 
  are 
  completely 
  

   in 
  discordance 
  with 
  these 
  classifications,, 
  and 
  clearly 
  1 
  demand 
  

   other 
  interpretations. 
  

  

  Bank 
  of 
  the 
  Trilobites. 
  

  

  As 
  to 
  the 
  rank 
  of 
  the 
  trilobites 
  in 
  a 
  classification 
  of 
  the 
  Crus- 
  

   tacea, 
  there 
  is 
  also 
  much 
  diversity 
  of 
  opinion. 
  They 
  have 
  long 
  

   been 
  regarded 
  as 
  an 
  order, 
  but 
  any 
  attempt 
  to 
  include 
  them 
  in 
  

   this 
  way 
  under 
  higher 
  groups, 
  such 
  as 
  the 
  Ento?nostraca, 
  Mal- 
  

   acostraca, 
  or 
  Palwocarida, 
  results 
  in 
  such 
  broad 
  generalities 
  

   and 
  looseness 
  of 
  definition 
  as 
  to 
  render 
  these 
  divisions 
  of 
  little 
  

  

  