﻿106 
  Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  separate 
  or 
  united 
  in 
  front, 
  and 
  carry 
  the 
  compound 
  sessile 
  

   eyes, 
  when 
  present 
  ; 
  -cephalic 
  appendages 
  pediform, 
  consisting 
  

   of 
  five 
  pairs 
  of 
  limbs, 
  all 
  biramous, 
  and 
  functioning 
  as 
  ambu- 
  

   latory 
  and 
  oral 
  organs, 
  except 
  the 
  simple 
  antennules, 
  which 
  are 
  

   purely 
  sensory. 
  Upper 
  lip 
  forming 
  a 
  well-developed 
  hypos- 
  

   toma 
  ; 
  under 
  lip 
  present. 
  Somites 
  of 
  the 
  thorax 
  movable 
  

   upon 
  one 
  another, 
  varying 
  in 
  number 
  from 
  two 
  to 
  twenty-nine. 
  

   Abdominal 
  segments 
  variable 
  in 
  number, 
  and 
  fused 
  to 
  form 
  a 
  

   caudal 
  shield. 
  All 
  segments, 
  thoracic 
  and 
  abdominal, 
  carry 
  a 
  

   pair 
  of 
  jointed 
  biramous 
  limbs. 
  All 
  limbs 
  have 
  their 
  coxal 
  

   elements 
  forming 
  gnathobases, 
  which 
  become 
  organs 
  of 
  mandu- 
  

   cation 
  on 
  the 
  head. 
  Respiration 
  integumental 
  and 
  by 
  branchial 
  

   fringes 
  on 
  the 
  exopodites. 
  Development 
  proceeding 
  from 
  a 
  

   protonauplius 
  form, 
  by 
  the 
  progressive 
  addition 
  of 
  segments 
  at 
  

   successive 
  moults. 
  

  

  Heretofore 
  it 
  has 
  been 
  impossible 
  to 
  give 
  an 
  adequate 
  diag- 
  

   nosis 
  of 
  the 
  Trilobita, 
  owing 
  to 
  the 
  absence 
  of 
  information 
  

   regarding 
  certain 
  important 
  characters, 
  and 
  the 
  obscurity 
  of 
  

   the 
  information 
  relating 
  to 
  some 
  other 
  features. 
  It 
  is 
  believed 
  

   that 
  enough 
  is 
  now 
  known 
  to 
  frame 
  a 
  definition 
  of 
  the 
  class, 
  

   which, 
  in 
  accuracy 
  and 
  completeness, 
  will 
  compare 
  favorably 
  

   with 
  any 
  based 
  upon 
  living 
  groups. 
  Such 
  a 
  definition 
  brings 
  

   out 
  the 
  fact 
  that 
  the 
  differences 
  between 
  the 
  trilobites 
  and 
  

   other 
  large 
  groups 
  are 
  clearly 
  recognizable, 
  and 
  do 
  not 
  consist 
  

   of 
  a 
  statement 
  of 
  anomalous 
  characters 
  whose 
  real 
  significance 
  

   is 
  unknown. 
  

  

  [To 
  be 
  continued.] 
  

  

  