﻿STRUCTURE 
  AND 
  CLASSIFICATION 
  OF 
  THK 
  ABAOHNIDA 
  . 
  221 
  

  

  previously 
  marked 
  by 
  separate 
  tergal 
  thickenings. 
  It 
  must 
  

   always 
  be 
  remembered 
  that 
  we 
  are 
  liable 
  (especially 
  in 
  the 
  

   case 
  of 
  fossilised 
  integuments) 
  to 
  attach 
  an 
  unwarranted 
  

   interpretation 
  to 
  the 
  mere 
  discontinuity 
  or 
  continuity 
  of 
  the 
  

   thickened 
  plates 
  of 
  chitinous 
  cuticle 
  on 
  the 
  back 
  of 
  an 
  Arthro- 
  

   pod. 
  These 
  plates 
  may 
  fuse, 
  and 
  yet 
  the 
  somites 
  to 
  which 
  

   they 
  belong 
  may 
  remain 
  distinct, 
  and 
  each 
  have 
  its 
  pair 
  of 
  

  

  EiG. 
  41. 
  — 
  Five 
  stages 
  in 
  the 
  development 
  of 
  the 
  trilobite 
  Sao 
  

   liirsuta. 
  A, 
  youngest 
  stage; 
  B, 
  older 
  stage 
  with 
  distinct 
  pygidial 
  

   carapace 
  ; 
  C, 
  stage 
  with 
  two 
  free 
  mesosomatic 
  somites 
  between 
  the 
  

   prosomatic 
  and 
  telsonic 
  carapaces; 
  D, 
  stage 
  with 
  seven 
  free 
  inter- 
  

   mediate 
  somites 
  ; 
  E, 
  stage 
  with 
  twelve 
  free 
  somites 
  ; 
  the 
  telsonic 
  

   carapace 
  has 
  not 
  increased 
  in 
  size 
  ; 
  «, 
  lateral 
  eye 
  ; 
  g, 
  so-called 
  facial 
  

   "suture" 
  (not 
  really 
  a 
  suture); 
  p, 
  telsonic 
  carapace. 
  (From 
  

   Korschelt 
  and 
  Heider, 
  after 
  Barrande.) 
  

  

  appendages 
  well 
  developed. 
  On 
  the 
  other 
  hand, 
  an 
  unusually 
  

   large 
  tergal 
  plate, 
  whether 
  terminal 
  or 
  in 
  the 
  series, 
  is 
  not 
  

   always 
  due 
  to 
  fusion 
  of 
  the 
  dorsal 
  plates 
  of 
  once-separated 
  

   somites, 
  but 
  is 
  often 
  a 
  case 
  of 
  growth 
  and 
  enlargement 
  of 
  a 
  

   single 
  somite 
  without 
  formation 
  of 
  any 
  trace 
  of 
  a 
  new 
  somite. 
  

   For 
  the 
  literature 
  of 
  Trilobites 
  see 
  22*. 
  

  

  Grade 
  B 
  (of 
  the 
  Arachnida). 
  NOMOMERISTICA. 
  

   — 
  Arachnida 
  in 
  Avhich, 
  excluding 
  from 
  consideration 
  the 
  eye- 
  

   bearing 
  prosthomere, 
  the 
  somites 
  are 
  primarily 
  (that 
  is 
  to 
  say, 
  

  

  