﻿220 
  E. 
  RAY 
  LANKESTER. 
  

  

  tion 
  that 
  the 
  great 
  development 
  of 
  metasomatic 
  carapace 
  is 
  

   a 
  primitive 
  and 
  not 
  a 
  late 
  character, 
  were 
  it 
  not 
  for 
  the 
  fact 
  

   that 
  Paradoxides 
  and 
  Atops, 
  with 
  an 
  inconspicuous 
  telsonic 
  

   carapace 
  and 
  numerous 
  free 
  somites, 
  are 
  also 
  Cambrian 
  in 
  

   age^ 
  the 
  latter, 
  indeed, 
  anterior 
  in 
  horizon 
  to 
  Agnostus. 
  

  

  On 
  the 
  other 
  hand, 
  it 
  may 
  well 
  be 
  doubted 
  whether 
  the 
  

   pygidial 
  or 
  posterior 
  carapace 
  is 
  pi'imarily 
  due 
  to 
  a 
  fusion 
  of 
  

   the 
  tergites 
  of 
  somites 
  which 
  were 
  previously 
  movable 
  and 
  

   well 
  developed. 
  The 
  posterior 
  carapace 
  of 
  the 
  Trilobites 
  and 
  

   of 
  Liraulus 
  is 
  probably 
  enough 
  in 
  origin 
  a 
  telsonic 
  carapace 
  — 
  

   that 
  is 
  to 
  say, 
  is 
  the 
  tergum 
  of 
  the 
  last 
  segment 
  of 
  the 
  body 
  

  

  Fig. 
  40. 
  — 
  Four 
  stages 
  in 
  the 
  development 
  of 
  the 
  trilobite 
  

   Agnostus 
  nudus. 
  A, 
  youngest 
  stage 
  witli 
  no 
  mesosomatic 
  

   somites. 
  B 
  and 
  C, 
  stages 
  with 
  two 
  mcsosonialic 
  somites 
  between 
  

   tlie 
  prosomatic 
  and 
  telsonic 
  carapaces; 
  D, 
  adult 
  condition, 
  still 
  with 
  

   only 
  two 
  free 
  mesosomatic 
  somites. 
  (From 
  Korschclt 
  and 
  lleider.) 
  

  

  which 
  carries 
  the 
  anus. 
  From 
  the 
  front 
  of 
  this 
  region 
  new 
  

   segments 
  are 
  produced 
  in 
  the 
  first 
  instance, 
  and 
  are 
  added 
  

   during 
  growth 
  to 
  the 
  existing 
  series. 
  This 
  telson 
  may 
  en- 
  

   large, 
  it 
  may 
  possibly 
  even 
  become 
  internally 
  and 
  sternally 
  

   developed 
  as 
  partially 
  separate 
  somites, 
  and 
  the 
  tergum 
  may 
  

   remain 
  without 
  trace 
  of 
  somite 
  formation, 
  or, 
  as 
  appears 
  to 
  

   be 
  the 
  case 
  in 
  Limulus, 
  the 
  telson 
  gives 
  rise 
  to 
  a 
  few 
  well- 
  

   marked 
  somites 
  (mesosoma 
  and 
  two 
  others), 
  and 
  then 
  en- 
  

   larges 
  without 
  further 
  trace 
  of 
  segmentation, 
  whilst 
  the 
  

   chitinous 
  integument 
  which 
  develops 
  in 
  increasing 
  thickness 
  

   on 
  the 
  terga 
  as 
  growth 
  advances 
  welds 
  together 
  the 
  unseg- 
  

   mented 
  telson 
  and 
  the 
  somites 
  in 
  front 
  of 
  it, 
  which 
  were 
  

  

  