﻿218 
  

  

  E. 
  RAY 
  LANKESTER. 
  

  

  demarcation 
  of 
  certain 
  areas 
  in 
  that 
  structure. 
  Sutures 
  are 
  

   stated 
  to 
  mark 
  off 
  some 
  of 
  these 
  pieces, 
  but 
  in 
  the 
  proper 
  

   sense 
  of 
  that 
  term, 
  as 
  applied 
  to 
  the 
  skeletal 
  structures 
  of 
  the 
  

   Vertebrata, 
  no 
  sutures 
  exist 
  in 
  the 
  chitinous 
  cuticle 
  of 
  Arthro- 
  

   poda. 
  That 
  any 
  partial 
  fusion 
  of 
  originally 
  distinct 
  chitinous 
  

   plates 
  takes 
  place 
  in 
  the 
  cephalic 
  shield 
  of 
  Trilobites, 
  com- 
  

   parable 
  to 
  the 
  partial 
  fusion 
  of 
  bony 
  pieces 
  by 
  suture 
  in 
  

   Vertebrata, 
  is 
  a 
  suggestion 
  contrary 
  to 
  fact. 
  

  

  The 
  Trilobites 
  are 
  known 
  only 
  as 
  fossils, 
  mostly 
  Silurian 
  and 
  

   pre-Silurlan 
  ; 
  a 
  few 
  are 
  found 
  in 
  Carboniferous 
  and 
  Permian 
  

   strata. 
  As 
  many 
  as 
  two 
  thousand 
  species 
  are 
  known. 
  Genera 
  

  

  Fig. 
  37. 
  

  

  Fig. 
  36. 
  

  

  Fig. 
  36. 
  — 
  Triartlirus 
  Becki, 
  Green. 
  Dorsal 
  view 
  of 
  second 
  

   thoracic 
  leg 
  witli 
  and 
  without 
  setae, 
  en, 
  inner 
  ramus 
  ; 
  ex, 
  outer 
  

   ramus. 
  (After 
  Beecher.) 
  

  

  Fig. 
  37.— 
  Deiphon 
  Forbesii, 
  Barr. 
  One 
  of 
  tlie 
  Cheiruridse. 
  

   Silurian, 
  Bohemia. 
  (From 
  Zittel's 
  ' 
  Palaeontology.) 
  

  

  with 
  small 
  metasomatic 
  carapace, 
  consisting 
  of 
  three 
  to 
  six 
  

   fused 
  segments 
  distinctly 
  marked 
  though 
  not 
  separated 
  by 
  soft 
  

   membrane, 
  are 
  Harpes, 
  Paradoxides, 
  and 
  Triarthi-us 
  (Fig. 
  34) 
  . 
  

   In 
  Calymene, 
  Homalonotus, 
  and 
  Phacops 
  (Fig. 
  38) 
  from 
  six 
  

   to 
  sixteen 
  segments 
  are 
  clearly 
  marked 
  by 
  ridges 
  and 
  grooves 
  

   in 
  the 
  metasomatic 
  tagma, 
  whilst 
  in 
  Ila^nus 
  (Fig. 
  39) 
  the 
  

   shield 
  so 
  formed 
  is 
  large, 
  but 
  no 
  somites 
  are 
  marked 
  out 
  on 
  

   its 
  surface. 
  In 
  this 
  genus 
  ten 
  free 
  somites 
  (mesosoma) 
  occur 
  

   between 
  the 
  prosomatic 
  and 
  metasomatic 
  carapaces. 
  Asaphus 
  

   and 
  Megalaspis 
  (Fig. 
  39) 
  are 
  similarly 
  constituted. 
  In 
  Agnos- 
  

  

  