﻿212 
  E. 
  KAY 
  LANKKSTEll. 
  

  

  pygidial 
  or 
  telsouic 
  shield 
  — 
  a 
  fusion 
  of 
  the 
  posterior 
  somites 
  

   of 
  the 
  body, 
  which 
  is 
  precisely 
  identical 
  in 
  character 
  with 
  

   the 
  metasomatic 
  carapace 
  of 
  Limuliis. 
  When 
  to 
  this 
  is 
  

   added 
  the 
  fact 
  that 
  a 
  post-anal 
  spine 
  is 
  developed 
  to 
  a 
  large 
  

   size 
  in 
  some 
  Trilobites 
  (Fig. 
  38), 
  like 
  that 
  of 
  Limulus 
  and 
  

   Scorpio, 
  and 
  that 
  lateral 
  spines 
  on 
  the 
  pleura 
  of 
  the 
  somites 
  

   are 
  frequent 
  as 
  in 
  Limulus, 
  and 
  that 
  neither 
  metasomatic 
  

   fusion 
  of 
  somites 
  nor 
  post-anal 
  spine, 
  nor 
  lateral 
  pleural 
  

   spines 
  are 
  found 
  in 
  any 
  Crustacean, 
  nor 
  all 
  three 
  together 
  in 
  

   any 
  Arthropod 
  besides 
  the 
  Trilobites 
  and 
  Limulus, 
  the 
  claim 
  

   of 
  the 
  Trilobites 
  to 
  be 
  considered 
  as 
  representing 
  one 
  order 
  of 
  

   a 
  lower 
  grade 
  of 
  Arachnida_. 
  comparable 
  to 
  the 
  grade 
  Ento- 
  

   mostraca 
  of 
  the 
  Crustacea, 
  seems 
  to 
  be 
  established. 
  

  

  The 
  fact 
  that 
  the 
  single 
  pair 
  of 
  prfeoral 
  appendages 
  of 
  

   Trilobites, 
  kuown 
  only 
  as 
  yet 
  in 
  one 
  genus, 
  is 
  in 
  that 
  particu- 
  

   lar 
  case 
  a 
  pair 
  of 
  uniramose 
  antennae, 
  does 
  not 
  reuder 
  the 
  

   association 
  of 
  Trilobites 
  and 
  Arachnids 
  improbable. 
  Although 
  

   the 
  prfBoral 
  pair 
  of 
  appendages 
  in 
  the 
  higher 
  Arachnida 
  is 
  

   usually 
  chelate, 
  it 
  is 
  not 
  always 
  so; 
  in 
  spiders 
  it 
  is 
  not 
  so; 
  

   nor 
  in 
  many 
  Acari. 
  The 
  biramose 
  structure 
  of 
  the 
  post-oral 
  

   limbs, 
  demonstrated 
  by 
  Beecher 
  in 
  the 
  Trilobite 
  Triarthrus, 
  

   is 
  no 
  more 
  inconsistent 
  Avith 
  its 
  claim 
  to 
  be 
  a 
  primitive 
  

   Arachnid 
  than 
  is 
  the 
  foliaceous 
  modification 
  of 
  the 
  limbs 
  in 
  

   Phyllopods 
  inconsistent 
  with 
  their 
  relationship 
  to 
  the 
  Arthros- 
  

   tracous 
  Crustaceans 
  such 
  as 
  Gammarus 
  and 
  Ouiscns. 
  

  

  Thus, 
  then, 
  it 
  seems 
  that 
  we 
  have 
  in 
  the 
  Trilobites 
  the 
  

   representatives 
  of 
  the 
  lower 
  phases 
  of 
  the 
  Arachnidan 
  pedi- 
  

   gree. 
  The 
  simple 
  anomomeristic 
  Trilobite, 
  with 
  its 
  equi- 
  

   formal 
  somites 
  and 
  equiformal 
  appendages, 
  is 
  one 
  term 
  of 
  

   the 
  series 
  which 
  ends 
  in 
  the 
  even 
  more 
  simple 
  but 
  degenerate 
  

   Acari. 
  Between 
  the 
  two 
  and 
  at 
  the 
  highest 
  point 
  of 
  the 
  arc, 
  

   so 
  far 
  as 
  morphological 
  differentiation 
  is 
  concerned, 
  stands 
  

   the 
  scorpion 
  ; 
  near 
  to 
  it 
  in 
  the 
  Trilobite's 
  direction 
  (that 
  is 
  on 
  

   the 
  ascending 
  side) 
  are 
  Limulus 
  and 
  the 
  Eurypterines 
  — 
  with 
  

   a 
  long 
  gap, 
  due 
  to 
  obliteration 
  of 
  the 
  record, 
  separating 
  them 
  

   from 
  the 
  Trilobite. 
  On 
  the 
  other 
  side 
  — 
  tending 
  downwards 
  

   from 
  the 
  scorpion 
  towards 
  the 
  Acari 
  — 
  are 
  the 
  Pedipalpi, 
  the 
  

  

  