﻿STllUCTUliE 
  AND 
  CLASSIFICATION 
  OF 
  THE 
  ARACHNIDA. 
  219 
  

  

  tus 
  (Fig. 
  40) 
  the 
  aiiterior 
  and 
  posterior 
  carapaces 
  constitute 
  

   almost 
  the 
  entire 
  body^ 
  the 
  two 
  carapaces 
  being- 
  connected 
  

   by 
  a 
  mid-region 
  of 
  only 
  two 
  free 
  somites. 
  It 
  has 
  been 
  held 
  

   that 
  the 
  forms 
  with 
  a 
  small 
  number 
  of 
  somites 
  marked 
  in 
  the 
  

   posterior 
  carapace, 
  and 
  numerous 
  free 
  somites 
  between 
  the 
  

   anterior 
  and 
  posterior 
  carapace, 
  must 
  be 
  considered 
  as 
  anterior 
  

   to 
  those 
  in 
  which 
  a 
  great 
  number 
  of 
  posterior 
  somites 
  are 
  

  

  Fig. 
  38. 
  

  

  Fig. 
  39. 
  

  

  Fig. 
  38. 
  — 
  Dalmanites 
  (Pliacops) 
  limulurus, 
  Green. 
  One 
  of 
  

   the 
  Pliacopidse. 
  from 
  the 
  Silurian, 
  New 
  York. 
  (From 
  Zittel.) 
  

  

  Fig. 
  39. 
  — 
  Megalaspis 
  extenuatus. 
  One 
  of 
  the 
  Asaphidse 
  

   allied 
  to 
  Ilaenus, 
  from 
  the 
  Ordoviciau 
  of 
  East 
  Gothland, 
  Sweden. 
  

   (From 
  Zittel.) 
  

  

  traceable 
  in 
  the 
  metasomatic 
  carapace, 
  and 
  that 
  those 
  in 
  which 
  

   the 
  traces 
  of 
  distinct 
  somites 
  in 
  the 
  posterior 
  or 
  metasomatic 
  

   carapace 
  are 
  most 
  completely 
  absent 
  must 
  be 
  regarded 
  as 
  

   derived 
  from 
  those 
  in 
  which 
  somites 
  are 
  well 
  marked 
  in 
  the 
  

   posterior 
  carapace 
  and 
  similar 
  in 
  appearance 
  to 
  the 
  free 
  

   somites. 
  The 
  genus 
  Agnostus, 
  which 
  belongs 
  to 
  the 
  last 
  

   category, 
  occurs 
  abundantly 
  in 
  Cambrian 
  strata, 
  and 
  is 
  one 
  

   of 
  the 
  earliest 
  forms 
  known. 
  This 
  would 
  lead 
  to 
  the 
  supposi- 
  

  

  