﻿STIIUOTCRE 
  AND 
  CLASSIFICATION 
  OF 
  THE 
  ARAOHNIDA. 
  231 
  

  

  known. 
  They 
  became 
  extinct 
  in 
  PaltBozoic 
  times, 
  and 
  are 
  

   chiefly 
  found 
  in 
  the 
  Upper 
  Silurian, 
  though 
  extending 
  up- 
  

   wards 
  as 
  far 
  as 
  the 
  Carboniferous. 
  They 
  may 
  be 
  regarded 
  

   as 
  ^' 
  macrourous" 
  Xiphosura; 
  that 
  is 
  to 
  say, 
  Xiphosura 
  in 
  

   which 
  the 
  nomomeristic 
  number 
  of 
  eighteen 
  well-developed 
  

   somites 
  is 
  present, 
  and 
  the 
  posterior 
  ones 
  form 
  a 
  long 
  tail-like 
  

   region 
  of 
  the 
  body. 
  There 
  still 
  appears 
  to 
  be 
  some 
  doubt 
  

  

  Pig. 
  47. 
  — 
  Pterygotus 
  osi 
  lien 
  sis, 
  Schmidt. 
  Silurian 
  of 
  

   Rootzikil. 
  Restoration 
  of 
  the 
  ventral 
  surface, 
  one 
  third 
  the 
  natural 
  

   size, 
  after 
  Schmidt, 
  a, 
  camerostome 
  or 
  epistoma; 
  m, 
  chiiariam 
  or 
  

   nietasternite 
  of 
  tlie 
  prosoma 
  (so-called 
  metastoma) 
  ; 
  oc, 
  the 
  com- 
  

   pound 
  eyes; 
  1 
  to 
  8, 
  segments 
  of 
  the 
  sixth 
  prosomatic 
  appendage; 
  

   I' 
  to 
  V, 
  first 
  five 
  opisthosoniatic 
  somites; 
  7', 
  sixth 
  opisthoso- 
  

   matic 
  somite. 
  Observe 
  the 
  powerful 
  gnatliobases 
  of 
  the 
  sixth 
  pair 
  

   of 
  prosomatic 
  limbs 
  and 
  the 
  median 
  plates 
  behind 
  }ii. 
  Tiie 
  dotted 
  

   line 
  on 
  somite 
  I 
  indicates 
  the 
  position 
  of 
  the 
  genital 
  operculum, 
  

   which 
  was 
  probably 
  provided 
  with 
  branchial 
  lamellse. 
  (From 
  Zittel's 
  

   ' 
  Palaeontology. 
  ') 
  

  

  whether 
  in 
  the 
  sub-order 
  Eurypteromorpha 
  the 
  first 
  pair 
  of 
  

   prosomatic 
  appendages 
  (Fig. 
  46) 
  is 
  atrophied, 
  or 
  whether, 
  if 
  

   pi"esent, 
  it 
  has 
  the 
  form 
  of 
  a 
  pair 
  of 
  tactile 
  palps 
  or 
  of 
  minute 
  

  

  