﻿28 
  A. 
  F. 
  Foerste 
  — 
  Relations 
  of 
  the 
  

  

  fig. 
  7B, 
  in 
  the 
  absence 
  of 
  the 
  diagonal 
  furrow 
  on 
  the 
  pos- 
  

   terior 
  pair 
  of 
  ribs 
  and 
  in 
  the 
  slight 
  expansion 
  of 
  the 
  

   posterior 
  part 
  of 
  the 
  axial 
  lobe. 
  However, 
  the 
  disap- 
  

   pearance 
  laterally 
  of 
  the 
  ends 
  of 
  the 
  furrow 
  limiting 
  the 
  

   posterior 
  margin 
  of 
  the 
  second 
  axial 
  ring 
  is 
  a 
  feature 
  not 
  

   known 
  in 
  Hoplolichas, 
  and, 
  in 
  the 
  absence 
  of 
  any 
  knowl- 
  

   edge 
  of 
  the 
  cranidium 
  of 
  the 
  Cape 
  Girardeau 
  specimen, 
  

   it 
  seems 
  very 
  doubtful 
  whether 
  the 
  latter 
  is 
  a 
  genuine 
  

   species 
  of 
  Hoplolichas. 
  Under 
  these 
  circumstances 
  the 
  

   Cape 
  Girardeau 
  specimen 
  is 
  regarded 
  provisionally 
  as 
  

   an 
  aberrant 
  form 
  of 
  Acrolichas 
  cucullus 
  (Meek 
  and 
  

   Worthen), 
  with 
  which 
  it 
  is 
  associated 
  stratigraphically. 
  

   It 
  forms 
  number 
  10771 
  in 
  the 
  Walker 
  Museum, 
  at 
  Chi- 
  

   cago 
  University. 
  The 
  writer 
  is 
  greatly 
  indebted 
  to 
  

   Prof. 
  A. 
  D. 
  Hole 
  of 
  Earlham 
  College, 
  at 
  Richmond, 
  Indi- 
  

   ana, 
  for 
  the 
  privilege 
  of 
  using 
  this 
  and 
  other 
  specimens 
  

   from 
  the 
  museum 
  of 
  Chicago 
  University,, 
  since 
  these 
  

   specimens 
  form 
  part 
  of 
  the 
  material 
  discussed 
  in 
  his 
  

   paper 
  on 
  American 
  Ordovician 
  Lichadidas 
  of 
  the 
  Missis- 
  

   sippi 
  Valley, 
  still 
  unpublished. 
  

  

  3. 
  American 
  species 
  definitely 
  referred 
  to 
  the 
  European 
  genus 
  

  

  Hemiarges. 
  

  

  Another 
  genus 
  found 
  in 
  the 
  Ordovician 
  formations 
  of 
  

   both 
  Europe 
  and 
  America 
  is 
  Hemiarges, 
  proposed 
  by 
  

   Gtirich 
  in 
  1901, 
  with 
  Lichas 
  wesenbergensis 
  Schmidt 
  as 
  

   the 
  genotype. 
  This 
  genus, 
  however, 
  belongs 
  to 
  an 
  en- 
  

   tirely 
  different 
  division 
  of 
  the 
  Lichadidse 
  from 
  that 
  of 
  

   any 
  of 
  the 
  forms 
  discussed 
  in 
  the 
  preceding 
  lines, 
  

   the 
  difference 
  in 
  structure 
  being 
  most 
  marked 
  in 
  case 
  

   of 
  the 
  pygidium. 
  Corydocephalus 
  belongs 
  to 
  the 
  same 
  

   subdivision 
  as 
  Hemiarges 
  but 
  is 
  not 
  identical 
  generically. 
  

   In 
  Corydocephalus 
  the 
  second 
  pair 
  of 
  lateral 
  glabellar 
  

   lobes 
  is 
  prominent 
  and 
  rises 
  to 
  the 
  same 
  level 
  as 
  the 
  

   median 
  lobe 
  ; 
  the 
  anterior 
  pair 
  of 
  lobes 
  is 
  distinctly 
  less 
  

   prominent 
  than 
  the 
  second 
  pair 
  and 
  is 
  depressed 
  in 
  the 
  

   space 
  between 
  the 
  second 
  pair 
  and 
  the 
  frontal 
  extension 
  

   of 
  the 
  median 
  lobe; 
  the 
  third 
  or 
  posterior 
  pair 
  of 
  lobes 
  

   is 
  small 
  but 
  is 
  readily 
  recognizable. 
  In 
  Hemiarges, 
  on 
  

   the 
  contrary, 
  the 
  anterior 
  pair 
  of 
  lobes 
  is 
  the 
  more 
  prom- 
  

   inent 
  pair; 
  both 
  the 
  second 
  and 
  posterior 
  pairs 
  are 
  much 
  

   smaller 
  and 
  usually 
  are 
  weakly 
  defined 
  ; 
  the 
  second 
  pair, 
  

   in 
  fact, 
  usually 
  can 
  not 
  be 
  differentiated 
  distinctly 
  from 
  

  

  