﻿American 
  Ordovician 
  Lichadidce. 
  37 
  

  

  destroyed 
  by 
  the 
  fact 
  that 
  we 
  do 
  not 
  know 
  exactly 
  what 
  

   horizon 
  here 
  is 
  signified 
  by 
  Middle 
  Nashville. 
  Appar- 
  

   ently 
  it 
  included 
  not 
  only 
  Catheys 
  but 
  also 
  Leipers 
  fos- 
  

   sils. 
  Ulrich, 
  in 
  the 
  Columbia 
  Folio, 
  in 
  1903, 
  lists 
  no 
  

   Trenton 
  species, 
  but 
  only 
  Lichas 
  halli, 
  from 
  the 
  Leipers 
  

   equivalent 
  of 
  the 
  Maysville 
  formation 
  in 
  Central 
  Ten- 
  

   nessee. 
  Since 
  the 
  first 
  known 
  specimen 
  of 
  Lichas 
  halli 
  

   Foerste 
  was 
  figured 
  by 
  Hall 
  erroneously 
  under 
  Lichas 
  

   trentonensis, 
  there 
  is 
  a 
  possibility 
  that 
  Safford, 
  following 
  

   Hall, 
  had 
  a 
  specimen 
  of 
  Lichas 
  halli 
  before 
  him 
  when 
  he 
  

   listed 
  Lichas 
  trentonensis. 
  No 
  Lichadidse 
  have 
  been 
  

   recorded 
  from 
  central 
  or 
  southern 
  Kentucky, 
  although 
  

   the 
  species 
  known 
  from 
  the 
  Corryville 
  member 
  of 
  the 
  

   Maysville, 
  in 
  the 
  Cincinnati 
  area, 
  including 
  Lichas 
  halli, 
  

   undoubtedly 
  occur 
  also 
  on 
  the 
  Kentucky 
  side 
  of 
  the 
  Ohio 
  

   river. 
  

  

  In 
  Chazyan 
  times, 
  Acrolichas 
  is 
  known 
  only 
  in 
  the 
  

   St. 
  Lawrence 
  and 
  Champlain 
  basins, 
  extending 
  thence 
  

   southward 
  as 
  far 
  as 
  the 
  Virginia 
  embayment. 
  

  

  In 
  late 
  Black 
  River, 
  and 
  in 
  Trenton 
  times, 
  Acrolichas. 
  

   trentonensis 
  occurs 
  as 
  far 
  south 
  as 
  New 
  Jersey; 
  no 
  

   species 
  is 
  known 
  in 
  these 
  times 
  from 
  Ohio, 
  Indiana, 
  or 
  

   Kentucky, 
  and 
  the 
  occurrence 
  of 
  Acrolichas 
  in 
  Tennessee 
  

   in 
  Trenton 
  strata 
  may 
  be 
  regarded 
  as 
  very 
  doubtful. 
  In 
  

   Kimmswick 
  strata 
  A 
  crolichas 
  is 
  known 
  as 
  far 
  south 
  as 
  

   Alexander 
  county, 
  at 
  the 
  southern 
  end 
  of 
  Illinois. 
  Far- 
  

   ther 
  north, 
  however, 
  Acrolichas 
  cucullus 
  ranges 
  from 
  Illi- 
  

   nois 
  and 
  Missouri 
  to 
  Minnesota, 
  Wisconsin, 
  and 
  Lake 
  

   "Winnipeg, 
  and 
  thence 
  eastward 
  to 
  the 
  Ottawa 
  area 
  of 
  

   Canada. 
  Hemiarges 
  in 
  a 
  similar 
  manner 
  is 
  represented 
  

   by 
  a 
  species 
  in 
  Missouri, 
  and 
  Hemiarges 
  paulianus 
  ranges 
  

   from 
  Minnesota 
  as 
  far 
  eastward 
  as 
  the 
  Ottawa 
  area. 
  

  

  From 
  the 
  Catheys, 
  Cynthiana, 
  Utica, 
  Eden, 
  and 
  Mays- 
  

   ville 
  formations 
  Acrolichas 
  is 
  absent. 
  When 
  the 
  genus 
  

   reappears 
  again 
  in 
  Richmond 
  times 
  its 
  range 
  again 
  is 
  

   northern. 
  Acrolichas 
  harrisi 
  occurs 
  in 
  the 
  Liberty 
  mem- 
  

   ber 
  in 
  Ohio 
  and 
  Indiana. 
  Acrolichas 
  clermontensis 
  and 
  

   A. 
  rhino 
  cer 
  as 
  are 
  found 
  in 
  the 
  Maquoketa 
  of 
  Iowa, 
  

   and 
  Acrolichas 
  bicornis 
  is 
  found 
  in 
  the 
  Maquoketa 
  of 
  

   Minnesota. 
  

  

  Since 
  the 
  genus 
  must 
  have 
  been 
  preserved 
  somewhere 
  

   in 
  the 
  interval 
  between 
  Trenton 
  and 
  Richmond 
  times, 
  the 
  

   area 
  of 
  retreat 
  could 
  scarcely 
  have 
  been 
  southward. 
  

   Why 
  not 
  northward? 
  Northward, 
  where 
  so 
  often 
  it 
  

  

  