﻿36 
  A. 
  F. 
  Foerste 
  — 
  Relations 
  of 
  the 
  

  

  The 
  other 
  American 
  species 
  here 
  are 
  referred 
  to 
  Aero- 
  

   lichas. 
  

  

  11. 
  The 
  Bohemian 
  Silurian 
  genera 
  Cory 
  otocephalus 
  and 
  

   Dicranopeltis. 
  

  

  In 
  Silnrian 
  times 
  the 
  number 
  of 
  Lichad 
  genera 
  com- 
  

   mon 
  to 
  both 
  America 
  and 
  Europe 
  increased, 
  and 
  included 
  

   not 
  only 
  the 
  typical 
  Baltic 
  genus 
  Lichas 
  but 
  also 
  the 
  

   Bohemian 
  genera 
  Corydocephalus 
  and 
  Dicranopeltis. 
  

   Neither 
  Corydocephalus 
  nor 
  Dicranopeltis 
  occur 
  in 
  the 
  

   Silurian 
  rocks 
  of 
  the 
  Baltic 
  area 
  so 
  that 
  these 
  genera 
  may 
  

   have 
  reached 
  us 
  by 
  some 
  more 
  southern 
  route. 
  . 
  They 
  are 
  

   represented 
  in 
  the 
  Saint 
  Clair 
  limestone 
  of 
  Arkansas, 
  in 
  

   the 
  Laurel 
  of 
  Indiana, 
  in 
  the 
  Racine 
  of 
  Illinois 
  and 
  Wis- 
  

   consin, 
  and 
  in 
  the 
  Cedarville 
  of 
  Ohio. 
  Corydocephalus 
  

   halli 
  Foerste 
  and 
  Dicranopeltis 
  nereus 
  (Hall) 
  occur 
  in 
  

   the 
  Rochester 
  shale 
  of 
  New 
  York. 
  Judging 
  from 
  their 
  

   earlier 
  appearance 
  in 
  Arkansas, 
  an 
  entrance 
  of 
  the 
  

   Bohemian 
  genera 
  into 
  American 
  waters 
  by 
  way 
  of 
  the 
  

   Gulf 
  of 
  Mexico 
  is 
  at 
  least 
  a 
  possibility. 
  The 
  type 
  of 
  

   Dicranopeltis 
  nereus 
  retains 
  only 
  the 
  thorax 
  and 
  pygid- 
  

   ium 
  ; 
  the 
  glabella 
  figured 
  as 
  though 
  belonging 
  to 
  this 
  type 
  

   in 
  reality 
  is 
  a 
  stray 
  fragment 
  of 
  a 
  species 
  of 
  Arctinurus, 
  

   probably 
  A. 
  boltoni, 
  lying 
  in 
  about 
  the 
  same 
  position 
  in 
  

   which 
  the 
  head 
  belonging 
  to 
  this 
  type 
  should 
  occur. 
  

   Arctinurus, 
  a 
  strictly 
  American 
  genus, 
  also 
  makes 
  its 
  

   appearance 
  in 
  the 
  St. 
  Clair 
  limestone 
  of 
  Arkansas. 
  The 
  

   Baltic 
  genus 
  Lichas 
  is 
  represented 
  most 
  nearly 
  in 
  Amer- 
  

   ican 
  strata 
  by 
  the 
  Silurian 
  species 
  at 
  present 
  referred 
  to 
  

   Metopolichas, 
  a 
  genus 
  restricted 
  to 
  the 
  Central 
  States, 
  

   and 
  making 
  its 
  first 
  appearance 
  in 
  the 
  Medinan. 
  

  

  In 
  place 
  of 
  Corydocephalus 
  the 
  term 
  Trochurus 
  was 
  

   employed 
  in 
  the 
  Bulletin 
  of 
  Denison 
  University, 
  vol. 
  17, 
  

   p. 
  251, 
  1917, 
  pi. 
  11, 
  ^g. 
  1 
  ; 
  pi. 
  12, 
  figs. 
  1-4. 
  

  

  12. 
  The 
  distribution 
  of 
  American 
  species 
  of 
  Acrolichas. 
  

  

  The 
  distribution 
  of 
  American 
  Ordovician 
  Lichads 
  is 
  

   essentially 
  a 
  northern 
  one. 
  The 
  Chazyan 
  species, 
  Acro- 
  

   lichas 
  minganensis, 
  ranges 
  as 
  far 
  south 
  as 
  Lexington, 
  

   Virginia. 
  Acrolichas 
  trentonensis, 
  a 
  Trenton 
  species, 
  is 
  

   listed 
  by 
  Safford 
  (Geol. 
  Tennessee, 
  1869, 
  p. 
  290) 
  from 
  

   the 
  Middle 
  Nashville. 
  The 
  value 
  of 
  this 
  reference 
  is 
  

  

  