﻿American 
  Ordovician 
  Lichadidcz. 
  35 
  

  

  anterior 
  and 
  second 
  pairs 
  of 
  lateral 
  lobes 
  have 
  coalesced 
  

   into 
  a 
  single 
  pair 
  of 
  very 
  prominent, 
  bulbous 
  lobes, 
  

   between 
  which 
  the 
  posterior 
  half 
  of 
  the 
  median 
  lobe 
  is 
  

   both 
  depressed 
  in 
  level 
  and 
  narrowed 
  in 
  width. 
  The 
  

   frontal 
  half 
  of 
  the 
  median 
  lobe, 
  moreover, 
  rises 
  obliquely 
  

   backward 
  into 
  a 
  protuberance, 
  whose 
  posterior 
  side 
  ter- 
  

   minates 
  abruptly 
  at 
  a 
  level 
  distinctly 
  higher 
  than 
  the 
  

   posterior 
  half 
  of 
  the 
  median 
  lobe. 
  

  

  10. 
  The 
  East 
  Baltic 
  genera 
  Homolichas, 
  Leiolichas, 
  and 
  

   Metopolichas. 
  

  

  Other 
  European 
  Lichad 
  genera, 
  hitherto 
  not 
  men- 
  

   tioned, 
  are 
  Homolichas, 
  proposed 
  by 
  Schmidt, 
  with 
  Lichas 
  

   depressus 
  Angelin 
  (PL 
  IV, 
  fig. 
  10) 
  as 
  type; 
  Leiolichas, 
  

   also 
  proposed 
  by 
  Schmidt, 
  with 
  Lichas 
  illcenoideus 
  Niesz- 
  

   kowski 
  (PL 
  IV, 
  fig. 
  6) 
  as 
  type; 
  and 
  Metopolichas, 
  pro- 
  

   posed 
  by 
  Reed, 
  with 
  Lichas 
  hubneri 
  Eichwald 
  (PL 
  IV, 
  fig. 
  

   1) 
  as 
  type. 
  Lichas 
  hubneri 
  is 
  found 
  in 
  the 
  Dubowiki 
  

   formation, 
  approximately 
  corresponding 
  to 
  our 
  upper 
  

   Chazyan 
  strata. 
  

  

  Recently 
  it 
  has 
  become 
  customary 
  to 
  assign 
  to 
  the 
  East 
  

   Baltic 
  Ordovician 
  genus 
  Metopolichas 
  several 
  typically 
  

   Silurian 
  Lichads, 
  whose 
  pygidia 
  are 
  characterized 
  by 
  the 
  

   absence 
  of 
  free 
  tips 
  on 
  the 
  posterior 
  pair 
  of 
  ribs, 
  the 
  

   axial 
  lobe 
  and 
  posterior 
  pair 
  of 
  ribs 
  being 
  bounded 
  pos- 
  

   teriorly 
  by 
  a 
  continuous 
  convex 
  margin, 
  only 
  the 
  first 
  

   and 
  second 
  pairs 
  of 
  ribs 
  having 
  short 
  free 
  tips. 
  There 
  is 
  

   no 
  evidence 
  whatever 
  that 
  the 
  genotype 
  of 
  Metopolichas, 
  

   Lichas 
  hubneri 
  (PL 
  IV, 
  fig. 
  1), 
  had 
  pygidia 
  of 
  this 
  struc- 
  

   ture. 
  On 
  the 
  contrary, 
  judging 
  from 
  the 
  straightness 
  of 
  

   the 
  posterior 
  part 
  of 
  the 
  glabellar 
  furrows 
  on 
  the 
  cra- 
  

   nidia 
  of 
  Lichas 
  hubneri, 
  these 
  cranidia 
  have 
  no 
  generic 
  

   relationship 
  to 
  those 
  of 
  the 
  Silurian 
  species 
  in 
  question. 
  

   In 
  the 
  absence 
  of 
  any 
  knowledge 
  of 
  the 
  pygidia 
  of 
  Lichas 
  

   hubneri, 
  the 
  choice 
  of 
  this 
  species 
  as 
  a 
  genotype 
  is 
  unfor- 
  

   tunate. 
  In 
  my 
  opinion 
  the 
  American 
  species 
  referred 
  to 
  

   Metopolichas 
  belong 
  to 
  typical 
  Lichas, 
  with 
  Lichas 
  lacin- 
  

   iatus 
  Wahlenberg 
  as 
  genotype. 
  

  

  As 
  a 
  matter 
  of 
  fact, 
  even 
  the 
  Ordovician 
  Lichads 
  of 
  

   America 
  show 
  remarkably 
  little 
  relationship 
  to 
  those 
  of 
  

   the 
  East 
  Baltic 
  provinces. 
  Hemiarges 
  is 
  definitely 
  

   known 
  to 
  be 
  present 
  in 
  American 
  strata. 
  Lichas 
  halli 
  

   appears 
  at 
  least 
  closely 
  related 
  to 
  the 
  type 
  of 
  Platylichas. 
  

  

  