﻿26 
  A. 
  F. 
  Foerste 
  — 
  Relations 
  of 
  the 
  

  

  Aet.' 
  II. 
  — 
  The 
  Generic 
  Relations 
  of 
  the 
  American 
  Ordo- 
  

   vician 
  Licliadidce; 
  by 
  Aug. 
  F. 
  Foeeste. 
  "With 
  Plates 
  

   I 
  to 
  IV. 
  

  

  1. 
  Two 
  prevalent 
  types 
  of 
  glabellce 
  among 
  European 
  Ordovician 
  

  

  Lichads. 
  

  

  The 
  glabella 
  of 
  most 
  European 
  Ordovician 
  Lichads 
  

   have 
  one 
  of 
  two 
  types 
  of 
  structure. 
  In 
  one 
  of 
  these 
  

   types, 
  the 
  posterior 
  margin 
  of 
  the 
  anterior 
  pair 
  of 
  lateral 
  

   glabellar 
  lobes 
  frequently 
  is 
  more 
  or 
  less 
  indistinctly 
  

   limited 
  from 
  the 
  second 
  pair, 
  the 
  second 
  pair 
  of 
  lobes 
  

   usually 
  is 
  indistinctly 
  differentiated 
  from 
  the 
  median 
  

   part 
  of 
  the 
  glabella, 
  and 
  the 
  posterior 
  pair 
  of 
  lobes, 
  even 
  

   when 
  of 
  small 
  size, 
  is 
  distinctly 
  defined 
  (Plate 
  III, 
  figs. 
  

   7A, 
  8A; 
  plate 
  IV, 
  figs. 
  1, 
  4A, 
  5A, 
  6A). 
  In 
  the-other 
  type, 
  

   the 
  anterior 
  and 
  second 
  pairs 
  of 
  lobes, 
  on 
  each 
  side 
  of 
  the 
  

   glabella, 
  have 
  coalesced 
  into 
  a 
  single 
  oblong 
  lobe, 
  poste- 
  

   rior 
  to 
  which 
  is 
  the 
  posterior 
  pair 
  of 
  lobes, 
  usuallv 
  of 
  

   small 
  size 
  (Plate 
  IV, 
  figs. 
  7A, 
  8A, 
  9A, 
  10A). 
  The 
  pres- 
  

   ence 
  of 
  the 
  posterior 
  pair 
  of 
  lateral 
  glabellar 
  lobes 
  is 
  

   common 
  to 
  both 
  types 
  of 
  structure. 
  In 
  a 
  third, 
  but 
  much 
  

   rarer 
  type, 
  all 
  of 
  the 
  lateral 
  lobes 
  of 
  the 
  glabella 
  have 
  

   coalesced 
  into 
  a 
  single 
  pair 
  of 
  side 
  lobes 
  (Plate 
  IV, 
  fig. 
  

   11 
  B). 
  

  

  2. 
  American 
  species 
  doubtfully 
  referred 
  to 
  the 
  European 
  genera 
  

   Platylichas 
  and 
  Hoplolichas. 
  

  

  In 
  American 
  Ordovician 
  Lichads, 
  posterior 
  lateral 
  

   glabellar 
  lobes 
  are 
  found 
  only 
  in 
  a 
  single 
  described 
  spe- 
  

   cies, 
  namely 
  Lichas 
  halli 
  Foerste 
  (Plate 
  II, 
  figs. 
  8A, 
  B), 
  

   from 
  the 
  Corryville 
  member 
  of 
  the 
  Maysville 
  formation, 
  

   at 
  Cincinnati, 
  Ohio. 
  This 
  species 
  appears 
  most 
  nearly 
  

   related 
  to 
  Lichas 
  margaritifer 
  XieszkoivsM 
  (Text 
  figs. 
  A, 
  

   B), 
  from 
  the 
  Borkholm 
  limestone 
  of 
  the 
  East 
  Baltic 
  

   areas. 
  The 
  latter 
  species 
  is 
  the 
  genotype 
  of 
  Platylichas, 
  

   proposed 
  by 
  Gtirich 
  in 
  1901. 
  The 
  pygidia 
  of 
  both 
  spe- 
  

   cies 
  are 
  closely 
  similar. 
  The 
  important 
  differences 
  are 
  

   found 
  in 
  the 
  lobation 
  of 
  the 
  glabella. 
  In 
  typical 
  Platy- 
  

   lichas 
  the 
  posterior 
  pair 
  of 
  lateral 
  glabellar 
  lobes 
  is 
  not 
  

   in 
  contact 
  with 
  the 
  anterior 
  pair, 
  and 
  it 
  seems 
  necessary 
  

   to 
  regard 
  the 
  intervening 
  area 
  as 
  consisting 
  of 
  the 
  second 
  

  

  