﻿American 
  Ordovician 
  Lichadidce. 
  45 
  

  

  1916, 
  p. 
  90, 
  pi. 
  30, 
  figs. 
  5-8) 
  sufficient 
  of 
  the 
  posterior 
  

   end 
  of 
  the 
  glabella 
  has 
  been 
  freed 
  from 
  the 
  matrix 
  to 
  

   give 
  a 
  better 
  clue 
  to 
  the 
  structure 
  of 
  this 
  part 
  of 
  the 
  

   cranidium 
  (PL 
  I, 
  figs. 
  4 
  A, 
  B, 
  restored 
  posteriorly). 
  A 
  

   different 
  interpretation 
  has 
  been 
  given 
  to 
  the 
  termina- 
  

   tion 
  of 
  the 
  tips 
  of 
  the 
  posterior 
  pair 
  of 
  ribs 
  (PL 
  I, 
  fig. 
  

   4 
  C). 
  In 
  the 
  free 
  tips 
  of 
  various 
  species 
  of 
  Acrolichas, 
  

   the 
  crest 
  of 
  some 
  of 
  the 
  granules 
  is 
  elongated 
  in 
  a 
  direc- 
  

   tion 
  diagonally 
  transverse 
  to 
  the 
  length 
  of 
  the 
  rib; 
  in 
  

   some 
  cases 
  a 
  narrow 
  ridge 
  extends 
  for 
  the 
  width 
  of 
  three 
  

   or 
  four 
  granules. 
  One 
  of 
  these 
  narrow 
  ridges 
  extends 
  

   nearly 
  across 
  the 
  width 
  of 
  the 
  free 
  tip 
  of 
  the 
  right 
  pos- 
  

   terior 
  rib, 
  in 
  the 
  type 
  here 
  discussed, 
  apparently 
  along 
  

   its 
  posterior 
  edge, 
  but 
  traces 
  of 
  the 
  surface 
  ornamenta- 
  

   tion 
  beyond 
  this 
  ridge 
  make 
  it 
  unnecessary 
  to 
  postulate 
  

   the 
  presence 
  of 
  truncated 
  free 
  tips 
  for 
  the 
  posterior 
  ribs, 
  

   and 
  the 
  interpretation 
  here 
  presented 
  is 
  offered 
  instead. 
  

  

  In 
  the 
  Kimmswick 
  limestone, 
  near 
  Batchtown, 
  in 
  Cal- 
  

   houn 
  county, 
  Illinois, 
  cranidia 
  of 
  an 
  Acrolichas 
  occur 
  

   (PL 
  II, 
  figs. 
  5 
  A, 
  B) 
  which 
  probably 
  represent 
  a 
  species 
  

   distinct 
  from 
  Acrolichas 
  conifrons, 
  but 
  the 
  differences 
  

   presented 
  by 
  the 
  cranidia 
  are 
  so 
  slight 
  that 
  they 
  are 
  

   difficult 
  of 
  recognition. 
  In 
  Acrolichas 
  conifrons, 
  the 
  

   frontal 
  lobe 
  is 
  more 
  vertical 
  and 
  less 
  convex 
  ; 
  the 
  median 
  

   lobe, 
  posterior 
  to 
  the 
  frontal 
  lobe, 
  also 
  is 
  less 
  convex, 
  

   and 
  there 
  is 
  a 
  slight 
  tendency 
  toward 
  median 
  angulation 
  

   of 
  the 
  upper 
  half 
  of 
  the 
  frontal 
  lobe, 
  best 
  seen 
  when 
  the 
  

   cranidium 
  is 
  viewed 
  from 
  above 
  (PL 
  I, 
  fig. 
  4 
  A). 
  All 
  

   of 
  these 
  features, 
  small 
  in 
  themselves, 
  combine 
  to 
  make 
  

   the 
  point 
  where 
  the 
  median 
  lobe 
  descends 
  into 
  the 
  frontal 
  

   lobe 
  more 
  angular 
  than 
  in 
  the 
  Kimmswick 
  specimens, 
  

   and 
  thus 
  warrant 
  the 
  term 
  conifrons. 
  The 
  pygidium 
  of 
  

   Acrolichas 
  conifrons, 
  however, 
  is 
  characterized 
  by 
  a 
  

   remarkably 
  abrupt 
  bend 
  at 
  the 
  middle 
  of 
  the 
  furrow 
  

   limiting 
  the 
  posterior 
  margin 
  of 
  the 
  second 
  ring 
  on 
  the 
  

   axial 
  lobe, 
  and 
  nothing 
  similar 
  to 
  this' 
  has 
  been 
  found 
  

   in 
  any 
  part 
  of 
  the 
  Kimmswick 
  limestone 
  in 
  the 
  Missis- 
  

   sippi 
  valley. 
  

  

  Lichas 
  harrisi, 
  PL 
  III, 
  figs. 
  6 
  A-C. 
  The 
  type, 
  ob- 
  

   tained 
  somewhere 
  near 
  Waynesville, 
  Ohio, 
  presumably 
  

   in 
  the 
  Waynesville 
  member 
  of 
  the 
  Richmond, 
  exposes 
  

   the 
  under 
  side 
  of 
  the 
  pygidium, 
  thorax, 
  and 
  hypostoma. 
  

   Other 
  pygidia, 
  having 
  the 
  same 
  general 
  aspect, 
  have 
  been 
  

   found 
  in 
  the 
  upper 
  part 
  of 
  the 
  Liberty 
  member 
  of 
  the 
  

  

  