﻿American 
  Ordovician 
  Lichadidce. 
  29 
  

  

  the 
  other 
  two. 
  This 
  produces 
  a 
  type 
  of 
  cranidmm 
  dis- 
  

   tinct 
  from 
  that 
  of 
  Corydocephalus. 
  In 
  America, 
  Hemi- 
  

   arges 
  is 
  represented 
  by 
  Lichas 
  (Arges) 
  wesenbergensis 
  

   paulianus 
  Clarke 
  (Geol. 
  Minnesota, 
  3, 
  pt. 
  2, 
  1894, 
  p. 
  744, 
  

   figs. 
  62-64; 
  see 
  also 
  PL 
  II, 
  figs 
  6A-C, 
  in 
  present 
  paper) 
  

   from 
  the 
  Prosser 
  limestone 
  of 
  Minnesota; 
  Arges 
  tuber- 
  

   culatus 
  Weller 
  (Geol. 
  Surv. 
  New 
  Jersey, 
  Pal. 
  3, 
  p. 
  199, 
  

   pi. 
  15, 
  figs. 
  11-13, 
  1903) 
  from 
  the 
  Trenton 
  of 
  New 
  Jersey; 
  

   and 
  an 
  nndescribed 
  species 
  from 
  the 
  Kimmswick 
  of 
  

   Missonri. 
  Hemiarges 
  wesenbergensis 
  occurs 
  in 
  the 
  

   Wesenberg 
  formation 
  of 
  the 
  East 
  Baltic 
  provinces, 
  a 
  

   horizon 
  corresponding 
  approximately 
  to 
  the 
  upper 
  

   Trenton 
  of 
  America. 
  

  

  4. 
  Acrolichas 
  proposed 
  for 
  twelve 
  American 
  and 
  one 
  Irish 
  

  

  species. 
  

  

  All 
  other 
  described 
  species 
  of 
  American 
  Ordovician 
  

   Lichads, 
  not 
  mentioned 
  in 
  the 
  preceding 
  lines, 
  belong 
  

   to 
  a 
  recently 
  named 
  genus 
  for 
  which 
  the 
  name 
  Acrolichas 
  

   was 
  proposed, 
  with 
  Lichas 
  cucullus 
  Meek 
  and 
  Worthen 
  

   as 
  the 
  genotype. 
  This 
  genus 
  possesses 
  the 
  following 
  

   characteristics 
  : 
  

  

  The 
  glabella 
  is 
  three-lobed, 
  with 
  one 
  median 
  and 
  two 
  

   apparently 
  simple 
  side 
  lobes. 
  These 
  side 
  lobes 
  have 
  re- 
  

   sulted 
  from 
  the 
  coalescence 
  of 
  the 
  first, 
  second, 
  and 
  third 
  

   pairs 
  of 
  lateral 
  glabellar 
  lobes, 
  as 
  seen 
  in 
  other 
  Lichad- 
  

   idse. 
  Posteriorly, 
  the 
  median 
  lobe 
  is 
  nearly 
  rectangular 
  

   or 
  widens 
  slightly 
  ; 
  anteriorly, 
  the 
  lobe 
  widens 
  rapidly 
  so 
  

   that 
  the 
  width 
  of 
  the 
  frontal 
  part 
  frequently 
  is 
  double 
  

   that 
  of 
  the 
  posterior 
  end. 
  The 
  side 
  lobes 
  are 
  reniform 
  

   in 
  outline, 
  the 
  convex 
  side 
  adjoining 
  the 
  middle 
  and 
  

   posterior 
  part 
  of 
  the 
  median 
  lobe. 
  Posteriorly, 
  these 
  

   lateral 
  lobes 
  border 
  for 
  their 
  entire 
  width 
  on 
  the 
  occipital 
  

   furrow. 
  The 
  occipital 
  ring 
  extends 
  for 
  a 
  short 
  distance 
  

   beyond 
  the 
  posterior 
  lateral 
  extremity 
  of 
  the 
  side 
  lobes 
  

   of 
  the 
  glabella. 
  Anterior 
  to 
  the 
  glabella 
  the 
  cranidium 
  

   presents 
  a 
  very 
  narrow 
  border, 
  frequently 
  not 
  exceeding 
  

   half 
  a 
  millimeter 
  in 
  length 
  in 
  specimens 
  of 
  average 
  size. 
  

  

  The 
  axial 
  lobe 
  of 
  the 
  pygidium 
  narrows 
  posteriorly 
  

   to 
  a 
  point 
  which 
  reaches 
  the 
  notch 
  separating 
  the 
  free 
  

   tips 
  of 
  the 
  two 
  posterior 
  ribs. 
  Usually 
  there 
  are 
  only 
  

   two 
  annulations 
  on 
  the 
  axial 
  lobe, 
  and 
  the 
  lateral 
  ends 
  

   of 
  that 
  furrow 
  which 
  limits 
  the 
  posterior 
  margin 
  of 
  the 
  

  

  