﻿American 
  Ordovician 
  Lichadidce. 
  39 
  

  

  have 
  consisted 
  of 
  five 
  consolidated 
  segments, 
  of 
  which 
  the 
  

   occipital 
  ring 
  formed 
  the 
  last, 
  the 
  three 
  pairs 
  of 
  lateral 
  

   glabellar 
  lobes 
  located 
  the 
  position 
  of 
  three 
  additional 
  

   segments, 
  and 
  the 
  frontal 
  extension 
  of 
  the 
  median 
  lobe 
  

   indicated 
  the 
  position 
  of 
  the 
  anterior 
  segment. 
  The 
  

   lateral 
  glabellar 
  lobes 
  of 
  these 
  hypothetical 
  ancestors 
  

   increased 
  in 
  size 
  anteriorly. 
  The 
  glabella 
  was 
  limited 
  

   laterally 
  by 
  the 
  dorsal 
  fnrrows 
  and 
  the 
  lateral 
  lobes 
  

   were 
  separated 
  from 
  each 
  other 
  by 
  pairs 
  of 
  lateral 
  gla- 
  

   bellar 
  fnrrows 
  (fig. 
  1). 
  At 
  an 
  early 
  stage 
  the 
  inner 
  ends 
  

   of 
  these 
  lateral 
  glabellar 
  fnrrows 
  must 
  have 
  been 
  joined 
  

   together 
  by 
  "connecting" 
  fnrrows 
  (fig. 
  2). 
  Most 
  of 
  the 
  

   later 
  changes 
  of 
  generic 
  valne 
  center 
  around 
  the 
  second 
  

   pair 
  of 
  lateral 
  glabellar 
  lobes. 
  

  

  In 
  Metalichas 
  and 
  Platylichas 
  (figs. 
  13, 
  14, 
  15) 
  that 
  

   part 
  of 
  the 
  dorsal 
  furrow 
  which 
  borders 
  on 
  the 
  outer 
  mar- 
  

   gin 
  of 
  the 
  second 
  pair 
  of 
  lobes 
  becomes 
  obsolete 
  and 
  this 
  

   second 
  pair 
  of 
  lobes 
  appears 
  to 
  merge 
  laterally 
  into 
  the 
  

   fixed 
  cheeks. 
  In 
  Platopolichas 
  and 
  Leiolichas 
  (fig. 
  11) 
  

   the 
  second 
  pair 
  of 
  lateral 
  furrows 
  and 
  the 
  middle 
  pair 
  of 
  

   connecting 
  furrows 
  have 
  disappeared, 
  and 
  in 
  typical 
  

   Metopolichas 
  (fig. 
  7) 
  not 
  even 
  a 
  trace 
  of 
  these 
  lateral 
  

   furrows 
  remains. 
  

  

  In 
  several 
  genera 
  the 
  first 
  pair 
  of 
  lateral 
  glabellar 
  fur- 
  

   rows 
  and 
  the 
  succeeding 
  pairs 
  of 
  connecting 
  furrows 
  have 
  

   coalesced 
  into 
  a 
  single 
  longitudinal 
  strongly 
  indented 
  

   furrow. 
  In 
  Hoploliclias, 
  Somolichas, 
  and 
  Conolichas 
  

   (fig. 
  6) 
  this 
  coalescence 
  is 
  accompanied 
  by 
  a 
  disappear- 
  

   ance 
  of 
  the 
  second 
  pair 
  of 
  glabellar 
  furrows. 
  The 
  dis- 
  

   appearance 
  also 
  of 
  the 
  third 
  pair 
  of 
  glabellar 
  furrows 
  

   would 
  lead 
  to 
  the 
  structure 
  found 
  in 
  Amphilichas 
  (fig. 
  

   10). 
  The 
  simultaneous 
  disappearance 
  of 
  both 
  the 
  second 
  

   and 
  third 
  pairs 
  of 
  lateral 
  glabellar 
  furrows 
  would 
  lead 
  to 
  

   the 
  structure 
  found 
  in 
  Acrolichas 
  (figs. 
  4, 
  5), 
  without 
  the 
  

   intermediate 
  stage 
  found 
  in 
  Hoplolichas 
  and 
  other 
  genera 
  

   (fig. 
  6) 
  in 
  which 
  the 
  posterior 
  lateral 
  lobes 
  are 
  still 
  pre- 
  

   served. 
  In 
  other 
  words, 
  Acrolichas 
  and 
  Amphilichas 
  

   may 
  have 
  arrived 
  at 
  identical 
  types 
  of 
  glabellar 
  structure 
  

   through 
  different 
  lines 
  of 
  development. 
  

  

  In 
  both 
  Acrolichas 
  and 
  Amphilichas 
  the 
  rear 
  connect- 
  

   ing 
  furrows 
  may 
  disappear 
  leaving 
  only 
  the 
  anterior 
  pair 
  

   of 
  glabellar 
  furrows 
  and 
  a 
  variable 
  amount 
  of 
  the 
  imme- 
  

   diately 
  adjacent 
  connecting 
  furrows. 
  

  

  In 
  various 
  species 
  of 
  Acrolichas 
  the 
  compound 
  nature 
  

  

  