﻿40 
  A. 
  F. 
  Foerste 
  — 
  Relations 
  of 
  the 
  

  

  of 
  the 
  single 
  longitudinal 
  pair 
  of 
  glabellar 
  furrows 
  can 
  be 
  

   recognized 
  by 
  slight 
  deviations 
  in 
  the 
  direction 
  of 
  these 
  

   furrows 
  and 
  by 
  slight 
  depressions 
  or 
  pits 
  in 
  their 
  bottom. 
  

   For 
  instance, 
  in 
  the 
  cranidiuni 
  of 
  Acroliclias 
  cucullus 
  

   figured 
  from 
  Hull, 
  opposite 
  Ottawa, 
  in 
  Canada 
  (PI. 
  I, 
  fig. 
  

   1A), 
  the 
  basal 
  part 
  of 
  the 
  median 
  lobe 
  is 
  widened 
  slightly 
  

   into 
  a 
  pedestal, 
  immediately 
  anterior 
  to 
  which 
  the 
  gla- 
  

   bellar 
  furrows 
  make 
  a 
  short 
  curve 
  with 
  the 
  concave 
  side 
  

   directed 
  outward 
  (see 
  also 
  fig. 
  3). 
  These 
  curved 
  por- 
  

   tions 
  correspond 
  to 
  the 
  second 
  pair 
  of 
  connecting 
  fur- 
  

   rows, 
  bounding 
  the 
  inner 
  side 
  of 
  the 
  second 
  pair 
  of 
  

   glabellar 
  lobes 
  ; 
  at 
  the 
  anterior 
  end 
  of 
  these 
  curved 
  por- 
  

   tions 
  there 
  usually 
  is 
  a 
  tendency 
  toward 
  a 
  slight 
  pit, 
  

   locating 
  the 
  inner 
  end 
  of 
  the 
  second 
  pair 
  of 
  lateral 
  fur- 
  

   rows. 
  Another 
  slight 
  pit, 
  locating 
  the 
  inner 
  end 
  of 
  the 
  

   first 
  pair 
  of 
  lateral 
  glabellar 
  furrows, 
  exists 
  near 
  the 
  

   point 
  where 
  the 
  anterior 
  part 
  of 
  the 
  glabella 
  begins 
  to 
  

   curve 
  strongly 
  downward. 
  Slight 
  changes 
  in 
  the 
  curva- 
  

   ture 
  of 
  the 
  surface 
  of 
  the 
  side 
  lobes 
  of 
  glabellas 
  suggest 
  

   that 
  the 
  anterior 
  pair 
  of 
  lateral 
  glabellar 
  furrows 
  was 
  

   very 
  oblique, 
  and 
  that 
  the 
  general 
  aspect 
  of 
  the 
  glabella 
  

   was 
  similar 
  to 
  that 
  of 
  Dicranopeltis. 
  Although 
  this 
  

   genus 
  is 
  not 
  known 
  until 
  Silurian 
  times, 
  the 
  structure 
  of 
  

   its 
  glabella 
  appears 
  to 
  have 
  been 
  very 
  primitive, 
  resem- 
  

   bling 
  that 
  of 
  the 
  supposed 
  ancestral 
  Lichad 
  (figs. 
  2, 
  12). 
  

   How 
  largely 
  hypothetical 
  the 
  derivation 
  of 
  Lichad 
  

   genera 
  must 
  remain 
  for 
  some 
  time 
  is 
  shown 
  by 
  the 
  far 
  

   departure 
  from 
  ancestral 
  types 
  shown 
  by 
  the 
  earliest 
  

   Lichads 
  known. 
  The 
  earliest 
  American 
  Lichads 
  are 
  of 
  

   Chazyan 
  age 
  and 
  belong 
  to 
  Acroliclias. 
  The 
  earliest 
  

   East 
  Baltic 
  forms 
  occur 
  in 
  the 
  Walchow 
  formation, 
  

   apparently 
  of 
  much 
  earlier 
  age 
  than 
  the 
  Chazyan, 
  and 
  

   they 
  belong 
  to 
  Metopolichas. 
  The 
  earliest 
  Bohemian 
  

   Lichads 
  belong 
  to 
  Platopoliclias, 
  and 
  these 
  also 
  appear 
  

   to 
  be 
  of 
  much 
  earlier 
  age 
  than 
  the 
  Chazyan. 
  

  

  14. 
  The 
  free 
  checks, 
  hypostoma, 
  and 
  thorax 
  of 
  Acroliclias. 
  

  

  Our 
  knowledge 
  of 
  the 
  Lichadidaa 
  usually 
  is 
  confined 
  to 
  

   the 
  cranidia 
  and 
  the 
  pygidia. 
  Rarely 
  are 
  these 
  found 
  

   connected 
  by 
  the 
  thorax. 
  In 
  the 
  type 
  of 
  Acroliclias 
  har- 
  

   risi 
  eleven 
  thoracic 
  segments 
  are 
  present. 
  In 
  the 
  type 
  of 
  

   Acroliclias 
  cornutus 
  only 
  ten 
  thoracic 
  segments 
  are 
  

   exposed, 
  but 
  the 
  cephalon 
  evidently 
  has 
  been 
  crowded 
  

  

  