﻿38 
  

  

  A. 
  F. 
  Foerste 
  — 
  Relations 
  of 
  the 
  

  

  appears 
  difficult 
  to 
  discriminate 
  between 
  Trenton 
  and 
  

   Eichmond 
  strata. 
  The 
  present 
  pancity 
  of 
  Trenton 
  and 
  

   Kichmond 
  strata 
  over 
  a 
  large 
  part 
  of 
  the 
  northern 
  part 
  

   of 
  the 
  North 
  American 
  continent 
  can 
  be 
  no 
  index 
  since 
  it 
  

  

  Hypothetical 
  2. 
  

   Ancestor 
  

  

  3 
  Acroli'chas 
  ^ 
  Acrolichas 
  5 
  

  

  HopioJ/chas 
  

   Homo 
  Sic 
  has 
  

   Conoirchas 
  

  

  6 
  Mstopojichas 
  

  

  7 
  

  

  ° 
  AmphlKcha-S 
  Amphilichas 
  

  

  JO 
  

  

  rtetalichas 
  

   StMathiae 
  

  

  W 
  

  

  15 
  

   Platylichcs(?) 
  

   haiii 
  

  

  of 
  generic 
  characteristics 
  

  

  11 
  12 
  

  

  P/atopolIchas 
  Hypothetical 
  Metalithas 
  

  

  LeioJichas 
  "Ancestor 
  Piatylichcs 
  

  

  Diagrams 
  illustrating 
  hypothetical 
  origin 
  

   among 
  Lichad 
  glabellae: 
  

  

  1. 
  Hypothetical 
  ancestral 
  glabella, 
  with 
  lateral 
  furrows. 
  2, 
  with 
  con- 
  

   necting 
  furrows. 
  3, 
  with 
  second 
  and 
  third 
  pairs 
  of 
  lateral 
  furrows 
  becom- 
  

   ing 
  obsolete; 
  giving 
  rise 
  to 
  4, 
  in 
  Acrolichas. 
  6, 
  8, 
  9, 
  similar 
  results 
  may 
  

   be 
  produced 
  by 
  the 
  disappearance 
  of 
  the 
  second, 
  pair 
  of 
  lateral 
  furrows, 
  

   followed 
  later 
  by 
  the 
  third 
  pair, 
  leading 
  to 
  Amphilichas. 
  5, 
  10, 
  in 
  both 
  

   Acrolichas 
  and 
  Amphilichas 
  the 
  posterior 
  connecting 
  furrows 
  may 
  disap- 
  

   pear. 
  7, 
  11, 
  a 
  similar 
  change 
  takes 
  place 
  in 
  Metopolichas, 
  Platopolichas, 
  

   and 
  Leiolichas. 
  12, 
  13, 
  14, 
  15, 
  Platyliclias 
  halli 
  may 
  have 
  been 
  derived 
  

   from 
  typical 
  Platylichas, 
  13, 
  the 
  chief 
  characteristic 
  of 
  the 
  genus 
  being 
  

   the 
  disappearing 
  of 
  part 
  of 
  the 
  dorsal 
  furrow. 
  

  

  is 
  well 
  known 
  that 
  the 
  fannas 
  of 
  the 
  distant 
  ontliers 
  so 
  

   far 
  fonnd 
  do 
  not 
  indicate 
  fannas 
  developed 
  in 
  restricted 
  

   basins, 
  bnt 
  appear 
  like 
  parts 
  of 
  the 
  much 
  larger 
  fannas 
  

   found 
  in 
  larger 
  areas 
  of 
  exposure 
  elsewhere. 
  

  

  13. 
  The 
  origin 
  of 
  generic 
  types 
  among 
  the 
  cranidia 
  of 
  the 
  

   Lichadidce. 
  

  

  The 
  glabella 
  of 
  the 
  ancestors 
  of 
  the 
  Ordovician 
  

   Lichads, 
  in 
  conformity 
  with 
  that 
  of 
  other 
  trilobites, 
  must 
  

  

  