﻿Williams 
  — 
  Devonian 
  Interval 
  in 
  northern 
  Arhansas. 
  145 
  

  

  brought 
  out 
  features 
  more 
  or 
  less 
  distinct 
  by 
  which 
  their 
  

   biological 
  relations 
  can 
  be 
  determined. 
  The 
  whole 
  of 
  this 
  list 
  

   may 
  require 
  some 
  specific 
  revision 
  whenever 
  better 
  specimens 
  

   come 
  to 
  light, 
  Enough 
  is 
  preserved 
  to 
  make 
  the 
  generic 
  iden- 
  

   tifications 
  fairly 
  certain. 
  In 
  determining 
  them, 
  great 
  pains 
  was 
  

   taken 
  to 
  discover, 
  if 
  possible, 
  signs 
  of 
  those 
  particular 
  charac- 
  

   ters 
  which 
  mark 
  the 
  difference 
  between 
  closely 
  allied 
  species 
  

   of 
  successive, 
  known 
  geological 
  horizons. 
  A 
  few 
  remarks 
  may 
  

   be 
  given 
  regarding 
  the 
  points 
  thus 
  ascertained 
  regarding 
  the 
  

   more 
  important 
  genera. 
  

  

  Lingula. 
  — 
  In 
  the 
  black 
  shale 
  strata, 
  and 
  in 
  the 
  black 
  nodules 
  

   of 
  the 
  greenish 
  shales 
  and 
  Sylamore 
  sandstone, 
  a 
  number 
  of 
  

   minute 
  Lingula 
  were 
  seen. 
  These 
  were 
  compared 
  with 
  a 
  large 
  

   series 
  of 
  specimens 
  from 
  the 
  typical 
  Genesee 
  shales 
  of 
  !N^ew 
  

   York, 
  Cleveland, 
  Bedford, 
  Berea, 
  Ohio, 
  Huron, 
  Portage, 
  

   Ithaca 
  black 
  shales, 
  Harpeth 
  shales 
  of 
  Tennessee, 
  and 
  from 
  

   similar 
  black-shale 
  rocks 
  of 
  Europe. 
  The 
  conclusion 
  from 
  the 
  

   study 
  is 
  that 
  the 
  forms 
  common 
  in 
  the 
  Genesee 
  black 
  shales 
  

   of 
  ]Srew 
  York 
  are 
  the 
  central 
  and 
  t^ypical 
  forms. 
  These 
  are 
  

   often 
  found 
  almost 
  alone. 
  The 
  most 
  frequent 
  associates 
  are 
  

   Conodont 
  teeth 
  and 
  a 
  small 
  Chonetes. 
  From 
  these 
  common 
  

   black-shale 
  forms 
  the 
  succeeding 
  Lingulas 
  differ 
  by 
  slight 
  and 
  

   indefinite 
  gradation, 
  until 
  they 
  reach 
  a 
  size 
  five 
  times 
  as 
  great 
  

   as 
  the 
  typical 
  Genesee 
  types, 
  and 
  vary 
  in 
  dimensions 
  and 
  in 
  

   thickness 
  of 
  shell. 
  

  

  The 
  change, 
  which 
  is 
  apparent 
  in 
  the 
  Kew 
  York 
  sections, 
  

   when 
  the 
  succession 
  is 
  distinct 
  and 
  clear, 
  is 
  from 
  the 
  very 
  

   minute 
  forms 
  in 
  the 
  pure 
  Genesee 
  shale, 
  to 
  larger, 
  broader 
  

   and 
  more 
  rounded 
  forms 
  in 
  the 
  same 
  shales 
  ; 
  the 
  variation 
  in 
  

   the 
  Genesee 
  specimens 
  reaches 
  a 
  doubling 
  or 
  tripling 
  of 
  the 
  

   typical 
  size, 
  and 
  with 
  modification 
  in 
  the 
  general 
  form. 
  In 
  

   the 
  blackish 
  shales 
  higher 
  up 
  at 
  about 
  the 
  Ithaca 
  horizon, 
  and 
  

   in 
  Portage 
  strata 
  when 
  they 
  are 
  prevalent, 
  and 
  still 
  higher 
  in 
  

   bands 
  of 
  shale 
  coming 
  in 
  in 
  the 
  midst 
  of 
  Chemung 
  strata, 
  in 
  

   western 
  Xew 
  York, 
  the 
  larger, 
  broader, 
  more 
  rounded 
  forms 
  

   are 
  seen. 
  In 
  the 
  black 
  shales 
  in 
  the 
  Ohio 
  sections, 
  represent- 
  

   ing 
  the 
  upper 
  part 
  of 
  the 
  Chemung 
  and 
  reaching 
  up 
  into 
  

   undoubted 
  Carboniferous 
  horizons, 
  the 
  same 
  form 
  is 
  traced, 
  

   increasing 
  somewhat 
  with 
  the 
  rise 
  in 
  horizon 
  and 
  as 
  they 
  mix 
  

   with 
  other 
  species. 
  Finally, 
  we 
  reach 
  a 
  form, 
  which 
  is 
  rejDre- 
  

   sented 
  fairly 
  well 
  by 
  Meek 
  and 
  Worthen's 
  Lingula 
  suhspatu- 
  

   lata. 
  The 
  fact 
  that 
  the 
  subspatulata 
  size 
  and 
  form 
  are 
  reached 
  

   in 
  the 
  prevalent 
  Tennessee 
  type 
  has 
  led 
  me 
  to 
  think 
  that 
  where 
  

   it 
  occurs 
  the 
  horizon 
  is 
  well 
  up 
  in 
  the 
  series 
  rather 
  than 
  in 
  a 
  

   mesodevonian 
  horizon. 
  

  

  In 
  the 
  Arkansas 
  case, 
  the 
  forms 
  seem 
  to 
  me 
  to 
  come 
  in 
  near 
  

   the 
  upper 
  limit 
  of 
  the 
  Devonian, 
  as 
  expressed 
  in 
  the 
  Xew 
  

  

  