﻿8. 
  Welle?" 
  — 
  Succession 
  of 
  Fossil 
  Faunas 
  in 
  Missouri. 
  191 
  

  

  The 
  fauna 
  of 
  the 
  beds 
  exposed 
  at 
  Station 
  16 
  is 
  intermediate 
  

   between 
  the 
  faunas 
  of 
  Zones 
  III 
  and 
  IV. 
  Both 
  8. 
  plenus 
  and 
  

   8. 
  grimesi 
  are 
  abundant, 
  though 
  8. 
  grimesi 
  is 
  the 
  more 
  com- 
  

   mon. 
  Some 
  other 
  species, 
  more 
  characteristic 
  of 
  Zone 
  IV, 
  are 
  

   present 
  and 
  therefore 
  this 
  fauna 
  has 
  been 
  placed 
  with 
  Zone 
  IV. 
  

   The 
  following 
  is 
  a 
  list 
  of 
  the 
  species 
  collected. 
  

  

  Chonetes 
  illinoisensis 
  Worth 
  en. 
  Orthis 
  sivallovi 
  Hall. 
  

  

  Spirifer 
  grimesi 
  Hall. 
  Productus 
  punctatus 
  Martin. 
  

  

  Spirifer 
  sp. 
  i 
  Athyris 
  incrassatus 
  Hall. 
  

  

  Syringothyris 
  plenus 
  Hall. 
  Zaphrentis 
  centralis 
  Wortlien. 
  

  

  Productus 
  setigerus 
  Hall. 
  Spirifer 
  lineatoides 
  Swallow. 
  

  

  Orthis 
  burlingtonensis 
  Hall. 
  { 
  Athyris 
  lamellosa 
  L'Eveille. 
  

  

  Spirifer 
  suborbicularis 
  Hall. 
  j 
  Spirifer 
  sp. 
  

  

  The 
  whole 
  fauna 
  of 
  this 
  zone 
  is 
  interesting 
  on 
  account 
  of 
  

   the 
  large 
  size 
  of 
  the 
  species. 
  A 
  constant 
  increase 
  in 
  the 
  aver- 
  

   age 
  size 
  of 
  the 
  members 
  of 
  the 
  fauna 
  as 
  a 
  whole, 
  from 
  Zone 
  I 
  

   to 
  Zone 
  IV 
  is 
  evident. 
  In 
  Zone 
  I 
  all 
  the 
  species 
  are 
  small, 
  

   while 
  in 
  Zone 
  IV 
  nearly 
  all 
  are 
  very 
  large. 
  This 
  increase 
  in 
  

   size 
  may 
  be 
  traced 
  out 
  even 
  in 
  a 
  single 
  species 
  or 
  a 
  race. 
  The 
  

   earliest 
  examples 
  of 
  Spirifer 
  grimesi 
  are 
  generally 
  small, 
  but 
  

   in 
  the 
  successively 
  higher 
  beds 
  the 
  average 
  size 
  increases 
  till 
  

   the 
  maximum 
  is 
  reached 
  in 
  Zone 
  IV. 
  During 
  this 
  increase 
  in 
  

   size 
  the 
  specific 
  characters 
  remain 
  unchanged, 
  but 
  above 
  Zone 
  

   IV, 
  as 
  the 
  species 
  becomes 
  again 
  reduced 
  in 
  size, 
  the 
  specific 
  

   characters 
  change 
  and 
  the 
  race 
  is 
  represented 
  by 
  8. 
  logani. 
  

  

  The 
  lineata 
  type 
  of 
  Spirifers 
  also 
  illustrates 
  this 
  same 
  prin- 
  

   ciple, 
  but 
  in 
  this 
  case, 
  with 
  the 
  increase 
  in 
  size, 
  new 
  specific 
  

   characters 
  are 
  assumed. 
  The 
  race 
  is 
  represented 
  in 
  Zones 
  I 
  

   and 
  II 
  by 
  8. 
  lineata, 
  which 
  is 
  scarcely 
  ever 
  more 
  than 
  one-half 
  

   inch 
  in 
  breadth. 
  In 
  Zone 
  III 
  8. 
  pseud 
  olineata 
  occurs. 
  This 
  

   is 
  a 
  larger 
  species 
  with 
  coarser 
  markings. 
  8. 
  lineatoides, 
  

   which 
  is 
  a 
  still 
  larger 
  and 
  coarser 
  form 
  of 
  the 
  same 
  race, 
  is 
  

   limited 
  to 
  Zone 
  IV. 
  As 
  this 
  race 
  decreases 
  in 
  size, 
  it 
  seems 
  to 
  

   pass 
  through 
  the 
  same 
  stages 
  by 
  which 
  it 
  attained 
  its 
  maximum 
  

   development, 
  but 
  in 
  a 
  reverse 
  order, 
  for 
  in 
  beds 
  above 
  Zone 
  

   IV, 
  8. 
  pseudolineata 
  occurs 
  which 
  cannot 
  be 
  distinguished 
  

   from 
  specimens 
  in 
  Zone 
  III. 
  Other 
  species 
  illustrating 
  the 
  

   same 
  principle, 
  and 
  reaching 
  their 
  maximum 
  development 
  at 
  

   nearly 
  the 
  same 
  horizon, 
  are 
  Athyris 
  lamellosa, 
  Orthis 
  swal- 
  

   lovi, 
  Zaphrentis 
  centralis, 
  and 
  Platyceras. 
  

  

  Zone 
  V. 
  — 
  Lying 
  above 
  Zone 
  III 
  at 
  Station 
  12, 
  and 
  also 
  at 
  

   Station 
  6, 
  is 
  a 
  bed 
  of 
  much 
  more 
  siliceous 
  limestone. 
  This 
  

   lower 
  chert 
  bed, 
  as 
  it 
  may 
  be 
  designated, 
  is 
  constituted 
  of 
  vari- 
  

   able 
  proportions 
  of 
  a 
  white, 
  calcareous 
  chert 
  in 
  lenticular 
  

   masses, 
  and 
  limestone. 
  The 
  limestone 
  is 
  variable 
  in 
  character, 
  

   but 
  in 
  general 
  is 
  much 
  finer 
  grained 
  and 
  harder 
  than 
  the 
  beds 
  

   below 
  or 
  above, 
  and 
  it 
  often 
  resembles 
  very 
  closely 
  the 
  fine 
  

  

  