﻿Report 
  of 
  the 
  State 
  Oeologisi. 
  951 
  

  

  (JS. 
  striatiformis, 
  Meek, 
  Waverly 
  sandstone 
  ; 
  S. 
  Logani, 
  Hall, 
  

   Keokuk 
  group; 
  S. 
  striatus, 
  Martin 
  ; 
  S. 
  Marcoui, 
  Waagen.) 
  

  

  (c) 
  Imbrex- 
  type. 
  Alate, 
  mucronate 
  shells, 
  with 
  narrow 
  cardi- 
  

   nal 
  area, 
  fine, 
  simple 
  (very 
  rarely 
  duplicate) 
  lateral 
  plications, 
  

   the 
  plications 
  on 
  fold 
  and 
  sinus 
  being 
  of 
  about 
  the 
  same 
  size 
  as 
  

   the 
  rest. 
  The 
  surface 
  is 
  frequently 
  lamellose. 
  

  

  (S. 
  Newberryi, 
  Hall, 
  Waverly 
  group 
  ; 
  S. 
  Marionensis, 
  Shumard, 
  

   Waverly 
  and 
  Choteau 
  groups 
  ; 
  S. 
  biplicatus, 
  Hall, 
  Kinderhook 
  

   group 
  ; 
  S. 
  imbrex, 
  Hall. 
  Burlington 
  limestone.) 
  

  

  (d) 
  Suborbicularis-type. 
  Forms 
  with 
  suborbicular 
  outline, 
  

   broad, 
  low 
  and 
  usually 
  simple 
  lateral 
  plications; 
  the 
  median 
  

   plications 
  are 
  few 
  and 
  indistinct. 
  

  

  {S. 
  suborbicular 
  is, 
  Hall, 
  Kinderhook-Keokuk 
  groups 
  ; 
  S. 
  subcar- 
  

   diiformis 
  Hall, 
  St. 
  Louis 
  group.) 
  

  

  (e) 
  Orestes-type. 
  Shells 
  of 
  small 
  size, 
  moderately 
  extended 
  on 
  

   the 
  hinge 
  ; 
  lateral 
  plications 
  simple 
  and 
  usually 
  few 
  in 
  number 
  ; 
  

   fold 
  and 
  sinus 
  angular 
  and 
  with 
  a 
  few 
  plications, 
  of 
  which 
  the 
  

   median 
  members 
  are 
  much 
  the 
  strongest. 
  Surface 
  usually 
  orna 
  

   mented 
  by 
  fine, 
  hair 
  like, 
  often 
  granulous 
  radiating 
  lines. 
  

  

  (S. 
  concinnus, 
  Hall, 
  Lower 
  Helderberg 
  group; 
  S. 
  Grieri, 
  Hall, 
  

   Corniferous 
  limestone 
  ; 
  S. 
  Orestes, 
  Hall, 
  lower 
  Upper 
  Devonian 
  ; 
  

   S. 
  Keokuk, 
  Hall, 
  Keokuk 
  group 
  ; 
  S. 
  opimus, 
  Hall, 
  Coal 
  measures.) 
  

  

  (f) 
  Divaricatus-type. 
  Species 
  with 
  hinge 
  not 
  extended, 
  low 
  

   fold 
  and 
  sinus 
  ; 
  numerous 
  fine 
  dichotomous 
  lateral 
  plications 
  not 
  

   differing 
  in 
  size 
  from 
  the 
  median 
  plications, 
  all 
  of 
  which 
  are 
  

   crossed 
  by 
  fine, 
  closely 
  set 
  concentric 
  lines 
  each 
  bearing 
  a 
  fimbria 
  

   of 
  short, 
  simple 
  spines. 
  

  

  (S. 
  divaricatus, 
  Hall, 
  Upper 
  Helderberg 
  and 
  Hamilton 
  groups.) 
  

  

  Section 
  Y. 
  Ostiolati. 
  (Plate 
  27, 
  figs. 
  1-17.) 
  Typical 
  examples, 
  

   Spirifer 
  ostiolatus, 
  Schlotheim, 
  S. 
  Oweni, 
  Hall. 
  

  

  Forms 
  with 
  the 
  median 
  fold 
  and 
  sinus 
  without 
  plications. 
  

  

  (S. 
  perextensus, 
  Meek 
  and 
  "Worthen 
  ; 
  S. 
  macrothyris, 
  Hall, 
  

   Corniferous 
  limestone 
  ; 
  S. 
  Marcyi, 
  Hall 
  ; 
  S. 
  audaculus, 
  Conrad 
  ; 
  

   S. 
  granulosus, 
  Conrad; 
  S. 
  Parry 
  anus, 
  Hall; 
  S. 
  asper, 
  Hall, 
  

   Hamilton 
  group.) 
  

  

  These 
  species, 
  in 
  the 
  degree 
  of 
  plication 
  of 
  the 
  sides 
  and 
  the 
  

   development 
  of 
  the 
  muscular 
  scars, 
  closely 
  resemble 
  the 
  members 
  

   of 
  the 
  foregoing 
  group. 
  As 
  a 
  rule 
  the 
  Ostiolati 
  are 
  stouter 
  

   shells, 
  shorter 
  on 
  the 
  hinge 
  and 
  more 
  ventricose 
  than 
  the 
  

  

  9 
  

  

  