﻿Report 
  of 
  the 
  State 
  Geologist. 
  991 
  

  

  The 
  external 
  surface 
  is 
  covered 
  with 
  numerous 
  fine 
  radiating 
  

   striae, 
  which 
  are 
  rarely 
  crossed 
  by 
  concentric 
  lines. 
  Shell- 
  

   substance 
  richly 
  punctate. 
  

  

  Type, 
  Retzia 
  vera, 
  Hall. 
  Kaskaskia 
  limestone. 
  

  

  Distribution. 
  Lower 
  — 
  upper 
  (?) 
  Carboniferous. 
  

  

  (?) 
  Acambona, 
  White. 
  1862. 
  

   (Plate 
  37, 
  figs. 
  21, 
  22.) 
  

  

  Shells 
  similar 
  to 
  Eumetria. 
  Beak 
  of 
  pedicle-valve 
  prominent, 
  

   incurved, 
  pointed 
  (?). 
  Deltarium 
  triangular 
  or 
  flat. 
  Brachial 
  

   valve 
  without 
  the 
  alate 
  cardinal 
  extensions 
  of 
  Eumetria. 
  Internal 
  

   pedicle-tube 
  slightly 
  developed. 
  Hinge-plate 
  with 
  two 
  short 
  

   processes 
  on 
  its 
  posterior 
  edge 
  which 
  extend 
  for 
  only 
  a 
  short 
  

   distance 
  into 
  the 
  umbonal 
  cavity. 
  

  

  Type, 
  Acambona 
  prima, 
  White. 
  Burlington 
  limestone. 
  

  

  Distribution. 
  Lower 
  Carboniferous. 
  

  

  Hustedia, 
  HaU. 
  1893. 
  

  

  (Plate 
  37, 
  figs. 
  13-20.) 
  

   Shells 
  distinguished 
  externally 
  from 
  Eumetria 
  by 
  their 
  much 
  

   coarser 
  plications. 
  The 
  essential 
  difference 
  from 
  that 
  genus 
  lies 
  

   in 
  the 
  structure 
  of 
  the 
  hinge-plate 
  and 
  the 
  umbonal 
  cavity 
  of 
  the 
  

   pedicle-valve. 
  The 
  latter 
  contains 
  an 
  internal 
  tube 
  attached 
  by 
  

   one 
  side 
  to 
  the 
  deltarium 
  and 
  split 
  along 
  the 
  opposite 
  side, 
  a 
  

   precisely 
  similar 
  structure 
  to 
  that 
  observed 
  in 
  Retzia 
  and 
  

   Acambona, 
  though 
  not 
  so 
  highly 
  developed 
  as 
  in 
  the 
  first 
  of 
  

   these. 
  The 
  hinge-plate 
  is 
  constituted 
  as 
  follows 
  : 
  It 
  is 
  erect 
  and 
  

   recurved 
  into 
  the 
  umbonal 
  cavity 
  of 
  the 
  pedicle-valve, 
  projecting 
  

   considerably 
  beyond 
  the 
  hinge-line 
  ; 
  the 
  upper 
  rx 
  

  

  face 
  is 
  convex 
  and 
  elevated 
  medially, 
  the 
  / 
  J/J 
  

  

  posterior 
  margin 
  sinuate 
  and 
  crescentic, 
  

   though 
  the 
  horns 
  of 
  the 
  crescent 
  are 
  very 
  

   short 
  ; 
  two 
  deep 
  converging 
  grooves 
  pass 
  over 
  

   the 
  upper 
  face, 
  and 
  outside 
  of 
  these 
  on 
  the 
  FlGS 
  - 
  fe-aes. 
  - 
  outline 
  

  

  1 
  x 
  7 
  profile 
  of 
  Hustedia 
  Mor- 
  

  

  lateral 
  margin 
  of 
  the 
  plate 
  are 
  strong 
  lobes 
  moni, 
  Marcou(s 
  P 
  ),with 
  en- 
  

   which 
  bear 
  the 
  erect, 
  slightlv 
  recurved 
  ^ 
  r 
  ff 
  tra 
  ° 
  sve 
  u 
  rse 
  s 
  ?<*ions 
  

  

  ' 
  ° 
  of 
  the 
  umbo 
  beneath 
  the 
  

  

  crura 
  ; 
  from 
  the 
  crural 
  bases 
  the 
  lateral 
  foramen; 
  showing 
  the 
  in- 
  

   margins 
  curve 
  downward 
  to 
  the 
  bottom 
  of 
  r^ 
  dTeTtTk'. 
  

   the 
  valve 
  and 
  form 
  the 
  socket 
  walls. 
  At 
  the 
  plates, 
  

   base 
  of 
  the 
  cardinal 
  process 
  and 
  in 
  the 
  median 
  line 
  arises 
  a 
  

   free, 
  slender, 
  ligulate 
  process 
  which 
  curves 
  upward 
  and 
  back- 
  

  

  49 
  

  

  