﻿Report 
  of 
  the 
  State 
  Geologist. 
  

  

  1097 
  

  

  Eichwaldia, 
  Billings. 
  1858. 
  

   (Plate 
  54, 
  figs. 
  12-22.) 
  

   Shells 
  subtriangular 
  in 
  outline, 
  with 
  biconvex 
  valves, 
  the 
  

   pedicle-valve 
  having 
  a 
  broad 
  median 
  sinus, 
  and 
  the 
  brachial 
  valve 
  

   a 
  corresponding 
  median 
  fold. 
  The 
  umbo 
  of 
  the 
  pedicle- 
  valve 
  is 
  

   acute 
  and 
  arched 
  over 
  the 
  opposite 
  valve, 
  though 
  not 
  closely 
  

   appressed 
  against 
  it. 
  As 
  far 
  as 
  has 
  been 
  ascertained, 
  the 
  umbonal 
  

   space 
  between 
  the 
  two 
  valves 
  is 
  open, 
  that 
  is, 
  there 
  is 
  no 
  del- 
  

   tidium 
  or 
  pair 
  of 
  deltidial 
  plates 
  extending 
  from 
  the 
  apex 
  down- 
  

  

  i 
  

  

  Fig. 
  662. 
  

  

  Fig. 
  663. 
  

  

  Fig. 
  664. 
  

  

  Fig. 
  665. 
  

  

  Interior 
  of 
  brachial 
  valve. 
  

   Vertical 
  section 
  of 
  sheU. 
  

   Enlargement 
  of 
  the 
  surface. 
  

  

  Eichtoaldia 
  reticulata, 
  Hall. 
  

   Fig. 
  659. 
  Dorsal 
  view. 
  Fig. 
  663. 
  

  

  Fig. 
  660. 
  Cardinal 
  view. 
  Fig. 
  664. 
  

  

  Fig. 
  661. 
  Longitudinal 
  section 
  of 
  two 
  valves. 
  Fig. 
  665. 
  

  

  Fig. 
  662. 
  Interior 
  of 
  pedicle- 
  valve. 
  

  

  Notation 
  : 
  f 
  , 
  " 
  bare 
  spot 
  " 
  or 
  foramen 
  ; 
  p(c), 
  deltidium 
  or 
  internal 
  plate 
  ; 
  C 
  , 
  umbonal 
  surface 
  

   of 
  pedicle-valve; 
  t, 
  teeth; 
  b, 
  dental 
  sockets; 
  j, 
  cardinal 
  process; 
  s, 
  median 
  septum 
  of 
  brachial 
  

   valve. 
  

  

  ward 
  ; 
  but 
  there 
  is 
  a 
  short 
  triangular 
  plate 
  or 
  diaphragm 
  which 
  

   begins 
  at 
  the 
  apex 
  of 
  this 
  valve 
  and 
  extends 
  forward 
  beyond 
  the 
  

   posterior 
  edge 
  of 
  the 
  brachial 
  valve, 
  and 
  thus 
  serves 
  the 
  purpose 
  

   of 
  the 
  deltidium, 
  though 
  deeply 
  depressed 
  within 
  the 
  cavity 
  of 
  

   the 
  pedicle-valve. 
  This 
  diaphragm 
  is 
  usually 
  quite 
  short 
  and 
  

   confined 
  to 
  the 
  apical 
  region, 
  but 
  it 
  may 
  extend 
  for 
  fully 
  one- 
  

   fifth 
  the 
  length 
  of 
  the 
  valve, 
  its 
  anterior 
  margin 
  being 
  free 
  and 
  

   its 
  lateral 
  margins 
  adherent 
  to 
  the 
  inner 
  cardinal 
  slopes. 
  The 
  

   138 
  155 
  

  

  