﻿Report 
  of 
  the 
  State 
  Geologist. 
  

  

  983 
  

  

  the 
  outer 
  opening 
  of 
  the 
  foramen 
  this 
  tube 
  appears 
  to 
  have 
  been 
  

   closed 
  on 
  all 
  sides, 
  but 
  further 
  toward 
  the 
  cardinal 
  margin 
  it 
  

   becomes 
  split 
  along 
  the 
  back 
  or 
  outer 
  surface, 
  diminishing 
  in 
  size 
  

   downward 
  and 
  disappearing 
  entirely 
  before 
  the 
  cardinal 
  margin 
  

   is 
  reached. 
  In 
  sections 
  made 
  across 
  the 
  vertical 
  foramen 
  it 
  is 
  

   seen 
  that 
  the 
  tube 
  extends 
  within 
  the 
  deltidial 
  covering, 
  and 
  

   forms 
  a 
  subcircular 
  enfolding 
  of 
  testaceous 
  matter 
  from 
  the 
  

   margins 
  of 
  the 
  foramen. 
  

  

  Fig. 
  349. 
  

  

  Fig. 
  350. 
  

  

  Fig. 
  351. 
  

  

  Fig 
  352. 
  

  

  Fig. 
  349. 
  — 
  SectionFjust 
  above^the 
  apex 
  of 
  the 
  brachial 
  valve; 
  showing 
  the 
  last 
  traces 
  of 
  the 
  

   deltidial 
  plates, 
  which 
  are 
  here 
  free. 
  

  

  Fig. 
  350.— 
  Section 
  at 
  the 
  apex 
  of 
  the 
  brachial 
  valve. 
  

  

  Fig. 
  351.— 
  Section 
  cutting 
  the 
  posterior 
  extension 
  of 
  the 
  median 
  and 
  lateral 
  lobes 
  of 
  the 
  

   hinge-plate 
  . 
  

  

  Fig. 
  352.— 
  The 
  same 
  features 
  'urther 
  down; 
  showing 
  also 
  the 
  appearance 
  of 
  the 
  teeth, 
  and 
  

   the 
  remnants 
  of 
  the 
  dental 
  lamellae 
  bordering 
  the 
  muscular 
  area 
  of 
  the 
  pedicle-valve. 
  

  

  Fig. 
  353. 
  

  

  Ftg. 
  354. 
  

  

  Fig. 
  355. 
  

  

  Fig. 
  356. 
  

  

  Fig. 
  353.— 
  Section 
  through 
  the 
  center'of 
  the 
  hinge-plate; 
  showing 
  also 
  the 
  development 
  of 
  

   the 
  dental 
  sockets. 
  ' 
  

  

  Fig. 
  354.— 
  Section 
  showing 
  the 
  ante-median 
  crest 
  of 
  the 
  hinge-plate. 
  

   Fig. 
  355.— 
  The 
  anterior 
  edge 
  of 
  the 
  hinge-plate 
  and 
  its 
  supporting 
  septum. 
  

   Fig. 
  356.— 
  Section 
  in 
  front 
  of 
  the 
  hinge-plate; 
  showing 
  the 
  crura 
  and 
  median 
  septum. 
  

  

  In 
  the 
  brachial 
  valve 
  the 
  hinge-plate 
  is 
  subquadrate 
  on 
  its 
  

   upper 
  surface, 
  its 
  posterior 
  margin 
  somewhat 
  crescentic, 
  the 
  

   horns 
  of 
  the 
  crescent 
  extending 
  into 
  the 
  umbonal 
  cavity 
  of 
  the 
  

   opposite 
  valve 
  ; 
  this 
  character, 
  however, 
  is 
  not 
  so 
  highly 
  devel- 
  

   oped 
  as 
  in 
  Etjmetria. 
  The 
  structure 
  of 
  this 
  plate 
  appears 
  to 
  be 
  

   essentially 
  similar 
  to 
  that 
  of 
  Exstedia 
  ; 
  at 
  all 
  events, 
  the 
  tent- 
  

   shaped 
  crural 
  supports 
  of 
  Etjmetria 
  are 
  absent 
  ; 
  there 
  is, 
  however, 
  

   no 
  trace 
  here 
  of 
  the 
  ligulate, 
  curved 
  process 
  which 
  occurs 
  in 
  

  

  41 
  

  

  