﻿Report 
  of 
  the 
  State 
  Geologist. 
  1003 
  

  

  bears 
  a 
  low 
  fold 
  corresponding 
  to 
  the 
  central 
  groove 
  of 
  the 
  

   opposite 
  valve. 
  On 
  the 
  interior 
  of 
  the 
  pedicle- 
  valve 
  the 
  shell 
  in 
  

   the 
  umbonal 
  region 
  is 
  very 
  thick, 
  and 
  in 
  this 
  thickened 
  portion 
  

   the 
  scar 
  of 
  the 
  pedicle 
  muscle, 
  and 
  in 
  front 
  of 
  it, 
  the 
  adductor 
  

   scar 
  is 
  excavated. 
  At 
  the 
  anterior 
  edge 
  of 
  the 
  muscular 
  area 
  

   the 
  shell 
  becomes 
  suddenly 
  and 
  abruptly 
  thinner, 
  and 
  thus 
  that 
  

   area 
  lies 
  on 
  a 
  well-developed, 
  solid 
  platform. 
  In 
  the 
  brachial 
  

   valve 
  the 
  hinge-plate 
  is 
  small 
  and 
  supported 
  by 
  a 
  low 
  median 
  

   septum 
  which 
  extends 
  about 
  two-thirds 
  the 
  length 
  of 
  the 
  valve. 
  

  

  The 
  crura 
  diverge 
  slightly 
  as 
  they 
  pass 
  downward, 
  making 
  a 
  

   very 
  low 
  curve 
  or 
  slight 
  angulation 
  at 
  their 
  union 
  with 
  the 
  

   primary 
  lamellae. 
  The 
  spiral 
  ribbon 
  is 
  very 
  delicate 
  and 
  quite 
  

   short, 
  making 
  but 
  two 
  and 
  one-half 
  or 
  three 
  volutions, 
  which 
  are 
  

   almost 
  circular. 
  These 
  spirals 
  are 
  coiled 
  in 
  planes 
  nearly 
  

   parallel 
  to 
  the 
  vertical 
  axial 
  plane 
  of 
  the 
  shell, 
  being 
  very 
  

   slightly 
  introverted, 
  and 
  the 
  primary 
  whorls 
  so 
  close 
  together 
  

   that 
  this 
  slight 
  introversion 
  brings 
  the 
  apices 
  into 
  approximation. 
  

   There 
  is 
  as 
  yet 
  no 
  satisfactory 
  proof 
  of 
  the 
  existence 
  of 
  a 
  jugum; 
  

   indeed, 
  the 
  evidence 
  derived 
  from 
  a 
  number 
  of 
  transparent 
  pre- 
  

   parations 
  is 
  decidedly 
  negative 
  upon 
  this 
  point. 
  Where 
  the 
  

   crura 
  are 
  attached 
  to 
  the 
  primary 
  lamella?, 
  the 
  ribbon 
  is 
  broad- 
  

   ened, 
  and 
  just 
  in 
  front 
  of 
  these 
  points 
  there 
  appear 
  to 
  have 
  been 
  

   two 
  short 
  convergent 
  apophyses 
  which 
  may 
  be 
  construed 
  as 
  dis- 
  

   crete 
  elements 
  of 
  a 
  jugum, 
  but 
  the 
  shell 
  appears 
  to 
  have 
  been 
  

   actually 
  a 
  jugate. 
  

  

  Subgenus 
  Protozyga, 
  Hall. 
  1893. 
  

   (Plate 
  40, 
  figs. 
  4, 
  5.) 
  

  

  Shells 
  small, 
  subplano-convex, 
  the 
  pedicle-valve 
  being 
  the 
  

   deeper. 
  The 
  ante-lateral 
  margins 
  of 
  the 
  valve 
  bear 
  evidence 
  of 
  

   broad, 
  coarse 
  plications. 
  

  

  The 
  brachial 
  valve 
  has 
  a 
  simply 
  divided 
  hinge-plate, 
  upon 
  these 
  

   divisions 
  resting 
  the 
  two 
  short 
  convergent 
  crura 
  ; 
  joining 
  the 
  lat- 
  

   ter 
  at 
  a 
  low 
  angle, 
  the 
  primary 
  lamellae 
  diverge 
  laterally, 
  con- 
  

   verge 
  slightly 
  toward 
  their 
  anterior 
  margins, 
  thence 
  curve 
  verti- 
  

   cally 
  upward, 
  nearly 
  touching 
  the 
  inner 
  surface 
  of 
  the 
  pedicle- 
  

   valve 
  and 
  very 
  gradually 
  approaching 
  each 
  other. 
  The 
  ribbon 
  is 
  

   continued 
  with 
  a 
  decided 
  internal 
  inclination, 
  until 
  it 
  completes 
  

  

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