﻿Report 
  of 
  tee 
  State 
  Geologist. 
  1001 
  

  

  narrow. 
  The 
  muscular 
  area 
  is 
  flabellate 
  and 
  extends 
  for 
  nearly 
  

   one-half 
  the 
  length 
  of 
  the 
  shell; 
  it 
  is 
  composed 
  of 
  two 
  elongate- 
  

   ovate 
  adductor 
  scars 
  inclosed 
  by 
  broad 
  and 
  radially 
  striated 
  

   diductors. 
  A 
  conspicuous 
  median 
  septum 
  begins 
  in 
  the 
  umbonal 
  

   region 
  and 
  extends 
  to 
  within 
  a 
  very 
  short 
  distance 
  of 
  the 
  anterior 
  

   edge 
  of 
  the 
  valve. 
  

  

  In 
  the 
  brachial 
  valve 
  the 
  hinge-plate 
  arises 
  from 
  the 
  bottom 
  of 
  

   the 
  shell 
  with 
  a 
  vertical 
  anterior 
  face 
  ; 
  but 
  just 
  above 
  the 
  plane 
  

   of 
  the 
  margins 
  of 
  the 
  valve 
  it 
  is 
  reflected 
  in 
  a 
  curve 
  so 
  abrupt 
  

   that 
  its 
  upper 
  face 
  becomes 
  horizontal. 
  The 
  anterior 
  face 
  is 
  con- 
  

   cave 
  and 
  quadrate 
  in 
  outline; 
  the 
  posterior 
  face 
  is 
  sub 
  triangular, 
  

   flat 
  or 
  concave, 
  and 
  is 
  frequently 
  bilobed 
  at 
  its 
  extremity. 
  In 
  

   profile 
  the 
  plate 
  has 
  a 
  hook-shaped 
  appearance, 
  and 
  its 
  posterior 
  

   extremity 
  is 
  elevated 
  considerably 
  above 
  the 
  beak 
  of 
  the 
  valve, 
  

   and 
  when 
  the 
  valves 
  are 
  in 
  articulation, 
  extending 
  quite 
  to 
  

   the 
  bottom 
  of 
  the 
  umbonal 
  cavity 
  of 
  the 
  pedicle- 
  valve. 
  The 
  

   crural 
  bases 
  are 
  situated 
  on 
  the 
  vertical 
  face 
  of 
  the 
  plate 
  

   just 
  at 
  the 
  point 
  of 
  recurvature. 
  The 
  crura 
  are 
  slender, 
  

   straight, 
  long 
  and 
  rod-like, 
  having 
  a 
  length 
  equal 
  to 
  fully 
  one- 
  

   fourth 
  that 
  of 
  the 
  shell. 
  They 
  are 
  attached 
  at 
  their 
  tips 
  to 
  

   the 
  inner 
  surfaces 
  of 
  the 
  primary 
  lamellae. 
  The 
  primary 
  lamellae 
  

   of 
  the 
  spiral 
  coils 
  are 
  greatly 
  incurred 
  and 
  their 
  apices 
  close 
  

   together 
  ; 
  their 
  umbonal 
  blades 
  are 
  very 
  broad. 
  The 
  jugum 
  

   originates 
  at 
  about 
  one-fourth 
  the 
  length 
  of 
  the 
  lamellae, 
  is 
  

   inclined 
  slightly 
  backward, 
  the 
  lateral 
  branches 
  uniting 
  directly 
  

   in 
  front 
  of 
  the 
  apices 
  of 
  the 
  lamellae, 
  and 
  forming 
  a 
  single 
  straight 
  

   stem, 
  which 
  is 
  continued 
  beyond 
  the 
  opposite 
  edge 
  of 
  the 
  coil 
  and 
  

   almost 
  to 
  the 
  inner 
  surface 
  of 
  the 
  pedicle- 
  valve. 
  The 
  spiral 
  rib- 
  

   bon 
  makes 
  from 
  six 
  to 
  ten 
  volutions, 
  and 
  the 
  cones 
  have 
  their 
  

   altitude 
  in 
  the 
  transverse 
  diameter 
  of 
  the 
  shell. 
  

  

  The 
  muscular 
  area 
  is 
  very 
  narrow 
  and 
  elongate, 
  the 
  posterior 
  

   adductor 
  scars 
  enveloping 
  the 
  extremities 
  of 
  the 
  anterior 
  

   adductors. 
  They 
  are 
  divided 
  into 
  pairs 
  by 
  a 
  median 
  septum 
  of 
  

   the 
  same 
  extent 
  as 
  that 
  of 
  the 
  pedicle- 
  valve. 
  Fine 
  racemose 
  vas- 
  

   cular 
  sinuses 
  are 
  sometimes 
  retained 
  over 
  the 
  pallial 
  region 
  of 
  

   both 
  valves. 
  

  

  The 
  external 
  surface 
  usually 
  bears 
  a 
  low 
  median 
  sinus 
  and 
  fold 
  

   on 
  the 
  pedicle-, 
  and 
  brachial 
  valves 
  respectively. 
  The 
  epidermal 
  

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