﻿Report 
  of 
  the 
  State 
  Geologist. 
  993 
  

  

  and 
  rather 
  abrupt 
  flattening 
  of 
  the 
  shell, 
  suggestive 
  of 
  a 
  cardinal 
  

   area. 
  The 
  umbonal 
  cavity 
  is 
  short 
  and 
  usually 
  much 
  thickened, 
  

   leaving 
  only 
  a 
  simple 
  passage 
  for 
  the 
  pedicle. 
  The 
  teeth 
  are 
  

   approximate 
  and 
  prominent, 
  arising 
  from 
  the 
  bottom 
  of 
  the 
  

   valve, 
  and 
  above 
  the 
  hinge-line 
  curved 
  backward 
  and 
  toward 
  each 
  

   other, 
  making 
  a 
  very 
  close 
  and 
  firm 
  articulation 
  with 
  the 
  other 
  

   valve. 
  The 
  dental 
  lamellae 
  are 
  not 
  continued 
  over 
  the 
  interior 
  

   of 
  the 
  valve. 
  The 
  muscular 
  area 
  is 
  well 
  defined 
  and 
  con- 
  

   sists 
  of 
  a 
  deep 
  posterior 
  area, 
  in 
  front 
  of 
  which 
  lies 
  a 
  

   flabelliform 
  scar, 
  extending 
  for 
  fully 
  one-half 
  the 
  length 
  of 
  

   the 
  shell. 
  

  

  In 
  the 
  brachial 
  valve 
  the 
  beak 
  is 
  not 
  prominent 
  and 
  the 
  false 
  

   area 
  is 
  absent. 
  A 
  small 
  chilidium 
  is 
  present 
  and 
  lies 
  against 
  the 
  

   vertical 
  posterior 
  wall 
  of 
  the 
  hinge-plate. 
  The 
  hinge-plate 
  is 
  

   greatly 
  elevated 
  ; 
  it 
  rests 
  upon 
  two 
  stout 
  supports 
  which 
  are 
  

   placed 
  very 
  closely 
  together, 
  leaving 
  no 
  opening 
  between 
  them 
  

   at 
  the 
  bottom 
  of 
  the 
  valve. 
  The 
  upper 
  face 
  of 
  the 
  plate 
  is 
  quad- 
  

   rate, 
  but 
  very 
  deeply 
  divided 
  by 
  a 
  median 
  longitudinal 
  groove, 
  

   and 
  less 
  conspicuously 
  by 
  a 
  transverse 
  groove 
  ; 
  the 
  surface 
  is 
  thus 
  

   divided 
  into 
  four 
  parts, 
  two 
  posterior 
  portions 
  which 
  extend 
  

   backward 
  into 
  the 
  umbonal 
  cavity 
  of 
  the 
  opposite 
  valve, 
  as 
  

   short, 
  stout 
  horns, 
  and 
  two 
  anterior 
  processes 
  which 
  are 
  

   broader 
  but 
  equally 
  elevated; 
  from 
  the 
  ante-lateral 
  margins 
  

   of 
  the 
  latter 
  arise 
  the 
  crura. 
  In 
  the 
  deep, 
  longitudinal 
  

   cleft 
  or 
  groove 
  of 
  the 
  plate 
  is 
  a 
  short, 
  convex 
  lobe, 
  termi- 
  

   nating 
  posteriorly 
  in 
  a 
  simple 
  or 
  double 
  extremity 
  ; 
  some- 
  

   times 
  this 
  part 
  is 
  absent. 
  The 
  whole 
  process 
  is 
  rendered 
  

   more 
  prominent 
  by 
  being 
  slightly 
  constricted 
  about 
  its 
  base. 
  It 
  is 
  

   supported 
  interiorly 
  by 
  a 
  short 
  median 
  septum, 
  which 
  is 
  frequently 
  

   obsolete. 
  The 
  dental 
  sockets 
  are 
  small 
  and 
  deep. 
  The 
  crura 
  are 
  

   broad, 
  thin 
  and 
  comparatively 
  short, 
  and 
  unite 
  with 
  the 
  primary 
  

   lamellae 
  in 
  a 
  sharp 
  lateral 
  curve 
  without 
  diminution 
  or 
  increase 
  in 
  

   size. 
  The 
  umbonal 
  blades 
  are 
  not 
  greatly 
  incurved 
  and 
  are 
  quite 
  

   as 
  narrow 
  as 
  any 
  portion 
  of 
  the 
  primary 
  ribbon. 
  The 
  jugum 
  

   takes 
  its 
  origin 
  well 
  forward 
  near 
  the 
  middle 
  of 
  the 
  spiral 
  cones 
  ; 
  

   the 
  lateral 
  branches 
  are 
  somewhat 
  broadened 
  at 
  their 
  origin, 
  but 
  

   become 
  slightly 
  constricted 
  and 
  twisted 
  just 
  above 
  their 
  bases, 
  

   125 
  51 
  

  

  