﻿202 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  curved, 
  usually 
  exposing 
  the 
  circular 
  apical 
  foramen, 
  beneath 
  which 
  

   the 
  deltidial 
  plates 
  are 
  frequently 
  retained. 
  Cardinal 
  slopes 
  of 
  both 
  

   valves 
  broad 
  and 
  not 
  distinctly 
  defined 
  ; 
  anterior 
  margin 
  subtruncate 
  

   and 
  gently 
  sinuate. 
  Hinge 
  plate 
  in 
  brachial 
  valve 
  concave, 
  divided 
  

   by 
  a 
  deep 
  central 
  concavity, 
  which 
  is 
  supported 
  by 
  a 
  medium 
  

   septum. 
  Brachidium 
  consisting 
  of 
  two 
  spiral 
  cones 
  arranged 
  base 
  

   to 
  base, 
  connected 
  by 
  a 
  V-shaped 
  jugum. 
  

  

  Whitfieldella 
  nitida 
  Hall 
  (Fig. 
  123). 
  Atrypa 
  nitida 
  Hall 
  

   (1852. 
  Pal. 
  N, 
  Y. 
  2:268, 
  pi. 
  55) 
  

  

  Distinguishing 
  characters. 
  Small 
  size; 
  

  

  yfik 
  ^^^ 
  strong 
  convexity 
  of 
  valves, 
  which 
  are 
  

  

  M'-^m 
  MH 
  nearly 
  equal, 
  pointed 
  incurved 
  beak 
  of 
  

  

  ^SkW 
  ^^ 
  pedicle 
  valve; 
  smooth 
  surface 
  except 
  for 
  

  

  ^ 
  concentric 
  growth 
  lines 
  and 
  strong 
  

  

  j^^^ 
  ^l^k 
  wrinkles 
  of 
  growth; 
  often 
  slight 
  sinus 
  

  

  %J^m 
  ^^^H 
  near 
  the 
  front 
  of 
  both 
  valves, 
  causing 
  

  

  slight 
  frontal 
  emargination. 
  

  

  Fig. 
  123 
  Whitfieldella 
  nitida 
  -r^ 
  , 
  , 
  i 
  , 
  • 
  , 
  ^i 
  . 
  •, 
  

  

  round 
  abundantly 
  m 
  the 
  Clinton 
  lenses 
  

   and 
  less 
  commonly 
  in 
  the 
  Rochester 
  shale 
  at 
  Niagara. 
  

  

  This 
  species 
  varies 
  from 
  broadly 
  to 
  narrowly 
  ovate; 
  the 
  thick- 
  

   ness 
  is 
  frequently 
  greater 
  than 
  the 
  width 
  and 
  coarse 
  thickenings 
  

   and 
  wrinkles 
  show 
  changes 
  in 
  growth. 
  This 
  may 
  characterize 
  

   senile 
  individuals. 
  

  

  Whitfieldella 
  nitida 
  var. 
  oblata 
  Hall 
  (Fig. 
  124). 
  Atrypa 
  

   nitida 
  var. 
  oblata 
  Hall 
  (1852. 
  Fa/. 
  A^. 
  F. 
  2:269, 
  pi. 
  55) 
  

  

  Distinguishing 
  characters. 
  Broadly 
  

   ovate 
  form, 
  angle 
  between 
  cardinal 
  

   slopes 
  often 
  90° 
  or 
  more; 
  moder- 
  

   ately 
  convex 
  valves, 
  subtriangular 
  in 
  

   cross-section; 
  uniformly 
  rounded 
  

   front 
  ; 
  surface 
  absolutely 
  smooth, 
  very 
  ^.^ 
  ^^^ 
  whitfleideiia 
  mtida 
  var. 
  obiata 
  

   deep 
  muscular 
  impressions. 
  

  

  Found 
  in 
  the 
  Clinton 
  lenses 
  and 
  the 
  Rochester 
  shale 
  associated 
  

   with 
  the 
  preceding 
  and 
  usually 
  more 
  abundant. 
  Also 
  at 
  Lockport, 
  

   etc. 
  (Hall). 
  

  

  