﻿172 
  Twenty-eighth 
  Eeport 
  on 
  the 
  State 
  Museum. 
  

  

  the 
  anterior 
  margins 
  ; 
  posterior 
  margins 
  flattened 
  ; 
  beak 
  acute, 
  

   not 
  elevated, 
  projecting 
  beyond 
  the 
  cardinal 
  margin 
  ; 
  cardinal 
  

   line 
  straight. 
  Substance 
  of 
  shell 
  very 
  thin, 
  showing 
  lamellose 
  

   lines 
  of 
  growth 
  ; 
  margin 
  depressed, 
  crenulated 
  from 
  the 
  im- 
  

   pression 
  of 
  the 
  surface 
  striae. 
  

  

  Most 
  of 
  the 
  specimens 
  are 
  obscure 
  casts 
  of 
  the 
  interior, 
  and 
  

   present 
  few 
  characters 
  beyond 
  the 
  general 
  form. 
  

  

  Compared 
  with 
  A. 
  aphcea 
  Hall, 
  a 
  species 
  with 
  which 
  it 
  

   is 
  associated 
  in 
  Wisconsin 
  and 
  Illinois, 
  it 
  is 
  longer, 
  with 
  the 
  

   beaks 
  more 
  elongate 
  and 
  attenuate. 
  It 
  many 
  respects 
  it 
  re- 
  

   sembles 
  the 
  Myalina 
  mytiliformis 
  Hall, 
  of 
  the 
  Clinton 
  

   group 
  (Pal. 
  N". 
  Y., 
  vol. 
  ii, 
  p. 
  100); 
  but 
  it 
  is 
  proportionately 
  

   wider, 
  and 
  the 
  beaks 
  more 
  attenuate. 
  

  

  MODIOLOPSIS 
  Hall. 
  

   Modiolopsis 
  perlatus. 
  

  

  Plate 
  27, 
  Figs. 
  3,4. 
  

  

  Modiolopsis 
  perlatw 
  HaIlL. 
  Doc. 
  Edit. 
  28th 
  Rep. 
  St. 
  Mus. 
  Nat. 
  Hist. 
  Explana- 
  

   tion 
  of 
  pi. 
  27, 
  figs. 
  3, 
  4. 
  1876. 
  

  

  Shell 
  ovate, 
  moderately 
  convex, 
  the 
  greatest 
  convexity 
  a 
  

   little 
  anterior 
  to 
  the 
  middle 
  of 
  the 
  shell. 
  Beaks 
  slightly 
  

   elevated 
  above 
  the 
  hinge-] 
  ine 
  and 
  scarcely 
  more 
  than 
  one-fifth 
  

   the 
  length 
  of 
  the 
  shell 
  from 
  the 
  anterior 
  end 
  ; 
  the 
  greatest 
  

   width 
  of 
  the 
  shell 
  about 
  midway 
  between 
  the 
  beak 
  and 
  the 
  

   posterior 
  extremity. 
  The 
  straight 
  hinge 
  -line 
  extends 
  about 
  

   half 
  the 
  distance 
  from 
  the 
  beak 
  to 
  the 
  posterior 
  extremity, 
  

   thence 
  curving 
  into 
  the 
  posterior 
  slope. 
  Basal 
  margin 
  a 
  little' 
  

   contracted 
  immediately 
  below 
  the 
  beak, 
  and 
  thence 
  curving 
  

   into 
  the 
  broadly 
  rounded 
  posterior 
  extremity. 
  Surface 
  marked 
  

   by 
  fine 
  lines 
  of 
  growth 
  with 
  a 
  few 
  stronger 
  undulations. 
  

  

  Length 
  of 
  the 
  shell 
  three 
  and 
  a 
  half 
  centimetres 
  ; 
  width 
  

   two 
  and 
  a 
  half 
  centimetres. 
  

  

  This 
  shell 
  diners 
  from 
  characteristic 
  forms 
  of 
  M. 
  subalatus 
  

   in 
  the 
  more 
  ascending 
  direction 
  of 
  the 
  hinge-line 
  from 
  the 
  beak, 
  

   the 
  broader 
  and 
  more 
  regularly 
  rounded 
  posterior 
  extremity, 
  

   and 
  the 
  lesser 
  constriction 
  of 
  the 
  basal 
  margin 
  beneath 
  the 
  

   beak. 
  

  

  