﻿170 
  TWEXTY-EIGHTH 
  REPORT 
  OX 
  THE 
  STATE 
  MUSEUM. 
  

  

  elliptical, 
  gibbous 
  in 
  the 
  upper 
  part, 
  more 
  attenuate 
  in 
  front, 
  

   cardinal 
  slopes 
  flattened 
  in 
  most 
  specimens, 
  sometimes 
  roun- 
  

   ded. 
  Ventral 
  beak 
  small, 
  acute, 
  flattened 
  on 
  the 
  back, 
  and 
  

   closely 
  incurved 
  ; 
  the 
  front 
  half 
  of 
  the 
  ventral 
  valve 
  marked 
  

   by 
  a 
  broad, 
  shallow, 
  sometimes 
  undefined 
  sinus. 
  Dorsal 
  valve 
  

   usually 
  more 
  ventricose, 
  beak 
  obtusely 
  pointed 
  and 
  strongly 
  

   incurred, 
  a 
  low, 
  broad, 
  scarcely 
  defined 
  mesial 
  elevation 
  

   marks 
  the 
  center 
  and 
  often 
  occupies 
  nearly 
  one-third 
  the 
  

   entire 
  width 
  of 
  the 
  valve 
  ; 
  often 
  this 
  fold 
  is 
  obsolete, 
  leaving 
  

   the 
  valve 
  regularly 
  arcuate 
  froni 
  side 
  to 
  side, 
  while 
  in 
  other 
  

   specimens 
  the 
  borders 
  are 
  depressed 
  and 
  the 
  shell 
  flattened 
  in 
  

   front. 
  

  

  Surface 
  of 
  the 
  shell, 
  except 
  a 
  small 
  space 
  on 
  the 
  umbo 
  of 
  

   the 
  ventral 
  valve, 
  covered 
  by 
  a 
  finely 
  reticulate 
  marking, 
  with 
  

   elongate, 
  generally 
  hexagonal 
  pits 
  or 
  openings, 
  with 
  thin 
  and 
  

   sharp 
  ridges 
  between: 
  these 
  markings 
  vary 
  in 
  different 
  speci- 
  

   mens, 
  and 
  also 
  on 
  different 
  parts 
  of 
  the 
  same 
  individual, 
  being 
  

   generally 
  finest 
  on 
  the 
  cardinal 
  slopes. 
  The 
  small 
  triangular 
  

   space 
  near 
  the 
  ventral 
  beak, 
  which 
  is 
  destitute 
  of 
  marking, 
  

   has 
  the 
  appearance 
  of 
  having 
  been 
  exfoliated, 
  but 
  since 
  this 
  

   is 
  an 
  invariable 
  character 
  in 
  all 
  the 
  individuals 
  examined, 
  

   varying 
  in 
  size 
  with 
  the 
  size 
  of 
  the 
  shell, 
  it 
  is 
  probably 
  de- 
  

   pendent 
  upon 
  organic 
  causes. 
  

  

  This 
  species 
  resembles 
  Atrypa 
  [EicJiioaldia] 
  corattifera* 
  

   Pal. 
  X. 
  Y., 
  vol. 
  ii, 
  p. 
  281, 
  but 
  differs 
  in 
  being 
  much 
  broader 
  and 
  

   less 
  ventricose, 
  and 
  also 
  in 
  the 
  greater 
  breadth 
  and 
  less 
  prom- 
  

   inence 
  of 
  the 
  mesial 
  fold. 
  

  

  