﻿960 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  not 
  extend 
  to 
  the 
  bottom 
  of 
  the 
  valve. 
  Between 
  these 
  arises 
  a 
  

   large 
  median 
  septum 
  which 
  is 
  horizontally 
  expanded 
  at 
  its 
  summit 
  

   and 
  toward 
  its 
  interior 
  margin. 
  In 
  the 
  brachial 
  valve 
  the 
  dental 
  

   sockets 
  are 
  deep, 
  the 
  hinge-plate 
  very 
  large, 
  formed 
  by 
  the 
  hori- 
  

   zontal 
  expansion 
  and 
  union 
  of 
  the 
  crural 
  plates 
  and 
  producing 
  a 
  plat- 
  

  

  Figs. 
  299-301. 
  —Suessia 
  costata, 
  Deslongchamps; 
  showing 
  the 
  structure 
  of 
  the 
  pedicle 
  and 
  

   brachial-valves. 
  (Suess.) 
  

  

  form 
  which 
  extends 
  to 
  the 
  middle 
  of 
  the 
  valve. 
  This 
  plate 
  bears 
  

   the 
  impressiom 
  of 
  the 
  adductor 
  muscles. 
  The 
  primary 
  lamella 
  

   of 
  the 
  spiral 
  coils 
  are 
  united 
  by 
  a 
  jugum 
  bearing 
  a 
  median 
  process 
  

   directed 
  anteriorly. 
  

  

  Type, 
  Suessia 
  costata, 
  E. 
  Deslongchamps. 
  

  

  Distribution. 
  Jurassic. 
  

  

  Whitfieldella, 
  Hall. 
  1893. 
  

   (Plate 
  32, 
  figs. 
  1-9.) 
  

  

  Shells 
  usually 
  of 
  small 
  size 
  ; 
  valves 
  subequally 
  convex, 
  ovate 
  

   or 
  elongate 
  in 
  outline. 
  Umbo 
  of 
  the 
  pedicle- 
  valve 
  not 
  high 
  

   or 
  greatly 
  incurved, 
  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. 
  In 
  the 
  typical 
  

   forms 
  there 
  is 
  a 
  faint 
  sinus 
  on 
  both 
  valves 
  near 
  the 
  anterior 
  

   margin, 
  otherwise 
  the 
  surface 
  is 
  smooth. 
  On 
  the 
  interior 
  the 
  

   muscular 
  impressions 
  of 
  the 
  pedicle-valve 
  are 
  similar 
  to 
  those 
  

   of 
  Meristella. 
  In 
  the 
  brachial 
  valve 
  the 
  hinge-plate 
  is 
  concave, 
  

   divided 
  by 
  a 
  deep 
  central 
  concavity 
  which 
  is 
  supported 
  by 
  a 
  

   median 
  septum. 
  On 
  either 
  side 
  are 
  lobes 
  bearing 
  the 
  bases 
  of 
  

   the 
  crura. 
  The 
  brachidium 
  consists 
  of 
  two 
  spiral 
  cones 
  arranged 
  

  

  18 
  

  

  