﻿1030 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  forming 
  an 
  umbonal 
  pit. 
  Cardinal 
  process 
  represented 
  by 
  two 
  

   faint 
  processes 
  beneath 
  the 
  beak. 
  

  

  Figs. 
  440, 
  441.— 
  Hemithyris 
  psittacea, 
  Gmelin 
  (sp.) 
  o, 
  pedicle-valve; 
  b, 
  brachial 
  valve; 
  c, 
  pos- 
  

   terior 
  adductors; 
  d, 
  anterior 
  adductors; 
  e, 
  diductors; 
  /, 
  pedicle 
  muscles. 
  (Davidson.) 
  

  

  Type, 
  Hemithyris 
  psittacea, 
  G-melin 
  (sp.). 
  (Recent.) 
  

   Distribution. 
  Tertiary 
  (?) 
  — 
  Recent. 
  

  

  Acanthothyris, 
  D'Orbigny. 
  1850. 
  

   Shells 
  like 
  Hemithyris, 
  but 
  having 
  the 
  surface 
  covered 
  with 
  

   spines, 
  and 
  the 
  dental 
  plates 
  well 
  developed. 
  

  

  Fig. 
  442.— 
  Acanthothyris 
  spinosa, 
  Schlotheim 
  (sp.). 
  (Woodward.) 
  

  

  Type, 
  Acanthothyris 
  spinosa, 
  Schlotheim 
  (sp.). 
  (Jurassic.) 
  

   Distribution. 
  Jurassic 
  — 
  Recent. 
  

  

  Syntrophia, 
  Hall. 
  1893. 
  

   (Plate 
  45, 
  figs. 
  1-3.) 
  

   Shells 
  transversely 
  elongate, 
  biconvex, 
  with 
  straight 
  hinge- 
  

   line 
  whose 
  length 
  nearly 
  equals 
  the 
  greatest 
  diameter 
  of 
  the 
  

   valves; 
  each 
  valve 
  medially 
  divided 
  by 
  an 
  open 
  delthyrium 
  

   External 
  surface 
  smooth, 
  with 
  fine 
  concentric 
  lines 
  visible 
  only 
  

   about 
  the 
  margins 
  ; 
  the 
  inner 
  shell-layers 
  show 
  a 
  strongly 
  fibrous 
  

   radiating 
  structure 
  without 
  punctation. 
  The 
  pedicle-valve 
  bears 
  

   a 
  more 
  or 
  less 
  clearly 
  developed 
  median 
  sinus 
  and 
  the 
  brachial 
  

   valve 
  a 
  broad, 
  indistinct 
  fold. 
  

  

  On 
  the 
  interior 
  the 
  teeth 
  are 
  very 
  small, 
  lying 
  at 
  the 
  extremi- 
  

   ties 
  of 
  the 
  delthyrial 
  margins 
  and 
  supported 
  by 
  dental 
  plates 
  

  

  88 
  

  

  