﻿Report 
  of 
  the 
  State 
  Geologist. 
  969 
  

  

  External 
  surface 
  of 
  the 
  valves 
  smooth 
  or 
  with 
  concentric 
  striae. 
  

   Shell- 
  structure 
  fibrous, 
  impunctate. 
  

   Type, 
  Merisfa 
  Icevis, 
  Hall. 
  Lower 
  Helderberg 
  group. 
  

   Distribution. 
  Devonian. 
  

  

  Dioristella, 
  Bittner. 
  1890. 
  

  

  Smooth 
  shells 
  having 
  a 
  jugum 
  whose 
  lateral 
  branches 
  return 
  

   upon 
  themselves, 
  somewhat 
  as 
  in 
  Meristella. 
  

  

  Fig, 
  310.— 
  Jugum 
  of 
  Dioristella 
  indistincta, 
  Beyrich 
  (sp.). 
  (Bittxkb.) 
  

  

  Type, 
  Dioristella 
  indistincta, 
  Beyrich 
  (sp.). 
  

   Distribution. 
  Triassic. 
  

  

  Charionella, 
  Billings. 
  1861. 
  

   (Plate 
  34. 
  figs. 
  12-14.) 
  

  

  Shells 
  meristoid 
  in 
  exterior 
  and 
  probably 
  in 
  the 
  structure 
  of 
  

   the 
  brachidium 
  and 
  jugum. 
  Hinge-plate 
  greatly 
  modified, 
  its 
  

   appearance 
  being 
  that 
  of 
  two 
  ridges, 
  strongly 
  recurved 
  at 
  their 
  

   edges, 
  passing 
  along 
  the 
  margins 
  of 
  the 
  delthyrium 
  and 
  inclos- 
  

   ing 
  the 
  dental 
  sockets 
  ; 
  these 
  are 
  supported 
  by 
  thin 
  lamellae 
  

   which 
  converge 
  toward 
  the 
  bottom 
  of 
  the 
  valve 
  ; 
  the 
  crura 
  aris- 
  

   ing 
  from 
  the 
  extremities 
  of 
  the 
  lateral 
  ridges 
  are 
  short 
  and 
  

   curved 
  outward 
  ; 
  the 
  central 
  portion 
  of 
  the 
  plate 
  must 
  be 
  con- 
  

   sidered 
  as 
  absent, 
  or 
  as 
  very 
  concave 
  and 
  merged 
  in 
  the 
  sub- 
  

   stance 
  of 
  the 
  valve. 
  

  

  Type, 
  Chrrionella 
  scitula, 
  Hall 
  (sp.) 
  Corniferous 
  limestone. 
  

  

  Pentagonia, 
  Cozzens. 
  1846. 
  

  

  (Plate 
  34, 
  figs. 
  15-20.) 
  

  

  Synonym; 
  Gonioccelia, 
  Hall, 
  1861. 
  

  

  Shells 
  with 
  a 
  very 
  broad 
  sinus 
  on 
  the 
  pedicle 
  valve, 
  which 
  is 
  

   limited 
  by 
  divergent 
  carina?, 
  outside 
  of 
  which 
  the 
  cardinal 
  or 
  

   lateral 
  slopes 
  are 
  very 
  abrupt. 
  On 
  the 
  brachial 
  valve 
  is 
  a 
  

   rounded 
  median 
  fold, 
  which 
  may 
  be 
  divided 
  on 
  its 
  summit 
  by 
  a 
  

   narrow 
  sinus, 
  and 
  in 
  the 
  umbo-lateral 
  region 
  are 
  two 
  short 
  folds 
  

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