﻿1048 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  The 
  muscular 
  impressions 
  occupy 
  an 
  elongate 
  area 
  below 
  the 
  

   hinge-plate, 
  and 
  are 
  divided 
  by 
  a 
  median 
  ridge, 
  but 
  are 
  only 
  

   obscurely 
  divisible 
  into 
  their 
  elementary 
  scars. 
  The 
  lateral 
  por- 
  

   tions 
  of 
  this 
  valve 
  frequently 
  bear 
  a 
  series 
  of 
  vascular 
  sinuses 
  in 
  

   the 
  pallial 
  region. 
  

  

  Surface 
  smooth 
  or 
  with 
  concentric 
  lines 
  crowded 
  near 
  the 
  

   margins 
  of 
  the 
  valves. 
  Shell- 
  substance 
  punctate. 
  

  

  Type, 
  Centronella 
  glans-fagea, 
  Hall. 
  Upper 
  Helderberg 
  

   group. 
  

  

  Distribution. 
  Devonian. 
  

  

  Subgenus 
  Oriskania, 
  Hall. 
  1893. 
  

   (Plate 
  52, 
  figs. 
  11-13.) 
  

  

  Shells 
  large 
  with 
  the 
  characteristic 
  naviculoid 
  form 
  and 
  smooth 
  

   exterior 
  of 
  Centr 
  nella. 
  The 
  form 
  of 
  the 
  brachidium 
  has 
  not 
  

   been 
  determined, 
  but 
  there 
  is 
  every 
  reason 
  to 
  infer 
  that 
  it 
  differs 
  

   in 
  no 
  essential 
  features 
  from 
  that 
  of 
  Centronella. 
  The 
  hinge- 
  

  

  Fig. 
  463. 
  Fig. 
  464. 
  Fig. 
  465. 
  

  

  Oriskania 
  navicella, 
  Hall. 
  

  

  Fig. 
  463. 
  A 
  cardinal 
  view; 
  showing 
  the 
  hinge-plate 
  and 
  the 
  elevation 
  of 
  the 
  narrow 
  median 
  

   crest 
  or 
  cardinal 
  process. 
  The 
  specimen 
  also 
  retains 
  the 
  teeth 
  of 
  the 
  pedicle- 
  

   valve 
  and 
  a 
  portion 
  of 
  the 
  dental 
  lamellae, 
  though 
  the 
  latter 
  are 
  broken 
  near 
  the 
  

   surface 
  of 
  the 
  valve. 
  

  

  Fig. 
  464. 
  An 
  enlargement 
  of 
  the 
  hinge-plate; 
  showing 
  the 
  crural 
  lobes 
  and 
  the 
  extent 
  of 
  the 
  

   cardinal 
  process 
  . 
  

  

  Fig. 
  465. 
  A 
  profile 
  of 
  the 
  same 
  specimen; 
  showing 
  the 
  thickness 
  of 
  the 
  plate 
  and 
  the 
  uncinate 
  

   form 
  of 
  the 
  cardinal 
  process. 
  X 
  3. 
  

  

  plate 
  is 
  elongate-triangular, 
  continuous 
  between 
  the 
  crural 
  bases, 
  

   and 
  bears 
  a 
  median 
  vertical 
  crest, 
  or 
  cardinal 
  process, 
  which 
  

   begins 
  at 
  the 
  apex, 
  rises 
  rapidly 
  in 
  height, 
  and 
  extends 
  for 
  fully 
  

   one-half 
  the 
  length 
  of 
  the 
  plate 
  on 
  its 
  upper 
  edge, 
  but 
  at 
  its 
  base 
  

  

  106 
  

  

  