﻿Report 
  of 
  the 
  State 
  Geologist. 
  955 
  

  

  Amboccelia, 
  Hall. 
  1860. 
  

   (Plate 
  31, 
  figs. 
  8-17) 
  

  

  Shells 
  small, 
  concavo-, 
  or 
  plano-convex. 
  Marginal 
  outline 
  

   nearly 
  semi-circular. 
  Hinge-line 
  long 
  and 
  straight, 
  its 
  length 
  

   nearly 
  or 
  quite 
  equaling 
  the 
  greatest 
  transverse 
  diameter 
  of 
  the 
  

   shell. 
  

  

  Pedicle-valve 
  greatly 
  elevated 
  ; 
  umbo 
  arched 
  and 
  incurved 
  ; 
  

   with 
  a 
  narrow 
  median 
  groove 
  which 
  becomes 
  fainter 
  or 
  disappears 
  

   toward 
  the 
  anterior 
  margin. 
  Cardinal 
  area 
  well 
  defined 
  and 
  

   arched 
  ; 
  divided 
  medially 
  by 
  an 
  open 
  delthyrium 
  whose 
  lateral 
  

   margins 
  bear 
  incomplete 
  deltidial 
  plates. 
  Teeth 
  prominent, 
  erect, 
  

   strongly 
  recurred 
  at 
  the 
  tips 
  ; 
  not 
  supported 
  by 
  dental 
  plates. 
  

   Muscular 
  area 
  quite 
  restricted, 
  consisting 
  of 
  narrow, 
  elongate 
  

   diductors, 
  inclosing 
  an 
  almost 
  linear 
  adductor. 
  The 
  entire 
  area 
  

   is 
  sometimes 
  divided 
  by 
  a 
  faint 
  median 
  ridge. 
  The 
  interior 
  sur- 
  

   face 
  about 
  the 
  muscular 
  area 
  is 
  strongly 
  pitted. 
  

  

  Brachial 
  valve 
  convex 
  at 
  the 
  beak, 
  becoming 
  depressed 
  over 
  

   the 
  pallial 
  region 
  and 
  reflexed 
  near 
  the 
  margin. 
  Cardinal 
  area 
  

   comparatively 
  broad 
  and 
  standing 
  at 
  nearly 
  right 
  angles 
  to 
  the 
  

   area 
  of 
  the 
  opposite 
  valve. 
  Delthyrium 
  open, 
  the 
  deltidial 
  cov- 
  

   ering 
  attaining 
  the 
  same 
  degree 
  of 
  development 
  as 
  in 
  the 
  pedicle- 
  

   valve. 
  Cardinal 
  process 
  narrow 
  and 
  much 
  elongated, 
  resting 
  on 
  

   the 
  bottom 
  of 
  the 
  valve 
  except 
  at 
  its 
  posterior 
  extremity, 
  which 
  is 
  

   simply 
  bifurcated. 
  Crural 
  plates 
  erect, 
  parallel 
  ; 
  taking 
  their 
  origin 
  

   in 
  the 
  deltidial 
  plates 
  and 
  extending 
  about 
  one-fourth 
  the 
  distance 
  

   across 
  the 
  valve. 
  The 
  spirals 
  are 
  attached 
  by 
  long 
  crura, 
  the 
  

   ribbon 
  making 
  a 
  few 
  volutions 
  only, 
  thus 
  forming 
  loose 
  coils, 
  

   directed 
  laterally. 
  The 
  jugum 
  has 
  apparently 
  the 
  same 
  incipient 
  

   condition 
  of 
  development 
  as 
  in 
  Spirifer. 
  According 
  to 
  (Ehlert, 
  

   the 
  spiral 
  ribbon 
  bears 
  spinules 
  on 
  its 
  outer 
  margins. 
  Muscular 
  

   impressions 
  anterior 
  and 
  composed 
  of 
  four 
  well-defined 
  adductor 
  

   scars. 
  

  

  Surface 
  smooth 
  or 
  with 
  fine 
  concentric 
  striae 
  crossed 
  by 
  

   indistinct 
  radiating 
  lines 
  ; 
  rarely 
  spinous. 
  Shell 
  substance 
  

   fibrous, 
  impunctate. 
  

  

  Type, 
  Amboccdia 
  umbonata, 
  Conrad 
  (sp.), 
  Hamilton 
  group. 
  

   . 
  Distribution. 
  Devonian 
  — 
  Carboniferous. 
  

  

  13 
  

  

  