﻿Report 
  of 
  the 
  State 
  Geologist. 
  1033 
  

  

  which 
  converge, 
  and 
  uniting, 
  are 
  supported 
  by 
  a 
  short 
  median 
  

   septum, 
  forming 
  thus 
  a 
  well-defined 
  spondylium. 
  

  

  In 
  the 
  brachial 
  valve 
  the 
  hinge-structure 
  is 
  similar 
  to 
  that 
  of 
  

   Camarotcechia, 
  the 
  crural 
  plates 
  converging 
  and 
  forming 
  a 
  short, 
  

   very 
  small 
  median 
  cavity, 
  which 
  is 
  supported 
  by 
  a 
  long 
  septum. 
  

   The 
  crura 
  are 
  short 
  and 
  the 
  lateral 
  divisions 
  of 
  the 
  hinge-plate 
  

   small. 
  No 
  cardinal 
  process 
  exists. 
  

  

  Type, 
  Oamarella 
  Volborthi, 
  Billings. 
  

  

  Distribution. 
  Cambrian 
  (?) 
  — 
  Lower 
  Silurian. 
  

  

  Parastrophia, 
  Hall. 
  1893. 
  

   (Plate 
  45, 
  figs. 
  20-27.) 
  

  

  Shells 
  broad, 
  tranversely 
  oval 
  in 
  outline; 
  surface 
  with 
  low 
  

   rounded 
  plications 
  which 
  are 
  stronger 
  on 
  fold 
  and 
  sinus, 
  

   apparent 
  on 
  the 
  lateral 
  slopes 
  only 
  near 
  the 
  margins 
  of 
  the 
  

   valves. 
  Brachial 
  valve 
  much 
  the 
  more 
  convex, 
  its 
  umbo 
  pro- 
  

   jecting 
  conspicuously 
  beyond 
  that 
  of 
  the 
  pedicle-valve. 
  Cardinal 
  

   margin 
  straight 
  and 
  moderately 
  long 
  ; 
  no 
  evidence 
  of 
  cardinal 
  

   area 
  in 
  either 
  valve. 
  

  

  In 
  the 
  pedicle- 
  valve 
  the 
  delthyrium 
  is 
  broadly 
  triangular 
  and 
  

   is 
  usually 
  filled, 
  partially 
  or 
  wholly, 
  by 
  the 
  beak 
  of 
  the 
  opposite 
  

   valve. 
  On 
  the 
  interior 
  the 
  dental 
  lamellae 
  make 
  a 
  strong 
  

   spondylium 
  which 
  reaches 
  almost 
  to 
  the 
  bottom 
  of 
  the 
  valve, 
  

   being 
  supported 
  by 
  a 
  very 
  low 
  median 
  septum 
  extending 
  nearly 
  

   one 
  half 
  the 
  length 
  of 
  the 
  shell. 
  

  

  In 
  the 
  brachial 
  valve 
  there 
  are 
  two 
  vertical 
  crural 
  plates 
  not 
  

   connected 
  by 
  a 
  cardinal 
  process. 
  These 
  are 
  slightly 
  convex 
  on 
  

   their 
  inner 
  surfaces 
  and 
  at 
  their 
  point 
  of 
  greatest 
  convexity 
  they 
  

   unite 
  with 
  two 
  longitudinal 
  and 
  gradually 
  convergent 
  lamellae, 
  

   which 
  form 
  a 
  spondylium 
  narrower 
  than 
  that 
  of 
  the 
  opposite 
  

   valve 
  and 
  supported 
  by 
  a 
  very 
  low 
  median 
  septum 
  somewhat 
  

   longer 
  than 
  that 
  of 
  the 
  pedicle-valve. 
  

  

  Type, 
  Parastropliia 
  he?niplicata, 
  Hall 
  (sp.). 
  (Trenton 
  limestone.) 
  

  

  Distribution. 
  Lower 
  — 
  Upper 
  Silurian. 
  

  

  Anastrophia, 
  Hall. 
  1867. 
  

   (Plate 
  45, 
  figs. 
  28-35.) 
  

  

  Shells 
  with 
  strongly 
  reversed 
  convexity. 
  External 
  surface 
  

   covered 
  with 
  numerous 
  fine 
  and 
  sharp 
  dichotomizing 
  ribs, 
  extend- 
  

   ing 
  to 
  the 
  apices 
  of 
  beaks 
  and 
  frequently 
  crossed 
  by 
  delicate 
  con- 
  

   130 
  91 
  

  

  