﻿Report 
  of 
  the 
  State 
  Geologist. 
  1023 
  

  

  to 
  that 
  of 
  JEypothtkis 
  ; 
  the 
  median 
  septum 
  of 
  the 
  brachial 
  valve 
  

   is 
  extremely 
  faint 
  when 
  present, 
  but 
  is 
  usually 
  undeveloped. 
  

   Muscular 
  impressions 
  not 
  large 
  but 
  well 
  defined 
  and 
  clearly 
  

   subdivided. 
  Vascular 
  sinuses 
  sometimes 
  retained 
  on 
  the 
  pedicle- 
  

   valve, 
  always 
  obscure 
  on 
  the 
  brachial 
  valve. 
  

  

  (Type, 
  Pugnax 
  acuminatus, 
  Martin. 
  Devonian 
  — 
  Carboniferous.) 
  

  

  Eatonia, 
  Hall. 
  1857. 
  

   (Plate 
  44, 
  figs. 
  30-10.) 
  

  

  Concavo-convex 
  shells 
  with 
  median 
  fold 
  and 
  sinus, 
  and 
  plicated 
  

   or 
  radiate-lineate 
  exterior. 
  Anterior 
  margin 
  deeply 
  sinuate. 
  

   From 
  the 
  beak 
  of 
  the 
  pedicle-valve 
  diverge 
  two 
  lateral 
  cardinal 
  

   ridges 
  which 
  limit 
  a 
  more 
  or 
  less 
  distinct 
  false 
  area. 
  On 
  the 
  

   interior 
  the 
  teeth 
  are 
  adnascent 
  to 
  the 
  lateral 
  walls 
  of 
  the 
  valve, 
  

   all 
  traces 
  of 
  supporting 
  lamellae 
  being 
  absent. 
  Muscular 
  area 
  

   large, 
  flabellate 
  and 
  deeply 
  excavated 
  in 
  the 
  substance 
  of 
  the 
  

   shell. 
  Pedicle-impression 
  broad, 
  traversed 
  medially 
  by 
  a 
  longi- 
  

   tudinal 
  groove; 
  diductors 
  extending 
  for 
  about 
  one-half 
  the 
  length 
  

   of 
  the 
  shell, 
  their 
  outer 
  margins 
  being 
  elevated 
  ; 
  they 
  enclose 
  a 
  

   pair 
  of 
  small 
  central 
  adductor 
  scars 
  whose 
  posterior 
  margins 
  are 
  

   raised 
  into 
  prominent 
  myophores. 
  The 
  scars 
  are 
  divided 
  by 
  a 
  

   slight 
  median 
  septum 
  which 
  is 
  continued 
  posteriorly 
  ; 
  this 
  septum 
  

   being 
  often 
  rendered 
  very 
  conspicuous 
  by 
  the 
  growth 
  of 
  the 
  shell 
  

   about 
  the 
  apophyses 
  of 
  the 
  cardinal 
  process 
  of 
  the 
  opposite 
  valve, 
  

   and 
  in 
  the 
  extreme 
  cases 
  its 
  development 
  is 
  such 
  that 
  it 
  rises 
  

   above, 
  and 
  incloses 
  the 
  adductor 
  scars, 
  the 
  latter 
  being 
  excavated 
  

   in 
  its 
  substance. 
  

  

  In 
  the 
  brachial 
  valve 
  the 
  dental 
  sockets 
  are 
  long 
  and 
  narrow, 
  the 
  

   cardinal 
  process 
  very 
  large 
  and 
  composed 
  of 
  a 
  stout, 
  erect 
  stem 
  

   resting 
  upon 
  a 
  rather 
  short 
  median 
  septum, 
  and 
  divided 
  at 
  its 
  

   summit 
  into 
  two 
  long, 
  divergent, 
  tooth-like 
  branches, 
  whose 
  

   upper 
  faces 
  extend 
  to 
  the 
  interior 
  surface 
  of 
  the 
  opposite 
  valve; 
  

   hence 
  their 
  greatest 
  elevation 
  is 
  at 
  their 
  anterior 
  extremities, 
  

   whence 
  they 
  slope 
  toward 
  the 
  beak 
  of 
  the 
  valve, 
  usually 
  uniting 
  

   before 
  that 
  point 
  is 
  reached. 
  The 
  surface 
  of 
  attachment 
  of 
  each 
  

   of 
  these 
  apophyses 
  is 
  medially 
  grooved. 
  Below 
  them, 
  and 
  at 
  the 
  

   base 
  of 
  the 
  central 
  stem, 
  arise 
  the 
  crura, 
  which 
  are 
  long, 
  straight 
  

   and 
  slender, 
  with 
  expanded 
  extremities. 
  The 
  muscular 
  scars 
  are 
  

   clearly 
  defined 
  and 
  consist 
  of 
  a 
  pair 
  of 
  small 
  posterior 
  adductors, 
  

  

  81 
  

  

  