﻿1024 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  and 
  in 
  front 
  of 
  them 
  a 
  larger 
  pair 
  whose 
  surface 
  is 
  radiately 
  

   striated, 
  the 
  entire 
  area 
  being 
  elongate-oval. 
  Vascular 
  impres- 
  

   sions 
  are 
  occasionally 
  retained 
  in 
  the 
  pedicle-valve. 
  

  

  Type, 
  Eatonia 
  medialis, 
  Yanuxem 
  (sp.). 
  

  

  Distribution. 
  Lower 
  Devonian. 
  

  

  Cyclorhina, 
  Hall. 
  1893. 
  

   (Plate 
  42, 
  figs. 
  27-31.) 
  

  

  Shells 
  of 
  comparatively 
  large 
  size 
  at 
  maturity, 
  subtriangular 
  in 
  

   outline 
  ; 
  biconvex, 
  the 
  convexity 
  of 
  the 
  brachial 
  valve 
  being 
  the 
  

   greater. 
  Fold 
  and 
  sinus 
  very 
  broad, 
  and 
  developed 
  in 
  the 
  usual 
  

   manner, 
  on 
  brachial 
  and 
  pedicle- 
  valves 
  respectively. 
  

  

  On 
  the 
  pedicle-valve 
  the 
  apex 
  is 
  obtuse, 
  not 
  elevated, 
  and 
  is 
  

   very 
  broadly 
  truncated 
  by 
  a 
  large 
  circular 
  foramen, 
  which, 
  even 
  

   in 
  the 
  earliest 
  growth-stages 
  observed, 
  is 
  enclosed 
  for 
  fully 
  five- 
  

   sixths 
  of 
  its 
  periphery 
  by 
  the 
  substance 
  of 
  the 
  valve. 
  The 
  del- 
  

   tidial 
  plates 
  are 
  incipient 
  at 
  maturity 
  and 
  scarcely 
  evident 
  in 
  

   young 
  shells; 
  the 
  delthyrial 
  margins 
  are 
  extremely 
  divergent. 
  

   The 
  cardinal 
  line 
  is 
  short 
  but 
  straight, 
  and 
  its 
  extremities 
  are 
  

   produced 
  on 
  each 
  side 
  to 
  form 
  a 
  short 
  alate 
  process 
  or 
  wing, 
  

   similar 
  to 
  those 
  in 
  the 
  genus 
  Ettmetria. 
  These 
  extensions 
  occur 
  

   on 
  both 
  valves, 
  and 
  are 
  very 
  apparent 
  in 
  the 
  younger 
  shells, 
  but 
  

   become 
  somewhat 
  obscured 
  with 
  the 
  increase 
  of 
  convexity 
  accom- 
  

   panying 
  maturity. 
  On 
  the 
  interior, 
  the 
  teeth 
  are 
  large 
  and 
  

   blunt, 
  and 
  attached 
  to 
  the 
  lateral 
  walls 
  of 
  the 
  shell, 
  though 
  they 
  

   also 
  rest 
  upon 
  the 
  thick 
  lamella} 
  similarly 
  attached 
  except 
  

   at 
  their 
  anterior 
  margins, 
  and 
  which 
  converge 
  downward 
  to 
  

   form 
  a 
  deep, 
  broad, 
  transversely 
  striated 
  pedicle-cavity. 
  The 
  

   thickened 
  lateral 
  margins 
  of 
  this 
  impression 
  are 
  continued 
  

   anteriorly 
  to 
  about 
  the 
  center 
  of 
  the 
  shell, 
  forming 
  an 
  elongate- 
  

   quadrate 
  diductor 
  scar 
  which 
  incloses 
  a 
  small 
  oval 
  adductor. 
  

  

  The 
  brachial 
  valve 
  has 
  a 
  convex 
  umbo, 
  showing 
  no 
  evidence 
  of 
  

   concavity 
  in 
  eariy 
  stages 
  of 
  growth. 
  Beneath 
  the 
  beak 
  is 
  a 
  very 
  

   fine, 
  vertical, 
  linear 
  cardinal 
  process 
  which 
  appears 
  to 
  be 
  contin- 
  

   uous 
  with 
  an 
  obscure 
  median 
  longitudinal 
  ridge, 
  traversing 
  about 
  

   one-half 
  the 
  length 
  of 
  the 
  valve. 
  Both 
  of 
  these 
  are 
  frequently 
  

   involved 
  in 
  the 
  shell-substance 
  and 
  evident 
  only 
  in 
  sections 
  of 
  the 
  

   shell. 
  The 
  hinge-plate 
  is 
  deeply 
  divided 
  medially, 
  each 
  lateral 
  

   portion 
  being 
  supported 
  by 
  a 
  deep 
  vertical 
  septum 
  resting 
  on 
  the 
  

   bottom 
  of 
  the 
  valve. 
  The 
  upper 
  surfaces 
  of 
  the 
  hinge 
  processes 
  

  

  82 
  

  

  