﻿Report 
  of 
  tee 
  State 
  Geologist. 
  973 
  

  

  inclined 
  downward 
  or 
  toward 
  the 
  beak 
  of 
  the 
  brachial 
  valve 
  for 
  

   a 
  short 
  distance, 
  thence 
  it 
  rises 
  abruptly 
  toward 
  the 
  umbo 
  of 
  the 
  

   pedicle-valve, 
  and 
  bifurcates 
  near 
  the 
  extremities 
  of 
  the 
  crura, 
  

   each 
  branch 
  following 
  the 
  curvature 
  of 
  the 
  primary 
  lamellae 
  and 
  

   continuing 
  for 
  only 
  a 
  part 
  of 
  the 
  distance 
  between 
  the 
  ends 
  of 
  

   the 
  crura 
  and 
  the 
  origin 
  of 
  the 
  jugum. 
  These 
  accessory 
  lamellae 
  

   vary 
  somewhat 
  in 
  form, 
  are 
  narrower 
  than 
  the 
  ribbon 
  of 
  the 
  coil, 
  

   and 
  lie 
  between 
  the 
  primary, 
  and 
  the 
  first 
  band 
  of 
  the 
  secondary 
  

   lamellae. 
  

  

  The 
  muscular 
  area 
  consists 
  of 
  a 
  long, 
  ovate 
  scar, 
  which 
  is 
  

   divided 
  into 
  a 
  subquadrate 
  posterior 
  pair, 
  and 
  a 
  subcordate 
  ante- 
  

   terior 
  pair 
  of 
  adductor 
  impressions. 
  These 
  are 
  separated 
  longi- 
  

   tudinally 
  by 
  a 
  very 
  faint 
  median 
  ridge. 
  On 
  casts 
  of 
  the 
  interior 
  

   the 
  filling 
  of 
  the 
  visceral 
  foramen 
  in 
  the 
  hinge-plate 
  frequently 
  

   shows 
  a 
  cross-striation 
  like 
  that 
  of 
  the 
  pedicle-cavity 
  of 
  the 
  oppo- 
  

   site 
  valve, 
  and 
  also 
  indicates 
  that 
  the 
  median 
  ridge 
  is 
  continued 
  

   throughout 
  the 
  extent 
  of 
  this 
  passage. 
  

  

  The 
  surface 
  of 
  the 
  valves 
  is 
  variously 
  ornamented 
  ; 
  in 
  the 
  typi- 
  

   cal 
  group, 
  at 
  each 
  concentric 
  growth-line, 
  there 
  is 
  a 
  broad 
  lamellar 
  

   expansion 
  ; 
  in 
  some 
  cases 
  this 
  expansion 
  is 
  striated 
  longitudinally, 
  

   or 
  it 
  may 
  be 
  divided 
  into 
  flat 
  spines, 
  which 
  merge 
  into 
  the 
  

   lamella 
  at 
  their 
  bases 
  ; 
  again 
  the 
  spines 
  may 
  be 
  long 
  and 
  tubular, 
  

   but 
  connected 
  by 
  the 
  laminar 
  expansions. 
  The 
  surface 
  frequently 
  

   appears 
  to 
  be 
  smooth 
  or 
  covered 
  only 
  with 
  concentric 
  striae, 
  and 
  

   in 
  one 
  of 
  the 
  largest 
  subdivisions 
  of 
  the 
  genus 
  (Seminula) 
  it 
  is 
  a 
  

   normal 
  condition, 
  while 
  in 
  other 
  divisions 
  it 
  is 
  often 
  wholly 
  

   casual. 
  

  

  Shell-substance 
  fibrous, 
  impunctate. 
  

  

  Type, 
  Athyris 
  concentrica, 
  von 
  Buch 
  (sp.). 
  Middle 
  Devonian. 
  

  

  Distribution. 
  Upper 
  Silurian 
  (?) 
  — 
  Carboniferous. 
  

  

  Subgenus 
  Cliothyris, 
  King. 
  1850. 
  

  

  (Plate 
  35,. 
  figs. 
  9, 
  10.) 
  

  

  Shells 
  with 
  surface 
  ornamentation 
  consisting 
  of 
  broad, 
  thin, 
  

   lamellar 
  expansions, 
  which 
  are 
  divided 
  almost, 
  sometimes 
  quite 
  

   to 
  their 
  bases 
  into 
  long, 
  flat 
  spinules 
  ; 
  the 
  hinge-plate 
  is 
  narrow 
  

   and 
  rather 
  acutely 
  triangular 
  ; 
  the 
  primary 
  lamellae 
  are 
  attached 
  

   to 
  the 
  crura 
  not 
  only 
  at 
  their 
  apices 
  but 
  for 
  a 
  short 
  distance 
  along 
  

  

  31 
  

  

  