﻿Report 
  of 
  the 
  State 
  Geologist. 
  1009 
  

  

  and 
  slender, 
  and 
  constitute 
  the 
  crural 
  bases. 
  Spirals 
  are 
  present, 
  

   but 
  their 
  direction 
  and 
  the 
  nature 
  of 
  the 
  loop 
  are 
  undetermined. 
  

   A 
  stout 
  median 
  ridge 
  supports 
  the 
  hinge-plate 
  and 
  divides 
  the 
  

   scars 
  of 
  the 
  adductor 
  muscles. 
  In 
  both 
  valves 
  the 
  lateral 
  por- 
  

   tion 
  of 
  the 
  umbonal 
  region 
  is 
  pitted. 
  The 
  plications 
  of 
  the 
  sur- 
  

   face 
  are 
  covered 
  by 
  fine, 
  sharp 
  and 
  elevated 
  concentric 
  striae. 
  

   Shell 
  substance, 
  fibrous, 
  impunctate. 
  

  

  Type, 
  Glintonella 
  vagabunda, 
  Hall. 
  

  

  Distribution. 
  Upper 
  Silurian 
  (Clinton 
  group). 
  

  

  Atrypina, 
  Hall. 
  1893. 
  

   (Plate 
  41, 
  figs. 
  1-6.) 
  

  

  Shell 
  small, 
  subovate 
  or 
  subcircular 
  in 
  marginal 
  outline, 
  piano-, 
  

   or 
  subconcavo-convex 
  in 
  contour; 
  surface 
  coarsely 
  and 
  sparsely 
  

   plicated 
  . 
  

  

  Pedicle-valve 
  with 
  the 
  umbo 
  prominent, 
  the 
  beak 
  abruptly 
  

   acute 
  and 
  more 
  or 
  less 
  incurved. 
  Foramen 
  apical, 
  and 
  deltidial 
  

   plates 
  normally 
  developed. 
  The 
  cardinal 
  margins 
  of 
  the 
  valve 
  

   are 
  somewhat 
  extended 
  in 
  the 
  typical 
  species, 
  though 
  the 
  hinge 
  

   itself 
  is 
  quite 
  short. 
  Teeth 
  divergent 
  and 
  unsupported, 
  taking 
  

   their 
  origin 
  on 
  the 
  lateral 
  cardinal 
  slopes, 
  and 
  very 
  slightly 
  

   recurved. 
  Muscular 
  scars 
  exceedingly 
  faint 
  ; 
  no 
  internal 
  septa 
  

   observable. 
  

  

  Brachial 
  valve 
  with 
  the 
  cardinal 
  process 
  small, 
  consisting 
  of 
  

   two 
  short 
  lobes, 
  which 
  meet 
  at 
  their 
  apices, 
  not 
  extending 
  back 
  

   of 
  the 
  hinge-line, 
  and 
  diverging 
  anteriorly. 
  The 
  surface 
  of 
  each 
  

   lobe 
  may 
  be 
  longitudinally 
  grooved, 
  but 
  the 
  inner 
  and 
  outer 
  

   divisions 
  thus 
  formed 
  are 
  confluent 
  at 
  

   their 
  outer 
  extremities. 
  The 
  anterior 
  face 
  

   of 
  the 
  process 
  is 
  abrupt 
  and 
  vertical, 
  its 
  

   lower 
  portion 
  being 
  continuous 
  with 
  the 
  

   socket 
  walls. 
  In 
  front 
  of 
  the 
  cardinal 
  pro- 
  

   cess, 
  but 
  not 
  supporting 
  it, 
  is 
  a 
  low 
  median 
  

   ridge, 
  on 
  either 
  side 
  of 
  which 
  are 
  obscure 
  

   muscular 
  imprints. 
  The 
  brachial 
  appar- 
  

   atus 
  consists 
  of 
  introverted 
  spirals 
  whose 
  . 
  

  

  I 
  Fig. 
  398.— 
  The 
  brachidium 
  of 
  

  

  bases 
  lie 
  against 
  the 
  lateral 
  slopes 
  of 
  the 
  Atrypina 
  disparate, 
  Hail, 
  

   pedicle- 
  valve 
  and 
  whose 
  apices 
  are 
  directed 
  toward 
  the 
  center 
  

   of 
  the 
  brachial 
  valve. 
  The 
  ribbon 
  is 
  loosely 
  coiled 
  and 
  makes 
  

   127 
  67 
  

  

  