﻿Report 
  of 
  the 
  State 
  Geologist. 
  1011 
  

  

  grooved, 
  forming 
  an 
  inner 
  and 
  outer 
  lobe. 
  The 
  latter 
  forms 
  the 
  

   upper 
  portion 
  of 
  the 
  socket- 
  wall 
  which 
  is 
  curved 
  downward 
  and 
  

   unites 
  with 
  the 
  lateral 
  surface 
  of 
  the 
  valve, 
  forming 
  a 
  broad 
  

   dental 
  socket 
  which 
  is 
  traversed 
  by 
  an 
  oblique 
  crenulated 
  ridge. 
  

   The 
  inner 
  lobes 
  of 
  the 
  hinge- 
  plate 
  are 
  short, 
  their 
  extremities 
  

   free, 
  bearing 
  the 
  crura. 
  

  

  These 
  crura 
  are 
  long 
  and 
  narrow, 
  diverge 
  laterally 
  and 
  are 
  

   attached 
  to 
  the 
  primary 
  lamellae 
  near 
  their 
  ante-lateral 
  curvature. 
  

   The 
  mode 
  of 
  attachment 
  

   is 
  peculiar, 
  the 
  crural 
  lam- 
  

   ellae 
  bending 
  upward 
  and 
  

   then 
  abruptly 
  downward, 
  

   greatly 
  widening 
  at 
  the 
  

   line 
  of 
  contact 
  and 
  touch- 
  

   ing 
  the 
  spiral 
  ribbon 
  only 
  

  

  at 
  its 
  OUter 
  margin. 
  The 
  FlG> 
  399— 
  Diagram 
  of 
  Atrypa 
  reticularis; 
  showing 
  

  

  6 
  the 
  form 
  and 
  structure 
  of 
  the 
  jugum 
  and 
  the 
  mode 
  of 
  

  

  demarkatlOn 
  between 
  the 
  attachment 
  of 
  the 
  crura 
  to 
  the 
  hinge-plate 
  and 
  the 
  

  

  crura 
  and 
  the 
  ribbon 
  of 
  P 
  rimar 
  y 
  lameii®. 
  

  

  the 
  coils 
  is, 
  therefore, 
  very 
  distinct. 
  The 
  spirals 
  have, 
  in 
  a 
  gen- 
  

   eral 
  sense, 
  their 
  bases 
  parallel 
  to 
  the 
  inner 
  surface 
  of 
  the 
  pedicle- 
  

   valve 
  and 
  the 
  apices 
  directed 
  toward 
  the 
  deepest 
  point 
  of 
  the 
  

   opposite 
  valve. 
  Their 
  axes 
  are 
  more 
  or 
  less 
  convergent, 
  so 
  that 
  

   the 
  approximate 
  surfaces 
  of 
  the 
  cones 
  are 
  flattened. 
  The 
  basal 
  

   section 
  of 
  these 
  cones 
  is 
  hemicordate, 
  the 
  anterior 
  extremity 
  

   being 
  much 
  the 
  narrower, 
  but 
  the 
  upper 
  volutions 
  are 
  more 
  

   nearly 
  elliptical. 
  The 
  ribbon 
  is 
  broad, 
  being 
  conspicuously 
  so- 
  

   on 
  the 
  anterior 
  curves 
  of 
  the 
  first 
  few 
  volutions, 
  each 
  one 
  

   extending 
  considerably 
  beyond 
  the 
  next 
  following. 
  These 
  

   anterior 
  curves 
  may 
  be 
  more 
  or 
  less 
  distinctly 
  fimbriated. 
  The 
  

   jugum 
  is 
  composed 
  of 
  two 
  processes 
  which 
  are 
  continuations 
  of 
  

   the 
  primary 
  lamellae 
  without 
  angulation. 
  These 
  processes 
  are 
  

   situated 
  posteriorly, 
  directed 
  toward 
  the 
  center 
  of 
  the 
  shell, 
  and 
  

   are, 
  in 
  effect, 
  the 
  starting 
  points 
  of 
  the 
  spirals. 
  They 
  have 
  the 
  

   following 
  structure 
  : 
  The 
  ribbon 
  maintains 
  its 
  usual 
  width 
  for 
  a 
  

   considerable 
  distance 
  within 
  the 
  point 
  of 
  attachment 
  to 
  the 
  

   crura, 
  then 
  narrows 
  rather 
  abruptly, 
  the 
  processes 
  ascending 
  as 
  

   they 
  approach 
  each 
  other. 
  Their 
  terminations 
  in 
  mature 
  shells 
  

   are 
  broadened, 
  thickened, 
  erect 
  and 
  recurved 
  at 
  the 
  tips, 
  having 
  

   a 
  clavate 
  appearance. 
  In 
  immature 
  growth-stages 
  or 
  undevel- 
  

  

  69 
  

  

  