﻿1010 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  but 
  three 
  or 
  four 
  volutions. 
  The 
  jugum 
  is 
  situated 
  posteriorly 
  

   and 
  constructed 
  as 
  in 
  Atrypa, 
  except 
  that 
  its 
  lateral 
  lamellae 
  

   appear 
  to 
  be 
  always 
  united 
  in 
  an 
  acute 
  angle, 
  which 
  is 
  directed 
  

   inward. 
  

  

  Muscular 
  impression 
  composed 
  of 
  large 
  flabellate 
  diductors, 
  

   enclosing 
  distinct 
  adductor 
  scars. 
  

  

  Type, 
  Airypina 
  imbricata, 
  Hall. 
  

  

  Distribution. 
  Upper 
  Silurian 
  — 
  Devonian. 
  

  

  Atrypa, 
  Dalman 
  1828. 
  

   (Plate 
  41, 
  figs. 
  7-17.) 
  

  

  Shells 
  subcircular 
  or 
  longitudinally 
  subo 
  val 
  in 
  outline. 
  Gibbous, 
  

   strongly 
  inequivalve. 
  Hinge-line 
  short, 
  straight; 
  cardinal 
  

   extremities 
  rounded. 
  Beaks 
  not 
  prominent. 
  

  

  Pedicle- 
  valve 
  the 
  smaller; 
  convex 
  in 
  the 
  umbonal 
  region, 
  but 
  

   depressed 
  and 
  often 
  deeply 
  sinuate 
  anteriorly. 
  Beak 
  small, 
  

   usually 
  incurved 
  in 
  advanced 
  growth- 
  stages, 
  concealing 
  the 
  

   foramen 
  and 
  deltidium. 
  The 
  foramen 
  is 
  triangular 
  in 
  young 
  

   shells, 
  extending 
  to 
  the 
  hinge-line, 
  but 
  becoming 
  gradually 
  closed 
  

   by 
  the 
  growth 
  of 
  deltidial 
  plates, 
  and 
  at 
  maturity 
  is 
  circular 
  and 
  

   apical, 
  encroaching 
  slightly 
  on 
  the 
  substance 
  of 
  the 
  valve. 
  The 
  

   plates 
  of 
  the 
  deltidium 
  are 
  not 
  coalesced 
  along 
  the 
  median 
  suture. 
  

   On 
  the 
  interior 
  the 
  umbonal 
  cavity 
  is 
  short 
  but 
  very 
  broad. 
  The 
  

   teeth 
  are 
  large, 
  widely 
  separated 
  and 
  doubly 
  grooved, 
  first 
  by 
  an 
  

   oblique 
  furrow 
  at 
  the 
  base, 
  into 
  which 
  is 
  fitted 
  a 
  crenulated 
  ridge 
  

   of 
  the 
  other 
  valve, 
  then 
  by 
  a 
  short 
  longitudinal 
  depression 
  on 
  the 
  

   summit; 
  the 
  tooth 
  is 
  doubly 
  curved 
  and 
  reflected, 
  making 
  the 
  

   articulation 
  of 
  the 
  valves 
  very 
  firm. 
  These 
  teeth 
  arise 
  from 
  the 
  

   inner 
  surface 
  of 
  the 
  lateral 
  slopes 
  of 
  the 
  valve, 
  and 
  are 
  hence 
  

   unsupported 
  by 
  lamellae. 
  The 
  muscular 
  impressions 
  are 
  sharply 
  

   defined; 
  the 
  triangular 
  pedicle-scar 
  is 
  followed 
  in 
  front, 
  by 
  a 
  

   median 
  elongate 
  double 
  scar 
  of 
  the 
  adductors, 
  outside 
  of 
  which 
  

   are 
  strong, 
  radiately 
  striate, 
  flabellate 
  diductors, 
  which 
  frequently 
  

   extend 
  beyond 
  the 
  middle 
  of 
  the 
  valve. 
  

  

  Brachial 
  valve 
  convex 
  or 
  rotund 
  in 
  the 
  middle, 
  with 
  a 
  median 
  

   fold 
  which 
  is 
  rarely 
  developed 
  except 
  toward 
  the 
  anterior 
  margin. 
  

   Beak 
  incurved 
  and 
  concealed. 
  ~No 
  cardinal 
  area. 
  The 
  hinge- 
  

   plate 
  is 
  composed 
  of 
  two 
  diverging 
  processes 
  which 
  may 
  or 
  may 
  

   not 
  meet 
  at 
  the 
  apex. 
  Each 
  of 
  these 
  processes 
  is 
  obliquely 
  

  

  68 
  

  

  