﻿Report 
  of 
  the 
  State 
  Geologist. 
  971 
  

  

  In 
  the 
  brachial 
  valve 
  the 
  hinge-plate 
  is 
  supported 
  by 
  a 
  median 
  

   septum 
  slightly 
  longer 
  than 
  that 
  of 
  the 
  opposite 
  valve, 
  and 
  the 
  

   narrow, 
  cordate 
  muscular 
  impression, 
  which 
  it 
  divides 
  medially, 
  

   is 
  considerably 
  thickened. 
  The 
  valve 
  bears 
  everted 
  spirals 
  

   similar 
  to 
  those 
  of 
  other 
  members 
  of 
  this 
  group, 
  but 
  the 
  speci- 
  

   mens 
  studied 
  were 
  not 
  in 
  a 
  condition 
  of 
  preservation 
  adapted 
  to 
  

   the 
  determination 
  of 
  th*- 
  structure 
  of 
  the 
  jugum. 
  

  

  Type, 
  Camarospira 
  JZucharix, 
  Rail. 
  Corniferous 
  limestone. 
  

  

  Athyris, 
  McCoy. 
  184 
  Jr. 
  

   (Plate 
  35, 
  figs. 
  1-8.) 
  

  

  Synonyms; 
  Spirigera, 
  D'Orbigny, 
  1847; 
  Euihyris, 
  Quenstedt, 
  

   1871. 
  

  

  Shells 
  subequally 
  biconvex; 
  outline 
  transversely 
  elliptical, 
  

   subcircular 
  or 
  elongate-subovate 
  ; 
  surface 
  medially 
  sinuate. 
  

  

  In 
  the 
  pedicle-valve 
  the 
  umbo 
  is 
  inconspicuous 
  and 
  incurved, 
  

   usual 
  Ly 
  concealing 
  the 
  foramen 
  and 
  deltidial 
  plates 
  ; 
  frequently, 
  

   the 
  former 
  is 
  exposed. 
  Cardinal 
  slopes 
  not 
  well 
  defined 
  in 
  the 
  

   typical 
  group. 
  The 
  convexity 
  of 
  the 
  valve 
  is 
  greatest 
  in 
  the 
  

   umbonal 
  region, 
  the 
  surface 
  sloping 
  evenly 
  to 
  the 
  sides, 
  and 
  

   becoming 
  depressed 
  on 
  the 
  median 
  line 
  into 
  a 
  sinus, 
  which 
  is 
  

   most 
  prominent 
  on 
  the 
  anterior 
  margin. 
  Beak 
  of 
  the 
  brachial 
  

   valve 
  not 
  prominent 
  ; 
  a 
  median 
  fold 
  corresponds 
  in 
  strength 
  to 
  

   the 
  sinus 
  of 
  the 
  opposite 
  valve. 
  

  

  In 
  the 
  interior 
  of 
  the 
  pedicle 
  valve 
  the 
  deltidial 
  plates 
  are 
  

   usually 
  absent; 
  the 
  teeth 
  are 
  prominent, 
  recurved 
  at 
  the 
  tips, 
  

   and 
  supported 
  by 
  stout 
  dental 
  lamellae, 
  which 
  are 
  not 
  produced 
  

   anteriorly 
  about 
  the 
  muscular 
  area. 
  Between 
  them 
  lies 
  a 
  deep, 
  

   transversely 
  striated 
  pedicle-cavity, 
  and 
  in 
  front 
  of 
  this 
  an 
  ovate 
  

   muscular 
  scar 
  extending 
  about 
  one-half 
  the 
  length 
  of 
  the 
  valve 
  

   and 
  divided 
  into 
  nabellate 
  diductors 
  (which 
  are 
  frequently 
  very 
  

   indistinct) 
  and 
  narrow, 
  cordate 
  adductors. 
  The 
  pallial 
  region 
  

   is 
  covered 
  with 
  ovarian 
  pittings 
  and 
  branching 
  sinuses. 
  

  

  In 
  the 
  brachial 
  valve 
  the 
  dental 
  sockets 
  are 
  broad 
  and 
  deep. 
  

   The 
  hinge 
  -plate 
  varies 
  considerably 
  in 
  form 
  ; 
  in 
  the 
  typical 
  divi- 
  

   sion 
  of 
  the 
  genus 
  it 
  is 
  sub 
  triangular 
  in 
  outline, 
  and 
  supported 
  by 
  

   stout 
  crural 
  plates. 
  The 
  median 
  portion 
  is 
  flat 
  or 
  concave, 
  the 
  

   lateral 
  margins 
  thickened 
  and 
  elevated. 
  At 
  the 
  apex 
  of 
  the 
  plate 
  

  

  29 
  

  

  