﻿Report 
  of 
  the 
  State 
  Geologist. 
  957 
  

  

  Cyrtina, 
  Davidson. 
  1858. 
  

   (Plate 
  29, 
  figs. 
  12-26.) 
  

  

  Shells 
  usually 
  of 
  small 
  size 
  and 
  semi-pyramidal 
  contour. 
  Pedi- 
  

   cle-valve 
  with 
  a 
  high, 
  vertical 
  or 
  arched 
  cardinal 
  area, 
  which 
  may 
  

   be 
  unsymmetrical 
  from 
  distortion 
  or 
  unequal 
  lateral 
  growth 
  ; 
  this 
  

   area 
  is 
  divided 
  medially 
  by 
  an 
  elongate-convex 
  deltidium, 
  which 
  

   may 
  be 
  perforated 
  at 
  any 
  point 
  below 
  the 
  apex, 
  by 
  a 
  circular, 
  

   direct 
  or 
  oblique 
  foramen, 
  or 
  be 
  without 
  any 
  evidence 
  of 
  such 
  

   foramen. 
  When 
  present 
  the 
  foramen 
  is 
  accompanied 
  by 
  a 
  sinus 
  

   on 
  the 
  deltidium, 
  extending 
  from 
  it 
  to 
  the 
  apex 
  of 
  the 
  valve 
  ; 
  

   even 
  when 
  this 
  foramen 
  has 
  been 
  closed 
  from 
  senile 
  deposition 
  of 
  

   testaceous 
  matter 
  this 
  foraminal 
  groove 
  may 
  remain. 
  

  

  The 
  exterior 
  surface 
  bears 
  a 
  median 
  sinus 
  and 
  more 
  or 
  less 
  dis- 
  

   tinct 
  lateral 
  plications. 
  On 
  the 
  interior 
  the 
  dental 
  lamellae 
  are 
  

   strongly 
  developed 
  and 
  converge 
  rapidly, 
  meeting 
  a 
  median 
  sep- 
  

   tum 
  from 
  the 
  bottom 
  of 
  the 
  valve. 
  The 
  union 
  consists 
  of 
  a 
  lat- 
  

   eral 
  junction 
  of 
  the 
  dental 
  lamellae 
  with 
  the 
  septum, 
  the 
  latter 
  

   continuing 
  for 
  a 
  short 
  distance 
  beyond 
  the 
  point 
  of 
  confluence 
  as 
  

   a 
  vertical 
  ridge, 
  always 
  apparent 
  in 
  the 
  bottom 
  of 
  the 
  spon- 
  

   dylium 
  thus 
  formed. 
  At 
  the 
  point 
  of 
  union 
  these 
  three 
  plates 
  

   constitute 
  a 
  tubular 
  chamber 
  which 
  has 
  no 
  external 
  opening 
  in 
  

   older 
  shells, 
  and 
  may 
  be 
  filled 
  by 
  organic 
  deposit. 
  The 
  dental 
  

   plates 
  are 
  shorter 
  than 
  the 
  septum, 
  the 
  latter, 
  at 
  its 
  base, 
  extend- 
  

   ing 
  beyond 
  the 
  center 
  of 
  the 
  valve, 
  its 
  anterior 
  margin 
  being 
  con- 
  

   cave 
  and 
  its 
  inner 
  extremity 
  acute 
  and 
  produced. 
  

  

  Brachial 
  valve 
  very 
  shallow, 
  with 
  narrow, 
  inconspicuous 
  car- 
  

   dinal 
  area. 
  Surface 
  plicated 
  as 
  in 
  the 
  opposite 
  valve. 
  Cardinal 
  

   process 
  consisting 
  of 
  a 
  double 
  apophysis 
  on 
  the 
  sides 
  of 
  which 
  

   are 
  strong, 
  divergent 
  crural 
  plates. 
  The 
  spiral 
  cones 
  are 
  elon- 
  

   gate-fusiform, 
  each 
  coil 
  attaining 
  its 
  greatest 
  diameter 
  just 
  below 
  

   the 
  center. 
  They 
  are 
  directed 
  obliquely 
  upward 
  and 
  backward 
  

   toward 
  the 
  middle 
  of 
  each 
  lateral 
  slope 
  of 
  the 
  pedicle-valve. 
  

   The 
  jugum 
  is 
  continuous, 
  its 
  branches 
  being 
  directed 
  upward 
  and 
  

   forward, 
  uniting 
  at 
  their 
  extremities. 
  The 
  muscular 
  impressions 
  

   comprise 
  two 
  oval 
  anterior, 
  and 
  fainter 
  posterior 
  scars. 
  The 
  sur- 
  

   face 
  ornamentation 
  consists 
  of 
  radial 
  plications 
  which 
  may 
  cover 
  

   both 
  fold 
  and 
  sinus 
  ; 
  in 
  rare 
  instances 
  the 
  lateral 
  plications 
  are 
  

   absent. 
  The 
  concentric 
  growth-lines 
  are 
  sometimes 
  fine 
  and 
  

  

  15 
  

  

  