﻿962 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum, 
  

  

  supporting 
  short, 
  straight 
  crura. 
  The 
  spiral 
  ribbon 
  makes 
  not 
  

   more 
  than 
  six 
  volutions, 
  forming 
  very 
  loose 
  coils. 
  There 
  is 
  no 
  

   median 
  septum. 
  

  

  Type, 
  Hyattella 
  congesta, 
  Conrad 
  (sp.). 
  

  

  Distribution. 
  Middle 
  Silurian. 
  

  

  Dayia, 
  Davidson. 
  1881. 
  

   (Plate 
  32, 
  figs. 
  21-25.) 
  

  

  Shells 
  small, 
  subtrihedral 
  in 
  contour, 
  with 
  a 
  very 
  convex 
  

   pedicle-valve 
  which 
  may 
  be 
  obscurely 
  keeled 
  along 
  the 
  middle 
  

   and 
  depressed 
  laterally. 
  Brachial 
  valve 
  convex 
  posteriorly, 
  but 
  

   becoming 
  concave 
  over 
  the 
  anterior 
  region, 
  and 
  bearing 
  a 
  well- 
  

   developed 
  median 
  sinus. 
  The 
  hinge-line 
  is 
  short 
  ; 
  the 
  cardinal 
  

   area 
  absent. 
  The 
  umbo 
  of 
  the 
  pedicle-valve 
  is 
  gibbous 
  and 
  its 
  

   apex 
  closely 
  incurved, 
  concealing 
  the 
  foramen. 
  Deltidial 
  plates 
  

   were 
  probably 
  developed, 
  but 
  they 
  appear 
  to 
  be 
  invariably 
  lost 
  in 
  

   separated 
  valves 
  The 
  delthyrium 
  is 
  wide, 
  the 
  teeth 
  divergent, 
  

   moderately 
  conspicuous 
  and 
  unsupported 
  by 
  lamellae. 
  In 
  the 
  

   bottom 
  of 
  the 
  valve 
  are 
  two 
  narrow, 
  divergent 
  muscular 
  grooves, 
  

   bordered 
  on 
  their 
  anterior 
  edges 
  by 
  thickened 
  ridges, 
  both 
  hav- 
  

   ing 
  the 
  shape 
  of 
  a 
  broad 
  inverted 
  V. 
  

  

  In 
  the 
  brachial 
  valve 
  the 
  character 
  of 
  the 
  hinge-plate 
  was 
  

   probably 
  simple, 
  and 
  supported 
  by 
  a 
  median 
  septum 
  traversing 
  

   about 
  one 
  half 
  the 
  length 
  of 
  the 
  valve; 
  on 
  either 
  side 
  of 
  this 
  

   septum 
  are 
  the 
  lateral 
  members 
  of 
  the 
  adductor 
  impression. 
  The 
  

   crura 
  are 
  short 
  and 
  straight 
  ; 
  the 
  primary 
  lamellae 
  are 
  attached 
  

   to 
  them 
  by 
  a 
  subrectangular 
  curve 
  and 
  pass 
  outward 
  just 
  within 
  

   the 
  margin 
  of 
  the 
  valve. 
  The 
  spirals 
  are 
  but 
  slightly 
  elevated 
  

   and 
  have 
  their 
  apices 
  directed 
  outward 
  toward 
  the 
  lateral 
  slopes 
  

   of 
  the 
  opposite 
  valve. 
  The 
  ribbon 
  makes 
  but 
  three 
  or 
  four 
  turns, 
  

   and 
  its 
  outer 
  anterior 
  edges 
  are 
  quite 
  coarsely 
  fimbriated. 
  The 
  

   jugum 
  is 
  situated 
  anteriorly, 
  taking 
  its 
  origin 
  near 
  the 
  upward 
  

   turn 
  of 
  the 
  primary 
  lamellae 
  ; 
  it 
  is 
  directed 
  upward 
  and 
  backward, 
  

   the 
  lateral 
  processes 
  meeting 
  at 
  or 
  just 
  behind 
  the 
  center 
  of 
  the 
  

   interior 
  cavity. 
  From 
  the 
  point 
  of 
  union 
  proceeds 
  a 
  short, 
  simple 
  

   process, 
  which 
  does 
  not 
  make 
  an 
  angle 
  with 
  the 
  rest 
  of 
  the 
  

   jugum. 
  

  

  Type, 
  Dayia 
  navicula, 
  Sowerby 
  (sp.). 
  Wenlock 
  limestone. 
  

  

  But 
  one 
  species 
  has 
  been 
  recognized. 
  

  

  