﻿964 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum, 
  

  

  Meristina, 
  Hall. 
  1867. 
  

  

  (Plate 
  33, 
  figs. 
  1-7.) 
  

  

  Synonym 
  ; 
  WMtfieldia, 
  Davidson, 
  1882. 
  

  

  Shells 
  biconvex, 
  the 
  greatest 
  depths 
  of 
  the 
  valves 
  being 
  sub- 
  

   equal. 
  General 
  expression 
  meristoid. 
  The 
  beak 
  of 
  the 
  pedicle- 
  

   valve 
  is 
  erect 
  in 
  youth, 
  but 
  so 
  greatly 
  incurved 
  at 
  maturity 
  as 
  to 
  

   totally 
  conceal 
  the 
  foramen 
  and 
  deltidium. 
  Cardinal 
  slopes 
  

   narrow 
  but 
  distinct, 
  forming 
  prominent 
  shoulders 
  which 
  may 
  be 
  

   traced 
  nearly 
  to 
  the 
  middle 
  of 
  the 
  lateral 
  margins. 
  A 
  low, 
  often 
  

   indistinct 
  median 
  ridge 
  extends 
  from 
  the 
  apex 
  forward 
  ; 
  at 
  about 
  

   the 
  middle 
  of 
  the 
  shell 
  it 
  is 
  divided 
  by 
  a 
  faint 
  groove, 
  becoming 
  

   broader 
  toward 
  the 
  margin 
  and 
  continued 
  into 
  a 
  subnasute 
  

   extension. 
  Lateral 
  slopes 
  scarcely 
  depressed. 
  

  

  The 
  brachial 
  valve 
  also 
  bears 
  a 
  low 
  median 
  ridge, 
  which 
  mani- 
  

   fests 
  itself 
  most 
  conspicuously 
  over 
  the 
  anterior 
  portion 
  of 
  the 
  

   shell. 
  

  

  On 
  the 
  interior 
  of 
  the 
  pedicle-valve 
  the 
  teeth 
  are 
  conspicuous 
  

   and 
  are 
  supported 
  by 
  thin 
  plates, 
  which 
  extend 
  to 
  the 
  bottom 
  of 
  

   the 
  valve 
  and 
  are 
  produced 
  forward 
  to 
  form 
  the 
  lateral 
  boundar- 
  

   ies 
  of 
  the 
  muscular 
  area. 
  Between 
  the 
  posterior 
  portion 
  of 
  these 
  

   plates 
  lies 
  the 
  deep 
  scar 
  of 
  the 
  pedicle-muscle, 
  which 
  is 
  separated 
  

   from 
  the 
  elongate 
  and 
  radially 
  striate 
  diductor 
  impression 
  by 
  a 
  

   prominent 
  callosity. 
  

  

  In 
  the 
  brachial 
  valve 
  the 
  hinge-plate 
  is 
  deeply 
  divided 
  in 
  the 
  

   middle 
  by 
  a 
  narrow 
  sulcus, 
  the 
  two 
  lateral 
  lobes 
  being 
  elevated, 
  

   and 
  supporting 
  the 
  crural 
  bases. 
  The 
  plate 
  is 
  thickened 
  on 
  the 
  

   under 
  side 
  and 
  supported 
  by 
  a 
  median 
  septum, 
  which 
  extends 
  for 
  

   one-half 
  the 
  length 
  of 
  the 
  valve. 
  The 
  crura 
  are 
  short 
  and 
  

   straight, 
  and 
  the 
  primary 
  lamella 
  of 
  the 
  spiral 
  ribbon 
  originate 
  

   from 
  them 
  at 
  a 
  sharp 
  angle, 
  diverge 
  laterally 
  as 
  they 
  turn 
  down- 
  

   ward, 
  passing 
  over 
  a 
  portion 
  of 
  the 
  secondary 
  volutions, 
  approach 
  

   each 
  other 
  toward 
  the 
  middle 
  of 
  their 
  length, 
  nearly 
  meeting 
  at 
  

   the 
  anterior 
  edge 
  of 
  the 
  median 
  septum, 
  thence 
  again 
  diverging 
  

   to 
  their 
  anterior 
  recurvature. 
  The 
  secondary 
  volutions 
  do 
  not 
  

   follow 
  precisely 
  the 
  curvature 
  of 
  the 
  primary 
  lamellse 
  and 
  the 
  

   resultant 
  cones 
  at 
  maturity 
  have 
  a 
  gracefully 
  undulated 
  surface. 
  

   The 
  jugum 
  consists 
  of 
  two 
  lateral 
  branches, 
  broad 
  at 
  their 
  origin, 
  

  

  22 
  

  

  