﻿Report 
  of 
  the 
  State 
  Geologist. 
  967 
  

  

  Meristella, 
  Hall. 
  1859. 
  

   (Plate 
  34, 
  figs. 
  1-11.) 
  

  

  Shells 
  having 
  the 
  same 
  general 
  external 
  characters 
  as 
  Merista. 
  

   Valves 
  convex, 
  often 
  inflated, 
  cardinal 
  areas 
  obscure. 
  The 
  

   umbo 
  of 
  the 
  pedicle-valve 
  is 
  incurved 
  at 
  maturity, 
  concealing 
  

   most, 
  if 
  not 
  all, 
  of 
  the 
  foramen 
  ; 
  in 
  early 
  stages 
  of 
  growth, 
  how- 
  

   ever, 
  the 
  beak 
  is 
  more 
  erect 
  and 
  exposes 
  the 
  deltidial 
  plates 
  in 
  

   an 
  elementary 
  condition 
  of 
  development. 
  The 
  anterior 
  margin 
  

   of 
  the 
  shell 
  is 
  sinuate, 
  and 
  usually 
  there 
  is 
  a 
  sinus 
  on 
  the 
  pedicle- 
  

   valve, 
  with 
  a 
  less 
  conspicuous 
  fold 
  on 
  the 
  brachial 
  valve 
  ; 
  some- 
  

   times 
  both 
  valves 
  bear 
  a 
  low 
  sinus, 
  or 
  the 
  sinus 
  on 
  the 
  pedicle- 
  

   valve 
  may 
  be 
  absent, 
  while 
  the 
  fold 
  on 
  the 
  brachial 
  valve 
  is 
  

   present, 
  thus 
  giving 
  the 
  shell 
  a 
  nasute 
  anterior 
  extension 
  ; 
  again, 
  

   fold 
  and 
  sinus 
  may 
  be 
  absent 
  on 
  both 
  valves. 
  

  

  In 
  the 
  interior 
  of 
  the 
  pedicle- 
  valve 
  the 
  delthyrium 
  is 
  wide, 
  its 
  

   margins 
  being 
  thickened 
  into 
  denial 
  ridges. 
  The 
  teeth 
  are 
  con- 
  

   spicuous, 
  often 
  much 
  thickened 
  and 
  curved 
  backward 
  at 
  their 
  

   tips, 
  interlocking 
  with 
  the 
  opposite 
  valve 
  in 
  such 
  a 
  manner 
  as 
  to 
  

   make 
  a 
  very 
  firm 
  articulation. 
  The 
  teeth 
  are 
  supported 
  by 
  

   lamellae 
  which 
  rest 
  upon 
  the 
  bottom 
  of 
  the 
  valve 
  and 
  are 
  con- 
  

   tinued 
  for 
  a 
  short 
  distance 
  about 
  the 
  posterior 
  margin 
  of 
  the 
  

   muscular 
  impression. 
  In 
  old 
  shells 
  this 
  portion 
  of 
  the 
  valve 
  

   becomes 
  greatly 
  thickened, 
  the 
  muscular 
  impression 
  correspond- 
  

   ingly 
  deepened 
  and 
  the 
  identity 
  of 
  the 
  dental 
  lamellae 
  is 
  

   obscured 
  by 
  their 
  becoming 
  merged 
  with 
  the 
  substance 
  of 
  the 
  

   valve. 
  The 
  pedicle-cavity 
  is 
  deep 
  and 
  frequently 
  shows 
  a 
  strong 
  

   muscular 
  scar. 
  The 
  impression 
  of 
  the 
  diductor 
  muscles 
  is 
  sub- 
  

   quadrate-ovate 
  or 
  sub 
  triangular 
  in 
  outline, 
  very 
  strongly 
  

   impressed 
  and 
  usually 
  clearly 
  divisible 
  into 
  its 
  two 
  lateral 
  com- 
  

   ponents. 
  The 
  central 
  adductor 
  scar 
  is 
  faint, 
  but 
  linear 
  when 
  

   retained. 
  The 
  lateral 
  scars 
  are 
  deeply 
  striated 
  longitudinally. 
  

   The 
  anterior 
  margin 
  of 
  the 
  muscular 
  area 
  is 
  frequently 
  obscure 
  

   but 
  is 
  not 
  infrequently 
  a 
  ridge 
  from 
  which 
  radiate 
  fine, 
  anas- 
  

   tomosing 
  pallial 
  sinuses. 
  In 
  the 
  post-lateral 
  regions 
  the 
  ovarian 
  

   sinuses 
  are 
  sometimes 
  retained. 
  

  

  In 
  the 
  brachial 
  valve 
  the 
  beak 
  is 
  depressed 
  and 
  sometimes 
  

   obscured 
  by 
  the 
  incurvature 
  of 
  the 
  umbo 
  of 
  the 
  opposite 
  valve. 
  

   The 
  dental 
  sockets 
  are 
  narrow 
  and 
  divergent. 
  The 
  hinge-plate 
  

   is 
  subject 
  to 
  some 
  unessential 
  variation 
  in 
  form. 
  Usually 
  it 
  is 
  

  

  25 
  

  

  