﻿Report 
  of 
  the 
  State 
  Geologist. 
  965 
  

  

  inclined 
  backward, 
  and 
  uniting 
  to 
  form 
  a 
  stem 
  which 
  bears 
  a 
  

   short 
  bifurcation 
  at 
  its 
  extremity. 
  

  

  The 
  muscular 
  area 
  is 
  elongate-ovate 
  and 
  more 
  or 
  less 
  distinctly 
  

   separated 
  into 
  anterior 
  and 
  posterior 
  scars. 
  Surface 
  of 
  the 
  

   valves 
  smooth 
  or 
  with 
  fine 
  concentric 
  growth 
  striae. 
  Shell- 
  

   substance 
  fibrous, 
  impunctate. 
  

  

  Type, 
  Meristina 
  Maria, 
  Hall. 
  Magara 
  group. 
  

  

  Distribution. 
  Silurian. 
  

  

  Glassina, 
  Hall. 
  1893. 
  

  

  Shells 
  small, 
  biconvex, 
  smooth. 
  Spirals 
  everted 
  ; 
  jugum 
  form- 
  

   ing 
  neither 
  athyroid 
  saddle 
  nor 
  upright 
  stem, 
  but 
  giving 
  off 
  at 
  

   the 
  junction* 
  of 
  the 
  lateral 
  branches 
  two 
  linear 
  processes 
  ; 
  the 
  

   whole 
  apparatus 
  having 
  thus 
  the 
  form 
  of 
  an 
  inclined 
  X 
  , 
  with 
  its 
  

   upper 
  tips 
  curved 
  outward. 
  

  

  Type, 
  Glassina 
  Iceviuscula, 
  Sowerby 
  (sp.). 
  Wenlock 
  limestone. 
  

  

  But 
  one 
  species 
  has 
  been 
  recognized. 
  

  

  Merista, 
  Suess. 
  1851. 
  

  

  (Plate 
  33, 
  figs. 
  8-15.) 
  

   Synonym 
  ; 
  Oamarium, 
  Hall, 
  1859. 
  

  

  Shells 
  transverse 
  or 
  elongate, 
  both 
  valves 
  generally 
  inflated 
  ; 
  

   anterior 
  margin 
  sinuate, 
  producing 
  a 
  fold 
  and 
  sinus 
  on 
  the 
  mar- 
  

   ginal 
  portion 
  of 
  the 
  brachial 
  and 
  pedicle- 
  valves 
  respectively. 
  

  

  In 
  the 
  pedicle-valve 
  the 
  apex 
  is 
  perforated 
  by 
  a 
  circular 
  fora- 
  

   men, 
  which, 
  however, 
  is 
  usually 
  concealed 
  at 
  maturity, 
  by 
  the 
  

   incurvature 
  of 
  the 
  beak 
  ; 
  deltidial 
  plates 
  rarely 
  retained. 
  On 
  the 
  

   interior 
  the 
  teeth 
  are 
  prominent 
  and 
  are 
  supported 
  by 
  dental 
  

   plates 
  which 
  extend 
  either 
  for 
  a 
  short 
  distance 
  into 
  the 
  interior 
  

   cavity 
  or 
  are 
  considerably 
  produced 
  at 
  their 
  bases 
  as 
  thickened 
  

   ridges. 
  Between 
  the 
  dental 
  plates 
  is 
  an 
  arched 
  free 
  plate 
  (the 
  

   " 
  shoe 
  -lifter 
  " 
  process) 
  attached 
  by 
  its 
  posterior 
  and 
  lateral 
  mar- 
  

   gins, 
  but 
  at 
  its 
  anterior 
  margin 
  extending 
  beyond 
  the 
  dental 
  

   lamellae 
  and 
  rising 
  in 
  a 
  low, 
  broad 
  curve. 
  In 
  rare 
  instances 
  this 
  

   process, 
  from 
  its 
  origin, 
  bears 
  a 
  sharp 
  median 
  carina 
  which 
  makes 
  

   the 
  anterior 
  margin 
  highly 
  angulate. 
  The 
  muscular 
  area 
  appears 
  

   to 
  be 
  limited 
  to 
  the 
  space 
  between 
  the 
  dental 
  lamellae 
  and 
  to 
  the 
  

   surface 
  of 
  the 
  " 
  shoe-lifter." 
  

  

  23 
  

  

  