﻿968 
  Forty- 
  seventh 
  Refort 
  on 
  tee 
  Staie 
  Museum. 
  

  

  triangular, 
  concave 
  on 
  the 
  upper 
  surface 
  and 
  divided 
  into 
  two 
  

   lobes 
  by 
  a 
  median 
  groove. 
  The 
  crura 
  take 
  their 
  origin 
  from 
  just 
  

   within 
  the 
  anterior 
  margins 
  of 
  the 
  lobes 
  thus 
  formed. 
  In 
  some 
  

   species 
  the 
  hinge-plate 
  is 
  more 
  subquadrate 
  in 
  outline, 
  the 
  varia- 
  

   tion 
  being 
  produced 
  by 
  the 
  development 
  of 
  post-lateral 
  expan- 
  

   sions. 
  This 
  plate 
  is 
  supported 
  by 
  a 
  median 
  septum, 
  which 
  

   extends 
  for 
  somewhat 
  more 
  than 
  one 
  -third 
  the 
  length 
  of 
  the 
  

   valve. 
  The 
  crura 
  are 
  short 
  and 
  straight, 
  and 
  the 
  primary 
  lamellae 
  

   of 
  the 
  brachidium 
  originate 
  from 
  them 
  at 
  an 
  acute 
  angle, 
  and 
  

   come 
  into 
  closest 
  apposition 
  at 
  the 
  anterior 
  extremity 
  of 
  the 
  

   median 
  septum. 
  In 
  the 
  mature 
  individual, 
  the 
  spiral 
  ribbon 
  

   makes 
  about 
  fifteen 
  volutions, 
  the 
  bases 
  of 
  the 
  cones 
  being 
  sub- 
  

   parallel 
  to 
  the 
  longitudinal 
  axis 
  of 
  the 
  shell 
  and 
  their 
  apices 
  

   directed 
  toward 
  its 
  lateral 
  margins. 
  In 
  their 
  general 
  shape 
  the 
  

  

  Figs. 
  308, 
  309 
  —The 
  jugum 
  of 
  Meristella 
  Walcotti. 
  

  

  cones 
  conform 
  to 
  the 
  character 
  of 
  the 
  interior 
  cavity, 
  and 
  in 
  the 
  

   less 
  convex 
  species 
  (M. 
  Walcotti, 
  M. 
  lento), 
  they 
  are 
  appressed 
  

   on 
  the 
  side 
  of 
  the 
  flatter 
  or 
  brachial 
  valve. 
  The 
  structure 
  of 
  the 
  

   jugum 
  is 
  the 
  same 
  as 
  described 
  for 
  the 
  genus 
  Merista, 
  with 
  this 
  

   difference, 
  however 
  : 
  the 
  circular 
  arms 
  of 
  the 
  jugum 
  curve 
  first 
  

   outward 
  in 
  the 
  horizontal 
  plane, 
  then 
  backward 
  and 
  abruptly 
  

   downward 
  to 
  the 
  inner 
  edges 
  of 
  the 
  primary 
  lamellae 
  ; 
  in 
  their 
  

   return 
  the 
  same 
  curvature 
  is 
  reversed 
  and 
  they, 
  therefore, 
  meet 
  

   the 
  stem 
  of 
  the 
  jugum 
  in 
  the 
  horizontal 
  plane, 
  their 
  point 
  of 
  

   union 
  being 
  invariably 
  above 
  the 
  point 
  of 
  coalescence 
  of 
  the 
  

   lateral 
  branches 
  of 
  the 
  jugum. 
  

  

  The 
  muscular 
  area 
  is 
  elongate-ovate, 
  and 
  extends 
  for 
  the 
  entire 
  

   length 
  of 
  the 
  median 
  septum 
  ; 
  the 
  four 
  adductor 
  scars 
  are 
  

   sometimes 
  distinctly 
  seen, 
  the 
  posterior 
  pair 
  being 
  broader 
  and 
  

   embracing 
  the 
  posterior 
  extremities 
  of 
  the 
  anterior 
  scars. 
  

  

  26 
  

  

  