﻿972 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  and 
  just 
  within 
  the 
  beak 
  of 
  the 
  valve 
  is 
  a 
  circular 
  perforation 
  

   (visceral 
  foramen), 
  which 
  is 
  continued 
  beneath 
  the 
  plate 
  into 
  the 
  

   cavity 
  of 
  the 
  valve. 
  The 
  anterior 
  margin 
  of 
  the 
  plate 
  is 
  straight 
  

   or 
  slightly 
  concave, 
  occasionally 
  trilobate, 
  and 
  the 
  crura 
  are 
  

   attached 
  at 
  the 
  extremities 
  of 
  the 
  lateral 
  ridges. 
  Sometimes 
  the 
  

   outline 
  of 
  the 
  hinge-plate 
  is 
  rendered 
  subquadrate 
  by 
  the 
  develop- 
  

   ment 
  of 
  two 
  post-lateral 
  expansions. 
  

  

  The 
  brachidium 
  consists 
  of 
  spiral 
  cones 
  lying 
  base 
  to 
  base, 
  with 
  

   their 
  apices 
  directed 
  laterally. 
  The 
  form 
  of 
  these 
  cones 
  varies 
  

   with 
  that 
  of 
  the 
  internal 
  cavity, 
  but 
  as 
  a 
  rule 
  they 
  are 
  much 
  com- 
  

   pressed 
  vertically, 
  the 
  posterior 
  curvature 
  being 
  short 
  and 
  convex, 
  

  

  Fig. 
  311. 
  

   Diagram 
  of 
  the 
  structure 
  of 
  the 
  brachidium 
  in 
  Athyris. 
  

   x. 
  Spiral 
  coil. 
  s. 
  Saddle. 
  

  

  a. 
  Primary 
  lamellae 
  of 
  spiral 
  coil. 
  m. 
  Fimbriated 
  extensions 
  of 
  saddle, 
  

  

  a'. 
  Secondary 
  lamellae. 
  t. 
  Stem 
  of 
  jugum. 
  

  

  a". 
  Umbonal 
  blades. 
  f. 
  Arms 
  of 
  jugum. 
  

  

  c 
  Crura. 
  6. 
  Accessory 
  lamellae. 
  

  

  I. 
  Jugum. 
  n. 
  Fimbriae 
  on 
  outer 
  margins 
  of 
  lamellae. 
  

  

  e. 
  Lateral 
  branches 
  of 
  jugum. 
  

  

  while 
  the 
  anterior 
  curve 
  is 
  long 
  and 
  sometimes 
  depressed. 
  The 
  

   crura 
  originate 
  from 
  the 
  hinge 
  plate 
  at 
  a 
  large 
  angle, 
  are 
  long 
  

   and 
  convergent, 
  the 
  primary 
  lamellas 
  beginning 
  at 
  their 
  extrem- 
  

   ities, 
  making 
  an 
  angular 
  curve 
  at 
  their 
  origin, 
  thence, 
  in 
  the 
  

   typical 
  species, 
  curving 
  deeply 
  upward 
  and 
  backward, 
  to 
  form 
  

   the 
  first 
  volution. 
  The 
  spirals 
  are 
  connected 
  by 
  a 
  jugum, 
  which 
  

   takes 
  its 
  origin 
  on 
  the 
  first 
  half 
  of 
  the 
  primary 
  lamellae, 
  the 
  two 
  

   lateral 
  lamellas 
  converging, 
  and 
  uniting 
  at 
  about 
  half 
  the 
  dis- 
  

   tance 
  across 
  the 
  base 
  of 
  the 
  cones, 
  to 
  form 
  a 
  broad 
  saddle 
  with 
  

   a 
  convex 
  upper 
  surface 
  ; 
  the 
  anterior 
  extremity 
  of 
  this 
  saddle 
  

   may 
  be 
  simple 
  or 
  divided 
  ; 
  its 
  posterior 
  portion 
  is 
  narrowed, 
  

  

  30 
  

  

  