﻿984 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum, 
  

  

  Hustedja, 
  but 
  the 
  median 
  portion 
  of 
  the 
  upper 
  face 
  is 
  

   convex 
  and 
  the 
  lateral 
  portions 
  deeply 
  grooved 
  and 
  bounded 
  on 
  

   the 
  outside 
  by 
  the 
  elevated 
  crural 
  bases. 
  The 
  hinge-plate 
  is 
  

   supported 
  by 
  a 
  strong 
  median 
  septum 
  which 
  extends 
  for 
  nearly 
  

   two-thirds 
  the 
  length 
  of 
  the 
  valve. 
  It 
  is 
  most 
  highly 
  elevated 
  

   near 
  the 
  middle 
  of 
  its 
  length 
  where 
  it 
  extends 
  vertically 
  about 
  

   one- 
  fifth 
  of 
  the 
  distance 
  across 
  the 
  internal 
  cavity; 
  thence 
  it 
  

   tapers 
  rapidly 
  to 
  its 
  anterior 
  extremity. 
  

  

  The 
  brachidium 
  has 
  been 
  reconstructed 
  from 
  serial 
  transverse 
  

   sections 
  of 
  the 
  shell 
  in 
  several 
  directions, 
  and 
  the 
  following 
  

   description 
  may 
  be 
  relied 
  upon 
  as 
  approximately 
  accurate. 
  The 
  

   umbonal 
  blades 
  of 
  the 
  primary 
  lamellae 
  are 
  comparatively 
  narrow 
  

   and 
  considerably 
  incurved 
  at 
  their 
  apices, 
  where 
  attached 
  to 
  the 
  

   long 
  crura, 
  as 
  in 
  Eumetria. 
  The 
  jugum 
  is 
  situated 
  well 
  forward, 
  

  

  Fig. 
  357. 
  Fig. 
  358. 
  

  

  The 
  jugum 
  of 
  Retzia 
  Adrieni, 
  de 
  Verneuil, 
  as 
  reconstructed 
  from 
  consecutive 
  sections. 
  

  

  just 
  behind 
  the 
  center 
  of 
  the 
  lamellae 
  ; 
  its 
  lateral 
  branches 
  are 
  

   erect 
  and 
  long 
  ; 
  they 
  narrow 
  with 
  a 
  slight 
  twist 
  just 
  above 
  their 
  

   origin, 
  as 
  in 
  the 
  genera 
  Rhynchospi 
  a 
  and 
  Trematos 
  pira, 
  then 
  

   broaden, 
  curving 
  outward 
  and 
  thence 
  inward 
  to 
  their 
  point 
  of 
  

   union 
  The 
  stem 
  is 
  short, 
  making 
  an 
  angle 
  with 
  the 
  lateral 
  

   branches, 
  and 
  is 
  directed 
  backward. 
  It 
  reaches 
  the 
  level 
  of 
  the 
  

   crura 
  at 
  a 
  considerable 
  distance 
  in 
  front 
  of 
  them 
  and 
  is 
  there 
  

   bifurcated, 
  each 
  arm 
  making 
  a 
  slight 
  double 
  or 
  sigmoid 
  curve. 
  

   These 
  arms 
  are, 
  however, 
  too 
  short 
  to 
  reach 
  the 
  umbonal 
  blades. 
  

   The 
  stem 
  itself 
  is 
  continued 
  for 
  a 
  short 
  distance 
  above 
  the 
  point 
  

   of 
  bifurcation. 
  The 
  spiral 
  ribbons 
  make 
  ten 
  or 
  eleven 
  volutions 
  

   in 
  full-grown 
  individuals. 
  Fimbriae 
  are 
  absent 
  from 
  both 
  the 
  

   spirals 
  and 
  jugum. 
  

  

  Type, 
  Retzia 
  Adrieni, 
  de 
  Yerneuil. 
  Middle 
  Devonian. 
  

  

  But 
  one 
  species 
  is 
  known. 
  

  

  42 
  

  

  