﻿1046 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  In 
  the 
  bottom 
  of 
  the 
  rostral 
  and 
  umbonal 
  cavity 
  is 
  a 
  broad, 
  

   usually 
  ill-defined 
  muscular 
  area, 
  from 
  which 
  radiates 
  a 
  series 
  of 
  

   vascular 
  ridges 
  and 
  depressions 
  extending 
  into 
  the 
  marginal 
  

   region 
  of 
  the 
  valve. 
  The 
  diductor 
  scars 
  are 
  situated 
  posteriorly 
  

   and 
  deeply 
  impressed 
  ; 
  between 
  and 
  in 
  front 
  of 
  them 
  is 
  a 
  narrow, 
  

   elongate 
  adductor 
  scar 
  which 
  is 
  rarely 
  divided 
  medially 
  and 
  

   often 
  extends 
  forward 
  to, 
  or 
  beyond 
  the 
  center 
  of 
  the 
  valve. 
  On 
  

   each 
  side 
  of 
  the 
  muscular 
  impression 
  is 
  a 
  thickened 
  area, 
  very 
  

   narrow 
  at 
  its 
  origin 
  in 
  the 
  rostral 
  region 
  or 
  pedicle-cavity 
  and 
  

   produced 
  into 
  divergent 
  ridges, 
  usually 
  two 
  on 
  each 
  side, 
  and 
  a 
  

   fifth 
  in 
  the 
  median 
  axis. 
  These 
  may 
  extend 
  to 
  the 
  margins 
  or 
  

   disappear 
  before 
  reaching 
  the 
  middle 
  of 
  the 
  valve 
  and 
  are 
  

   variously 
  subdivided 
  by 
  vascular 
  grooves 
  and 
  sinuses 
  emanating 
  

   from 
  them. 
  

  

  In 
  the 
  brachial 
  valve 
  the 
  hinge-plate 
  is 
  small, 
  similar 
  to 
  that 
  

   of 
  Eenssel^ekia 
  and 
  Amphigenia 
  in 
  general 
  form, 
  but 
  is 
  of 
  rela- 
  

   tively 
  less 
  size 
  than 
  in 
  the 
  former 
  genus 
  and 
  is 
  not 
  perforated 
  by 
  

   a 
  visceral 
  foramen 
  opening 
  beneath 
  the 
  apex. 
  Two 
  very 
  

   narrow, 
  almost 
  linear 
  and 
  closely 
  submarginal 
  dental 
  sockets 
  

   extend 
  nearly 
  to 
  the 
  apex; 
  within 
  them 
  lie 
  two 
  broad, 
  sub- 
  

   triangular 
  crural 
  plates, 
  which 
  are 
  divided 
  by 
  a 
  triangular 
  

   median 
  fissure 
  extending 
  to 
  the 
  bottom 
  of 
  the 
  valve. 
  The 
  inner 
  

   anterior 
  angles 
  of 
  these 
  plates 
  bear 
  the 
  slender 
  crural 
  processes, 
  

   the 
  extent 
  of 
  which 
  is 
  unknown. 
  In 
  mature 
  individuals 
  

   the 
  apical 
  portion 
  of 
  the 
  hinge-plate 
  is 
  peculiarly 
  constructed 
  ; 
  

   the 
  latter 
  areas 
  become 
  more 
  or 
  less 
  completely 
  united, 
  without 
  

   altogether 
  obliterating 
  the 
  median 
  triangular 
  fissure, 
  and 
  above 
  

   this 
  point 
  the 
  surface 
  is 
  excavated 
  into 
  a 
  spoon-shaped 
  cavity, 
  

   when 
  the 
  development 
  is 
  extreme, 
  or 
  is 
  transversely 
  angular 
  in 
  

   the 
  average 
  individual. 
  At 
  a 
  short 
  distance 
  from 
  the 
  hinge-plate 
  

   and 
  in 
  the 
  bottom 
  of 
  the 
  valve 
  there 
  arises 
  a 
  low 
  median 
  ridge, 
  

   which 
  continues 
  for 
  a 
  short 
  distance, 
  separating 
  the 
  obovate, 
  

   narrowly 
  flabelliform 
  scars 
  of 
  the 
  anterior 
  and 
  posterior 
  adductor 
  

   muscles. 
  The 
  anterior 
  scars 
  are 
  considerably 
  the 
  larger, 
  and 
  

   their 
  surface 
  is 
  longitudinally 
  striated. 
  The 
  vascular 
  grooves 
  

   and 
  ridges 
  are 
  more 
  obscurely 
  developed 
  than 
  in 
  the 
  pedicle- 
  

   ralve. 
  

  

  104 
  

  

  