﻿Report 
  of 
  the 
  State 
  Geologist. 
  819 
  

  

  hinge-plate, 
  that 
  the 
  so-called 
  visceral 
  foramen 
  which 
  perforates 
  

   it, 
  and 
  which 
  is 
  often 
  present 
  in 
  Athyris, 
  Renssel^eria, 
  

   Cryptonella, 
  etc., 
  is 
  a 
  remnant 
  of 
  this 
  aperture, 
  the 
  remainder 
  

   of 
  the 
  median 
  opening 
  having 
  become 
  filled 
  by 
  a 
  testaceous 
  

   secretion. 
  There 
  is 
  every 
  reason 
  to 
  believe 
  that 
  the 
  visceral 
  

   foramen 
  was 
  actually 
  traversed 
  by 
  the 
  lower 
  alimentary 
  canal, 
  

   and 
  if 
  this 
  were 
  true, 
  then 
  the 
  deep 
  and 
  narrow 
  median 
  chamber 
  

   bounded 
  by 
  the 
  crural 
  plates 
  must 
  also 
  have 
  enclosed 
  the 
  termi- 
  

   nal 
  portion 
  of 
  the 
  intestine 
  Within 
  it 
  lie 
  the 
  elongate 
  scars 
  of 
  

   the 
  adductor 
  muscles, 
  and 
  when 
  the 
  chamber 
  is 
  elevated 
  b} 
  T 
  the 
  

   completed 
  formation 
  of 
  a 
  cruralium, 
  these 
  scars 
  are 
  still 
  within 
  

   it, 
  as 
  in 
  the 
  case 
  of 
  the 
  spondylium. 
  It 
  is 
  therefore 
  the 
  morphic 
  

   equivalent 
  of 
  the 
  spondylium. 
  Its 
  supporting 
  median 
  septum, 
  

   when 
  present, 
  is 
  composed 
  of 
  two 
  lamellae, 
  each 
  representing 
  one 
  

   of 
  the 
  coalesced 
  or 
  adherent 
  crural 
  plates.* 
  

  

  The 
  unsupported 
  convex 
  internal 
  plate 
  or 
  " 
  shoe-lifter 
  " 
  in 
  the 
  

   pedicle-valve 
  of 
  Merista 
  and 
  Dicamara 
  must 
  be 
  interpreted 
  as 
  an 
  

   entirely 
  different 
  structure 
  from 
  the 
  spondylium. 
  It 
  is 
  not 
  pro- 
  

   duced 
  by 
  convergent 
  dental 
  plates, 
  but 
  these, 
  on 
  the 
  contrary, 
  

   are 
  divergent, 
  the 
  arched 
  plate 
  uniting 
  its 
  inner 
  edges. 
  Its 
  

   origin 
  and 
  the 
  reason 
  of 
  its 
  existence 
  are 
  still 
  obscure. 
  The 
  

   readiness 
  with 
  which 
  the 
  filling 
  of 
  the 
  cavity 
  between 
  this 
  plate 
  

   and 
  the 
  outer 
  wall 
  of 
  the 
  valve 
  separates 
  from 
  the 
  shell, 
  carrying 
  

   with 
  it 
  the 
  enclosing 
  walls, 
  leads 
  to 
  ,the 
  suggestion 
  that 
  the 
  

   " 
  shoe-lifter 
  " 
  may 
  be 
  the 
  innermost 
  lamina 
  of 
  the 
  shell 
  separated 
  

   from 
  the 
  rest 
  of 
  the 
  valve 
  and 
  leaving 
  it 
  thinner 
  in 
  this 
  region. 
  

   This 
  plate 
  bears 
  the 
  muscular 
  bands, 
  in 
  whole 
  or 
  in 
  part, 
  upon 
  

   its 
  convex 
  surface. 
  In 
  Eicbwaldia 
  it 
  has 
  been 
  observed 
  that 
  the 
  

   small 
  internal 
  plate 
  of 
  the 
  pedicle-valve 
  is 
  probably 
  a 
  modified 
  

   condition 
  of 
  the 
  deltidium, 
  as 
  the 
  pedicle 
  passes 
  beneath 
  it, 
  while 
  

   the 
  platform 
  in 
  AuLacoRHYNCHirs 
  may 
  prove 
  to 
  be 
  wholly 
  of 
  

   muscular 
  origin. 
  

  

  *In 
  ihe 
  pentamemids 
  th" 
  medin 
  spptum 
  of 
  th- 
  p 
  dicle-v 
  lve 
  supporting 
  the 
  spondylium, 
  is 
  

   formed 
  in 
  a 
  similar 
  manner 
  by 
  a 
  continuation 
  end 
  coa'tscence 
  of 
  the 
  dental 
  pistes, 
  arid 
  wher- 
  

   ever 
  the 
  median 
  supporting 
  septum 
  exists 
  in 
  this 
  group, 
  it 
  will 
  probably 
  b' 
  found 
  to 
  have 
  this 
  

   compositi 
  n 
  Median 
  and 
  )a 
  f 
  eral 
  septa, 
  however, 
  in 
  the 
  valv, 
  s 
  of 
  the 
  Br 
  chiopoda, 
  have 
  a 
  

   highly 
  diverse 
  origin 
  ia 
  different 
  case3 
  In 
  . 
  ost 
  instance, 
  except 
  wnere 
  bearing 
  spondyiia, 
  

   they 
  are 
  evidently 
  of 
  muscu'ar 
  origin 
  and 
  surfaces 
  of 
  muscular 
  attachment, 
  as 
  shown 
  in 
  

   Spirtferina 
  (s°e 
  figure 
  42, 
  page 
  53, 
  and 
  remarks 
  in 
  foot-notts, 
  P*rt 
  I, 
  p 
  49); 
  while 
  in 
  the 
  

   Trimerrllid-s: 
  they 
  appear 
  to 
  be 
  the 
  residuum 
  left 
  by 
  the 
  resorption 
  of 
  a 
  thick 
  testaceous 
  

   deposition 
  about 
  and 
  beneath 
  the 
  area 
  of 
  muscular 
  insertion. 
  

  

  