﻿946 
  Forty-seventh 
  Report 
  on 
  tee 
  State 
  Museum. 
  

  

  thus 
  forming 
  an 
  accessory 
  articulation 
  of 
  the 
  valves. 
  The 
  essen- 
  

   tial 
  articulation 
  is 
  effected 
  by 
  means 
  of 
  stout, 
  simple 
  teeth 
  lying 
  

   at 
  the 
  marginal 
  extremities 
  of 
  the 
  triangular 
  deltidial 
  covering 
  

   and 
  supported 
  by 
  dental 
  plates 
  which 
  are 
  usually 
  short, 
  but, 
  in 
  

   rare 
  types, 
  may 
  be 
  produced 
  even 
  to 
  the 
  anterior 
  margin 
  of 
  the 
  

   valve. 
  The 
  pedicle-passage 
  or 
  delthyrium 
  is 
  usually 
  open. 
  Nor- 
  

   mally 
  it 
  is 
  closed 
  by 
  a 
  pair 
  of 
  deltidial 
  plates 
  having 
  the 
  form 
  of 
  

   scalene 
  triangles, 
  which 
  develop 
  from 
  the 
  sides 
  of 
  the 
  delthyrium 
  

   and 
  meeting, 
  inclose 
  wholly 
  or 
  partially 
  a 
  circular 
  or 
  oval 
  pedicle 
  

   foramen. 
  At 
  normal 
  maturity 
  these 
  plates 
  become 
  anchylosed 
  

   along 
  the 
  median 
  suture 
  and 
  form 
  a 
  single 
  convex 
  plate 
  (the 
  so- 
  

   called 
  jpseudodeltidium). 
  

  

  The 
  usual 
  absence 
  of 
  the 
  deltidial 
  covering 
  may 
  be 
  due 
  either 
  

   to 
  accidental 
  removal 
  or 
  to 
  resorption 
  with 
  advancing 
  growth. 
  

   In 
  the 
  adult 
  and 
  senile 
  stages 
  of 
  development 
  many 
  species, 
  

   especially 
  in 
  the 
  line 
  of 
  development 
  to 
  Syrinoothyris, 
  form 
  a 
  

   testaceous 
  callosity 
  in 
  the 
  pedicle-cavity, 
  thickening 
  the 
  umbo 
  

   and 
  extending 
  across 
  the 
  delthyrium, 
  reaching, 
  in 
  extreme 
  cases, 
  

   nearly 
  to 
  the 
  cardinal 
  margin. 
  

  

  The 
  muscular 
  area 
  consists 
  of 
  a 
  sub 
  triangular 
  pedicle-impres- 
  

   sion 
  occupying 
  the 
  pedicle-cavity, 
  and 
  continuous 
  with 
  a 
  deeply 
  

   impressed 
  oval 
  or 
  obcordate 
  area, 
  which 
  is 
  posteriorly 
  situated 
  

   and 
  divisible 
  into 
  a 
  narrow 
  median 
  adductors 
  and 
  broad 
  lateral 
  

   diductors, 
  the 
  surface 
  of 
  the 
  latter 
  being 
  marked 
  by 
  radiating 
  or 
  

   racemose 
  furrows. 
  The 
  posterior 
  and 
  anterior 
  members 
  of 
  the 
  

   diductors 
  may 
  frequently 
  be 
  distinguished, 
  the 
  former 
  being 
  of 
  

   less 
  extent 
  and 
  their 
  surface 
  markings 
  somewhat 
  different 
  from 
  

   those 
  of 
  the 
  latter. 
  

  

  A 
  median 
  septum 
  in 
  this 
  valve 
  is 
  usually 
  absent, 
  though 
  not 
  

   infrequently 
  it 
  is 
  in 
  a 
  condition 
  of 
  incipient 
  development, 
  and 
  in 
  

   certain 
  species 
  having 
  the 
  aspect 
  of 
  Spiriferina, 
  it 
  forms 
  a 
  most 
  

   conspicuous 
  feature 
  of 
  the 
  interior. 
  

  

  In 
  the 
  brachial 
  valve 
  the 
  umbo 
  is 
  inconspicuous 
  ; 
  a 
  median 
  

   fold 
  corresponds 
  to 
  the 
  sinus 
  of 
  the 
  opposite 
  valve. 
  The 
  cardinal 
  

   area 
  is 
  narrow, 
  and 
  divided 
  by 
  a 
  broadly 
  triangular 
  delthyrium. 
  

   The 
  dental 
  sock* 
  ts 
  are 
  narrow, 
  moderately 
  deep 
  and 
  bounded 
  

   interiorly 
  by 
  highly 
  developed 
  socket 
  walls, 
  the 
  extremities 
  of 
  

   which 
  support 
  the 
  crural 
  bases. 
  

  

  4 
  

  

  