﻿Report 
  of 
  the 
  State 
  Geologisi. 
  1019 
  

  

  Rhynchotrema, 
  Hall. 
  1860. 
  

   (Plate 
  42, 
  figs. 
  12-16.) 
  

  

  Shells 
  large, 
  thick, 
  often 
  gibbous. 
  In 
  mature 
  conditions 
  the 
  

   deltidial 
  plates 
  are 
  of 
  great 
  size, 
  thickened 
  and 
  coalesced 
  with 
  

   the 
  bottom 
  of 
  the 
  valve, 
  their 
  outer 
  surface 
  being 
  concave. 
  The 
  

   pedicle 
  passage 
  encroaches 
  upon 
  the 
  substance 
  of 
  the 
  valve, 
  the 
  

   foramen 
  lying 
  behind 
  the 
  apex 
  and 
  the 
  passage 
  itself 
  inclosed 
  by 
  

   the 
  thickened 
  deltarium. 
  

  

  The 
  teeth 
  rest 
  upon 
  the 
  thickened 
  lateral 
  walls 
  of 
  the 
  valve, 
  

   and 
  there 
  appears 
  to 
  have 
  been 
  no 
  development 
  of 
  dental 
  lam- 
  

   ellae 
  unless 
  it 
  was 
  at 
  a 
  very 
  early 
  period 
  in 
  the 
  life 
  of 
  the 
  

   individual. 
  

  

  In 
  the 
  brachial 
  valve 
  there 
  is 
  a 
  thickened 
  median 
  septum 
  which 
  

   may 
  extend 
  for 
  more 
  than 
  one-half 
  the 
  length 
  of 
  the 
  shell 
  ; 
  and 
  

   it 
  is 
  upon 
  the 
  posterior 
  extremity 
  of 
  this 
  that 
  the 
  slender 
  median 
  

   cardinal 
  process 
  rests. 
  This 
  delicate 
  apophysis 
  is 
  frequently 
  dis- 
  

   torted 
  to 
  one 
  side 
  or 
  the 
  other. 
  The 
  bases 
  supporting 
  the 
  crura 
  

   are 
  divided 
  by 
  a 
  very 
  narrow 
  median 
  cleft, 
  and 
  are 
  remarkably 
  

   broad 
  and 
  stout, 
  abruptly 
  deflected 
  to 
  the 
  deep 
  dental 
  sockets. 
  

   The 
  crura 
  take 
  their 
  origin 
  from 
  the 
  central 
  portion 
  of 
  this 
  com- 
  

   paratively 
  broad 
  hinge-plate, 
  instead 
  of 
  from 
  the 
  margins 
  of 
  the 
  

   dental 
  sockets, 
  as 
  is 
  usually 
  the 
  case 
  in 
  the 
  palaeozoic 
  rhynchonel- 
  

   loids. 
  The 
  structure 
  of 
  the 
  hinge 
  apophyses 
  in 
  both 
  valves 
  is 
  a 
  

   persistent 
  character, 
  while 
  the 
  peculiarities 
  of 
  the 
  deltarium 
  are 
  

   variable 
  with 
  age 
  and 
  external 
  conditions. 
  The 
  muscular 
  impres- 
  

   sions 
  are 
  usually 
  strongly 
  developed, 
  there 
  being 
  beneath 
  the 
  del- 
  

   tidial 
  plates 
  a 
  deep 
  scar 
  of 
  the 
  pedicle 
  muscle, 
  while 
  the 
  adductor 
  

   impression 
  on 
  the 
  pedicle-valve 
  is 
  often 
  very 
  marked. 
  The 
  

   adductors 
  of 
  the 
  brachial 
  valve 
  and 
  the 
  diductors 
  of 
  the 
  pedicle- 
  

   valve 
  are 
  more 
  or 
  less 
  distinctly 
  defined. 
  

  

  Type, 
  Rhynchotrema 
  capax, 
  Conrad 
  (sp.). 
  

  

  Distribution. 
  Lower 
  Silurian. 
  

  

  Rhynchotreta, 
  Hall. 
  1879. 
  

   (Plate 
  42, 
  figs. 
  17-21.) 
  

   Trihedral, 
  strongly 
  plicated 
  shells 
  with 
  fold 
  and 
  sinus 
  normal, 
  

   in 
  adolescent 
  and 
  mature 
  stages 
  ; 
  long 
  and 
  broad 
  cardinal 
  slopes 
  ; 
  

   beak 
  erect, 
  acuminate 
  and 
  produced 
  on 
  the 
  pedicle-valve. 
  Fora- 
  

   men 
  at 
  maturity 
  apical, 
  its 
  upper 
  margin 
  encroaching 
  on 
  the 
  

  

  77 
  

  

  