﻿T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIHERELLID.E. 
  133 
  

  

  quires 
  the 
  pertaining 
  muscle 
  to 
  have 
  been 
  divided 
  at 
  its 
  extremity, 
  

   one 
  division 
  attached 
  to 
  the 
  hinge, 
  and 
  the 
  other 
  to 
  the 
  umbonal 
  

   cavity. 
  But 
  another 
  idea 
  regarding 
  the 
  cardinal 
  scar 
  (v) 
  has 
  been 
  

   adopted 
  by 
  Dall, 
  that 
  it 
  afforded 
  attachment 
  to 
  a 
  muscle 
  homologous 
  

   with 
  that 
  belonging 
  to 
  the 
  cardinal 
  process 
  of 
  the 
  Terebratulids. 
  

   Certainly 
  from 
  its 
  position, 
  and 
  especially 
  when 
  seated 
  on 
  an 
  eleva- 
  

   tion, 
  as 
  in 
  Trimerella 
  Lindstromi, 
  this 
  scar 
  has 
  much 
  the 
  appearance 
  

   of 
  being 
  identical 
  with 
  the 
  process 
  characteristic 
  of 
  the 
  hinge 
  in 
  the 
  

   family 
  just 
  referred 
  to 
  ; 
  but 
  we 
  think 
  a 
  more 
  correct 
  appreciation 
  of 
  

   its 
  true 
  relationship 
  is 
  involved 
  in 
  the 
  suggestion 
  that 
  it 
  is 
  homolo- 
  

   gous 
  with 
  the 
  small 
  pair 
  of 
  postlateral 
  or 
  converging 
  muscles, 
  of 
  

   which, 
  as 
  in 
  Discina, 
  a 
  member 
  passes 
  from 
  near 
  each 
  side 
  of 
  the 
  

   pedicle- 
  or 
  flat 
  valve 
  to 
  the 
  space 
  between 
  the 
  posterior 
  adductor 
  

   muscles 
  in 
  the 
  brachial 
  or 
  convex 
  valve. 
  This 
  suggestion, 
  it 
  must 
  

   be 
  understood, 
  does 
  not 
  necessitate 
  tbe 
  abandonment 
  of 
  the 
  idea 
  that 
  

   the 
  scar 
  (w, 
  w) 
  belonged 
  to 
  the 
  umbonal 
  muscle 
  : 
  only, 
  instead 
  of 
  

   this 
  muscle 
  being 
  divided, 
  with 
  a 
  division 
  implanted 
  on 
  each 
  side 
  of 
  

   the 
  pair 
  of 
  converging 
  muscles, 
  as 
  in 
  Discina, 
  it 
  may 
  have 
  been 
  

   simple 
  in 
  some 
  cases, 
  and 
  have 
  passed 
  between 
  the 
  latter 
  muscles. 
  

   Such 
  an 
  arrangement 
  would 
  be 
  the 
  reverse 
  of 
  what 
  prevails 
  in 
  Dis- 
  

   cina 
  ; 
  but 
  it 
  does 
  not 
  seem 
  to 
  be 
  opposed 
  by 
  any 
  valid 
  objections. 
  

  

  With 
  respect 
  to 
  the 
  slanting 
  muscles 
  of 
  Lingula, 
  we 
  strongly 
  sus- 
  

   pect 
  that 
  the 
  transmedial 
  pair 
  was 
  absent 
  in 
  the 
  Trimerellids 
  — 
  the 
  

   close 
  union 
  of 
  the 
  valves 
  at 
  the 
  hinge, 
  or 
  the 
  possession 
  of 
  a 
  dentary 
  

   apparatus, 
  rendering 
  impossible 
  any 
  lateral 
  movement 
  of 
  the 
  valves 
  

   in 
  their 
  posterior 
  or 
  umbonal 
  region, 
  such 
  being 
  the 
  function 
  of 
  the 
  

   transmedials. 
  

  

  The 
  three 
  remaining 
  pairs 
  of 
  slanting 
  muscles, 
  which 
  have 
  been 
  

   generically 
  designated 
  laterals, 
  and 
  severally 
  named 
  " 
  anteriors," 
  

   " 
  middles," 
  and 
  " 
  outsiders," 
  appear 
  to 
  us 
  to 
  have 
  characterized 
  the 
  

   fossil 
  family. 
  

  

  The 
  " 
  anterior 
  " 
  pair 
  of 
  Lingula 
  may 
  correspond 
  with, 
  in 
  the 
  Trime- 
  

   rellids, 
  the 
  anterior 
  scars, 
  n*, 
  of 
  the 
  brachial 
  valve, 
  and 
  the 
  ends 
  of 
  the 
  

   crescent 
  (s), 
  of 
  the 
  pedicle 
  one 
  ; 
  the 
  " 
  middle 
  " 
  pair 
  with 
  the 
  median 
  

   scars, 
  m, 
  of 
  the 
  pedicle-valve, 
  and 
  the 
  ends 
  of 
  the 
  crescent, 
  s, 
  of 
  the 
  

   brachial 
  one 
  ; 
  the 
  " 
  outside 
  " 
  pair 
  with 
  the 
  lateral 
  scars, 
  o, 
  of 
  the 
  

   pedicle-valve, 
  and 
  the 
  ends 
  of 
  the 
  crescent, 
  s, 
  of 
  the 
  brachial 
  one. 
  

   In 
  accordance 
  with 
  these 
  suggestions, 
  the 
  letter 
  s 
  (that 
  is, 
  in 
  the 
  

   brachial 
  valve) 
  denotes 
  a 
  compound 
  scar; 
  but, 
  instead 
  of 
  being 
  

   triple, 
  as 
  in 
  Lingula 
  (which, 
  it 
  must 
  be 
  borne 
  in 
  mind, 
  has, 
  in 
  addi- 
  

   tion, 
  one 
  of 
  tbe 
  terminations 
  of 
  the 
  transmedial 
  muscle 
  implanted 
  in 
  

   the 
  corresponding 
  part), 
  it 
  would 
  be 
  double. 
  

  

  Moreover, 
  in 
  Lingula 
  anatina 
  a 
  pair 
  of 
  slanting 
  muscles 
  is 
  im- 
  

   planted 
  in 
  the 
  brachial 
  valve, 
  adjacent 
  to, 
  or 
  upon 
  the 
  medio- 
  

   longitudinal 
  ridge, 
  a 
  member 
  on 
  each 
  side, 
  where 
  the 
  scars 
  left 
  by 
  

   the 
  muscles 
  are 
  generally 
  well 
  seen. 
  We 
  have 
  already 
  assumed 
  these 
  

   scars 
  to 
  correspond 
  with 
  those 
  lettered 
  n 
  in 
  figs. 
  4, 
  6, 
  that 
  is, 
  on 
  the 
  

  

  * 
  We 
  have 
  not 
  lettered 
  that 
  portion 
  of 
  the 
  platform 
  in 
  PI. 
  XII. 
  fig. 
  2, 
  where, 
  ap- 
  

   parently 
  the 
  scars 
  would 
  be 
  situated, 
  as 
  there 
  are 
  grounds, 
  presently 
  to 
  be 
  noticed, 
  

   for 
  believing 
  that 
  they 
  were 
  in 
  this 
  instance 
  transposed 
  to 
  the 
  median 
  plate 
  (I). 
  

  

  