﻿136 
  T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLIDS. 
  

  

  pedicle. 
  Besides 
  these 
  parts, 
  this 
  organ 
  is 
  attached 
  by 
  the 
  end 
  

   (compressed) 
  of 
  its 
  inner 
  fleshy 
  or 
  muscular 
  cylinder 
  to 
  an 
  ellipsoidal 
  

   space 
  in 
  front 
  of 
  the 
  narrow 
  lineated 
  band, 
  and 
  behind 
  the 
  umbonal 
  

   muscle. 
  The 
  part 
  similarly 
  situated 
  in 
  the 
  Trimerellids, 
  and 
  named 
  

   lozenge 
  (g, 
  figs. 
  1 
  & 
  7), 
  we 
  attribute 
  to 
  the 
  attachment 
  of 
  the 
  same 
  

   cylinder. 
  Referring 
  again 
  to 
  Lingula, 
  the 
  submarginal 
  portions 
  of 
  

   the 
  posterior 
  half 
  of 
  its 
  valves 
  are 
  distinguished 
  by 
  a 
  compound 
  scar 
  

   consisting 
  of: 
  — 
  a 
  line 
  on 
  the 
  hinge; 
  two 
  expansions, 
  one 
  near 
  each 
  

   of 
  the 
  postlateral 
  margins 
  ; 
  and 
  two 
  longitudinally 
  elliptical 
  spaces, 
  

   one 
  connected 
  with 
  each 
  of 
  the 
  expansions, 
  and 
  terminating 
  at 
  about 
  

   the 
  transverse 
  median 
  line 
  of 
  the 
  valves. 
  The 
  first 
  is 
  formed 
  by 
  

   the 
  postparietal 
  of 
  the 
  splanchnocoele 
  ; 
  the 
  second, 
  their 
  inner 
  side 
  

   by 
  the 
  post-lateroparietals 
  of 
  this 
  chamber, 
  and 
  their 
  outer 
  side 
  by 
  

   the 
  posterior 
  primary 
  vessels 
  belonging 
  to 
  the 
  pleurocoeles 
  ; 
  the 
  third 
  

   mark 
  the 
  attachment 
  of 
  the 
  slanting 
  muscles, 
  which 
  are 
  limited 
  on 
  

   their 
  outer 
  side 
  by 
  the 
  lateral 
  walls 
  of 
  the 
  splanchnocoele. 
  

  

  That 
  the 
  crescent 
  of 
  the 
  Trimerellids 
  is 
  made 
  up 
  of 
  the 
  same 
  parts 
  

   (though 
  subject 
  to 
  modifications, 
  as 
  exemplified 
  in 
  Dinobolus), 
  its 
  

   crown, 
  sides, 
  and 
  'ends 
  respectively 
  representing 
  the 
  first, 
  second,, 
  

   and 
  third, 
  is 
  a 
  proposition 
  which, 
  we 
  think, 
  requires 
  nothing 
  further 
  

   for 
  its 
  establishment. 
  

  

  In 
  Lingula 
  anatina 
  the 
  splanchnocoele 
  is 
  bounded 
  in 
  part 
  by 
  another 
  

   wall, 
  the 
  anteparietal. 
  The 
  transverse 
  scars 
  (t, 
  t) 
  in 
  Trimerella 
  

   occupy 
  a 
  similar 
  position, 
  evidencing 
  that 
  they 
  were 
  produced 
  by 
  the 
  

   same 
  wall. 
  A 
  further 
  agreement 
  consists 
  in 
  the 
  anteparietal 
  of 
  the 
  

   recent 
  shell 
  running 
  forward, 
  in 
  the 
  brachial 
  valve, 
  with 
  a 
  duckbill- 
  

   shaped 
  outline, 
  considerably 
  into 
  the 
  brachiocosle 
  ; 
  for 
  doubtless 
  a 
  

   similar 
  peculiarity 
  obtained 
  in 
  the 
  fossil 
  through 
  the 
  attachment 
  of 
  

   the 
  same 
  wall 
  to 
  the 
  sides 
  of 
  the 
  median 
  plate, 
  as 
  it 
  does 
  to 
  the 
  cor- 
  

   responding 
  ridge 
  of 
  Lingula, 
  with 
  this 
  difference, 
  however, 
  that 
  the 
  

   projection 
  would 
  be 
  longer, 
  and 
  its 
  base 
  narrower. 
  There 
  is 
  no 
  

   appearance 
  of 
  the 
  transverse 
  scars 
  starting 
  off 
  from 
  any 
  place 
  in 
  

   front 
  of 
  the 
  platform 
  : 
  they 
  run 
  close 
  up 
  to 
  the 
  median 
  plate, 
  thence 
  

   apparently 
  forward, 
  and 
  along 
  each 
  side 
  of 
  it 
  ; 
  which 
  would 
  neces- 
  

   sarily 
  cause 
  the 
  attached 
  portion 
  of 
  the 
  anteparietal 
  projection, 
  i. 
  e. 
  

   in 
  the 
  brachial 
  valve, 
  to 
  be 
  comparatively 
  narrow. 
  

  

  The 
  archlet 
  was 
  evidently 
  produced 
  by 
  the 
  attachment 
  of 
  the 
  

   anterior 
  pair 
  of 
  primary 
  vessels. 
  In 
  Lingula 
  anatina 
  these 
  vessels 
  

   do 
  not, 
  strictly 
  speaking, 
  unite 
  at 
  their 
  anterior 
  extremity 
  ; 
  and 
  it 
  

   appears 
  to 
  have 
  been 
  the 
  same 
  in 
  Trimerella, 
  as 
  the 
  archlet 
  seems 
  

   often 
  to 
  open 
  at 
  its 
  apex. 
  In 
  the 
  brachial 
  valve 
  of 
  the 
  Trimerellas 
  

   (T. 
  grandis, 
  PI. 
  XIII. 
  fig. 
  2 
  b) 
  the 
  archlet 
  has 
  also 
  a 
  remarkable 
  

   agreement 
  in 
  form 
  with 
  the 
  course 
  pursued 
  by 
  the 
  pair 
  of 
  primary 
  

   vessels 
  in 
  the 
  recent 
  species 
  referred 
  to. 
  The 
  opposite 
  valve 
  of 
  the 
  

   latter 
  shell 
  shows 
  the 
  same 
  vessels, 
  but 
  more 
  shortened 
  : 
  the 
  archlet 
  

   belonging 
  to 
  the 
  corresponding 
  valve 
  of 
  the 
  fossils 
  possesses 
  a 
  similar 
  

   peculiarity. 
  We 
  have 
  not 
  yet 
  met 
  with 
  any 
  impressions 
  producing 
  

   secondary 
  vessels. 
  

  

  It 
  will 
  be 
  understood 
  that 
  we 
  are 
  now 
  in 
  the 
  brachioccele 
  — 
  the 
  

   chamber 
  holding 
  the 
  arms 
  or 
  labial 
  appendages. 
  So 
  far 
  no 
  convoluted 
  

  

  