﻿166 
  T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMEEELIIDiE. 
  

  

  groove." 
  The 
  gutta-percha 
  squeezes 
  in 
  our 
  possession 
  show 
  very 
  

   faintly 
  what 
  has 
  been 
  taken 
  for 
  the 
  '•' 
  groove 
  " 
  (much 
  less 
  so 
  than 
  is 
  

   represented 
  by 
  Hall, 
  and 
  which 
  even 
  appears 
  too 
  much 
  like 
  a 
  notch 
  

   in 
  our 
  original 
  figure) 
  : 
  there 
  is 
  something 
  like 
  a 
  depression 
  ; 
  but, 
  

   considering 
  two 
  peculiarities 
  to 
  be 
  noticed 
  presently, 
  we 
  believe 
  it 
  

   to 
  be 
  nothing 
  more 
  than 
  such 
  as 
  occasionally 
  occurs 
  on 
  the 
  hinge 
  of 
  

   the 
  brachial 
  valve 
  of 
  Lingula 
  anatina. 
  

  

  The 
  peculiarities 
  alluded 
  to 
  belong 
  to 
  the 
  crescent 
  and 
  the 
  median 
  

   plate. 
  In 
  the 
  pedicle-valve 
  of 
  Trimerella, 
  as 
  will 
  be 
  recollected, 
  

   the 
  crown 
  of 
  the 
  crescent 
  has 
  a 
  forward 
  curve 
  in 
  the 
  middle, 
  due, 
  we 
  

   believe, 
  as 
  in 
  Lingula, 
  to 
  this 
  portion 
  having 
  been 
  forced 
  out 
  by 
  the 
  

   pedicle 
  ; 
  while 
  the 
  crescent 
  in 
  the 
  brachial 
  valve 
  has 
  a 
  pointed 
  

   crown, 
  with 
  the 
  point 
  directed 
  backwards 
  : 
  now 
  it 
  is 
  the 
  latter 
  

   peculiarity 
  that 
  plainly 
  presents 
  itself 
  in 
  Lingulops. 
  The 
  other 
  pecu- 
  

   liarity 
  equally 
  sustains 
  our 
  view 
  ; 
  for 
  in 
  the 
  Trimerellids, 
  as 
  also 
  in 
  

   Lingula, 
  the 
  pedicle-valve 
  has 
  a 
  short 
  or 
  rudimentary 
  median 
  plate, 
  

   whereas 
  in 
  their 
  brachial 
  valve 
  this 
  part 
  is 
  well 
  developed 
  and 
  

   elongated 
  as 
  it 
  evidently 
  is 
  in 
  Lingidops. 
  

  

  Believing 
  that 
  the 
  fossil 
  is 
  a 
  brachial 
  valve, 
  we 
  feel 
  tempted 
  to 
  

   offer 
  a 
  suggestion 
  in 
  connexion 
  with 
  the 
  semicircular 
  zone. 
  In 
  

   Lingula 
  anatina 
  a 
  broadish 
  flattened 
  organ, 
  called 
  the 
  setal 
  band, 
  

   is 
  attached, 
  chiefly 
  at 
  both 
  its 
  sides, 
  by 
  a 
  muscular 
  cord 
  to 
  the 
  valves 
  

   (differing 
  somewhat 
  in 
  each) 
  a 
  little 
  within 
  their 
  margins 
  : 
  in 
  the 
  

   brachial 
  valve 
  the 
  setal 
  band 
  is 
  uninterruptedly 
  (not 
  being 
  interfered 
  

   with 
  by 
  the 
  pedicle, 
  as 
  in 
  the 
  pedicle-valve) 
  carried 
  round 
  the 
  hinge 
  

   behind 
  the 
  umbonal 
  muscle. 
  Our 
  suggestion 
  is 
  that, 
  while 
  the 
  semi- 
  

   circular 
  zone 
  itself 
  formed 
  the 
  seat 
  of 
  the 
  mass 
  of 
  the 
  setal 
  band 
  *, 
  

   the 
  arched 
  fillet 
  was 
  produced 
  by 
  the 
  attachment 
  of 
  the 
  outer 
  mus- 
  

   cular 
  cord 
  and 
  associated 
  vessels 
  ; 
  also, 
  that 
  the 
  inner 
  scalloped 
  border 
  

   is 
  mainly 
  due 
  to 
  the 
  attachment 
  of 
  the 
  posterior 
  and 
  post-lateral 
  

   walls 
  of 
  the 
  splanchnoccele 
  f. 
  

  

  "We 
  have 
  yet 
  to 
  explain 
  the 
  peculiar 
  sinused 
  character 
  which 
  

   gives 
  the 
  semicircular 
  zone 
  such 
  a 
  striking 
  resemblance 
  to 
  a 
  moorish 
  

   arch. 
  The 
  inner 
  side 
  of 
  the 
  zone 
  is 
  cut 
  out 
  by 
  five 
  sinuses 
  — 
  one 
  in 
  

   the 
  middle, 
  and 
  two 
  on 
  each 
  side 
  — 
  only 
  opposite 
  ones 
  being 
  equal. 
  

   The 
  sinuses 
  are 
  each 
  the 
  outer 
  side 
  of 
  a 
  circular 
  scar, 
  its 
  inner 
  

   and 
  imperfect 
  side 
  lying 
  near 
  or 
  against 
  the 
  central 
  apophysis 
  

   or 
  platform 
  : 
  it 
  is 
  only 
  lately 
  that 
  we 
  have 
  been 
  able 
  to 
  make 
  out 
  

   the 
  form 
  of 
  the 
  sinuses. 
  Let 
  us 
  next 
  try 
  to 
  explain 
  their 
  origin. 
  

   The 
  middle 
  sinus, 
  we 
  have 
  little 
  doubt, 
  has 
  been 
  produced 
  by 
  the 
  

   umbonal 
  muscle 
  pressing 
  against 
  the 
  postparietal 
  or 
  posterior 
  wall 
  of 
  

   the 
  splanchnoccele. 
  It 
  might 
  be 
  suggested 
  that 
  the 
  lateral 
  sinuses 
  

   have 
  been 
  formed 
  in 
  the 
  same 
  way 
  by 
  other 
  muscles 
  ; 
  but 
  as 
  Lingida 
  

   is 
  not 
  characterized 
  by 
  any 
  similarly 
  situated 
  (at 
  least 
  the 
  brachial 
  

  

  * 
  The 
  entireness 
  of 
  the 
  zone 
  at 
  its 
  apex 
  forms 
  another 
  evidence 
  in 
  favour 
  of 
  

   the 
  valve 
  being 
  the 
  brachial 
  one. 
  

  

  t 
  See 
  " 
  On 
  some 
  characters 
  of 
  Lingula 
  anatina" 
  Annals 
  and 
  Mag. 
  of 
  Nat. 
  

   Hist., 
  4th 
  series, 
  vol. 
  i. 
  July 
  1873. 
  The 
  setal 
  band 
  and 
  its 
  modifications 
  in 
  the 
  

   brachial 
  valve 
  are 
  shown 
  in 
  the 
  two 
  woodcuts 
  already 
  given 
  in 
  a 
  preceding 
  page 
  

   of 
  the 
  present 
  memoir 
  (p. 
  132). 
  

  

  