﻿T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLID^. 
  127 
  

  

  possibly 
  have 
  been 
  small 
  in 
  quantity 
  compared 
  with 
  the 
  corneous 
  

   element*. 
  

  

  The 
  species 
  often 
  fail 
  in 
  neatness 
  or 
  regularity 
  of 
  form, 
  especially 
  

   in 
  their 
  umbonal 
  region. 
  This 
  part 
  is 
  often 
  massive 
  and 
  strongly 
  

   projecting, 
  and, 
  it 
  may 
  be, 
  ecurving, 
  incurving, 
  or 
  twisted 
  to 
  one 
  side. 
  

   Some 
  species 
  have 
  it 
  obtusely 
  and 
  regularly 
  rounded. 
  All 
  the 
  

   species 
  apparently 
  have 
  had 
  their 
  outer 
  surface 
  smooth, 
  or 
  marked 
  

   with 
  lines 
  of 
  growth, 
  coarse 
  in 
  some, 
  and 
  fine 
  in 
  others 
  : 
  slight 
  

   traces 
  of 
  diverging 
  longitudinal 
  lines 
  are 
  occasionally 
  visible. 
  

  

  Guided 
  by 
  certain 
  evidences 
  indicating 
  the 
  existence 
  of 
  two 
  of 
  the 
  

   chief 
  organs 
  belonging 
  to 
  shells 
  of 
  their 
  class, 
  it 
  is 
  readily 
  seen 
  that 
  

   the 
  Trimerellids 
  have 
  had 
  the 
  large 
  valve 
  pedunculated, 
  and 
  the 
  

   smaller 
  one 
  characterized 
  with 
  oral 
  or 
  (the 
  so-called) 
  brachial 
  ap- 
  

   pendages; 
  it 
  is 
  therefore 
  proposed 
  to 
  name 
  the 
  former 
  the 
  pedicle 
  

   (PI. 
  XII. 
  figs. 
  1, 
  3, 
  5), 
  and 
  the 
  latter 
  the 
  brachial 
  valvef 
  (PI. 
  XII. 
  

   figs. 
  2, 
  4, 
  6). 
  

  

  The 
  pedicle-valve 
  has 
  the 
  cardinal 
  face 
  of 
  its 
  umbo 
  flattened 
  so 
  as 
  

   to 
  form 
  a 
  triangular 
  space 
  or 
  area, 
  which 
  differs 
  remarkably 
  in 
  size 
  

   according 
  to 
  genera 
  and 
  species. 
  The 
  longitudinally 
  elongated 
  

   species 
  have 
  the 
  largest 
  area 
  ; 
  while 
  in 
  those 
  that 
  are 
  transversely 
  

   elongated 
  it 
  is 
  small. 
  As 
  in 
  other 
  Palliobranchs, 
  the 
  area 
  is 
  made 
  

   up 
  of 
  different 
  parts 
  ; 
  two, 
  however, 
  possess 
  exceptional 
  features. 
  

  

  The 
  deltidium 
  (PI. 
  XII. 
  figs. 
  1 
  a, 
  3, 
  7 
  a), 
  carved, 
  as 
  it 
  were, 
  out 
  

   of 
  the 
  central 
  portion 
  of 
  the 
  area, 
  is 
  comparatively 
  level, 
  in 
  relief, 
  

   or 
  an 
  excavation: 
  it 
  is 
  crowded 
  with 
  fine 
  imbricated 
  transverse 
  

   laminae, 
  in 
  general 
  arching 
  towards 
  either 
  the 
  point 
  of 
  the 
  beak 
  

   or 
  the 
  hinge-line. 
  At, 
  and 
  parallel 
  to, 
  its 
  broad 
  end 
  or 
  base, 
  

   there 
  is 
  a 
  narrow 
  inclined 
  band-like 
  space 
  or 
  deltidial 
  slope 
  (b), 
  

   distinguished 
  by 
  a 
  few 
  faint 
  longitudinal 
  lines 
  : 
  it 
  is, 
  however, 
  often 
  

   so 
  slightly 
  developed 
  as 
  to 
  be 
  with 
  difficulty 
  observed. 
  Next 
  to 
  the 
  

   deltidium, 
  or 
  on 
  each 
  side 
  of 
  it, 
  are 
  what 
  may 
  be 
  called 
  the 
  " 
  delti- 
  

  

  * 
  No 
  microscopic 
  sections 
  of 
  the 
  shell 
  have 
  been 
  made 
  ; 
  as 
  in 
  all 
  the 
  speci- 
  

   mens 
  possessing 
  remains 
  of 
  the 
  shell-substance 
  that 
  have 
  been 
  under 
  examina- 
  

   tion, 
  these 
  remains, 
  owing 
  to 
  their 
  being 
  a 
  methylosed 
  replacement 
  of 
  calcite, 
  or 
  

   dolomite, 
  are 
  not 
  in 
  a 
  condition 
  favourable 
  for 
  the 
  retention 
  of 
  any 
  original 
  

   histological 
  characters. 
  On 
  this 
  point, 
  see 
  " 
  Monograph 
  of 
  Spirifer 
  cuspidatus,'' 
  

   Annals 
  and 
  Mag. 
  of 
  Nat. 
  Hist. 
  4th 
  series, 
  vol. 
  ii. 
  July 
  1868 
  ; 
  and 
  " 
  Histology 
  of 
  

   the 
  Test 
  of 
  the 
  Class 
  Palliobranchiata," 
  Transactions 
  of 
  the 
  Royal 
  Irish 
  Aca- 
  

   demy, 
  vol. 
  xxiv. 
  pp. 
  453, 
  455. 
  

  

  t 
  The 
  pedicle- 
  and 
  brachial 
  valves 
  respectively 
  correspond 
  with 
  those 
  usually 
  

   called 
  ventral 
  and 
  dorsal. 
  While 
  describing 
  the 
  various 
  parts 
  which 
  characterize 
  

   the 
  Trimerellids, 
  our 
  references 
  will 
  be 
  confined 
  to 
  the 
  figures 
  or 
  diagrams 
  

   given 
  in 
  PL 
  XII., 
  unless 
  otherwise 
  stated 
  ; 
  and 
  we 
  shall 
  distinguish 
  similarly 
  

   situated 
  parts 
  (irrespective 
  of 
  whether 
  they 
  are 
  homologous, 
  or 
  analogous) 
  of 
  the 
  

   two 
  valves 
  — 
  those 
  of 
  the 
  pedicle- 
  [or 
  ventral] 
  valve 
  by 
  roman 
  letters, 
  and 
  those 
  

   of 
  the 
  brachial 
  [or 
  dorsal] 
  one 
  by 
  italics. 
  It 
  must 
  not 
  be 
  expected 
  that 
  certain 
  of 
  

   these 
  parts 
  will 
  be 
  found 
  unless 
  diligent 
  and 
  prolonged 
  investigations 
  are 
  devoted 
  

   to 
  them 
  : 
  they 
  are 
  to 
  be 
  sought 
  for, 
  not 
  only 
  by 
  allowing 
  the 
  light 
  to 
  fall, 
  on 
  

   places 
  where 
  they 
  occur, 
  in 
  every 
  favourable 
  direction, 
  but 
  even 
  at 
  different 
  

   times 
  of 
  the 
  day, 
  and 
  in 
  different 
  states 
  of 
  the 
  atmosphere. 
  We 
  do 
  not 
  say 
  that 
  

   our 
  diagram 
  figures 
  are, 
  strictly 
  speaking, 
  correct 
  representations 
  ; 
  but 
  we 
  offer 
  

   them 
  as 
  reliable 
  approximations, 
  subject 
  to 
  immaterial 
  corrections. 
  Such, 
  we 
  

   believe, 
  will 
  not 
  vitiate 
  the 
  descriptions 
  herein 
  given 
  of 
  the 
  interior 
  of 
  the 
  shells 
  

   under 
  consideration 
  . 
  

  

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