﻿T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLID^I. 
  147 
  

  

  lines 
  : 
  hinge 
  depressed, 
  or 
  concave 
  in 
  the 
  middle 
  : 
  cardinal 
  facet 
  

   large 
  ; 
  passing 
  down 
  with 
  a 
  considerable 
  inclination 
  to 
  the 
  cardinal 
  

   buttress, 
  from 
  which 
  it 
  is 
  separated 
  by 
  a 
  well-defined 
  edge 
  : 
  plat- 
  

   form 
  extending 
  to 
  about 
  half 
  the 
  length 
  of 
  the 
  valve 
  ; 
  strongly- 
  

   depressed 
  along 
  its 
  surface 
  ; 
  with 
  perpendicular 
  lateral 
  walls 
  : 
  vaults 
  

   extending 
  to 
  a 
  little 
  way 
  underneath 
  the 
  central 
  part 
  of 
  the 
  hinge 
  : 
  

   cardinal 
  buttress 
  low 
  ; 
  standing 
  approximately 
  at 
  a 
  right 
  angle 
  to 
  

   the 
  plane 
  of 
  the 
  shell 
  ; 
  as 
  wide 
  as 
  the 
  platform 
  : 
  umbonal 
  chambers 
  

   extending 
  to 
  some 
  distance 
  under 
  the 
  hinge 
  outside 
  of 
  the 
  deltidium. 
  

   Brachial 
  valve 
  deeper 
  and 
  more 
  convex 
  than 
  the 
  opposite 
  one 
  : 
  hinge 
  

   raised 
  in 
  the 
  middle 
  ; 
  with 
  a 
  strong 
  lamelliform 
  cardinal 
  process 
  : 
  

   platform 
  well 
  developed 
  ; 
  a 
  little 
  longer 
  than 
  that 
  of 
  the 
  opposite 
  

   valve 
  ; 
  its 
  vaidts 
  of 
  considerable 
  depth 
  : 
  median 
  plate 
  much 
  elongated. 
  

   Mr. 
  Billings, 
  to 
  whom 
  palaeontolgists 
  are 
  indebted 
  for 
  the 
  dis- 
  

   covery 
  of 
  this 
  species, 
  and 
  who 
  was 
  the 
  first 
  to 
  give 
  a 
  brief 
  descrip- 
  

   tion 
  of 
  it, 
  accompanied 
  by 
  a 
  figure 
  of 
  a 
  fragment 
  of 
  the 
  internal 
  cast 
  

   of 
  the 
  ventral 
  valve, 
  informs 
  us 
  that 
  the 
  largest 
  specimen 
  he 
  has 
  

   seen 
  is 
  three 
  inches 
  and 
  a 
  half 
  in 
  length, 
  by 
  three 
  inches 
  in 
  width. 
  

   He 
  also 
  states 
  " 
  there 
  is 
  sufficient 
  to 
  show 
  that 
  this 
  species 
  is 
  quite 
  

   distinct 
  from 
  T. 
  grandis. 
  If 
  a 
  section 
  were 
  made 
  across 
  the 
  beak 
  

   of 
  a 
  perfect 
  shell 
  of 
  T. 
  acuminata, 
  it 
  would 
  show 
  four 
  perforations 
  

   arranged 
  in 
  a 
  curve, 
  exactly 
  as 
  in 
  the 
  similar 
  section 
  of 
  the 
  Swedish 
  

   species 
  figured 
  by 
  Dr. 
  Lindstrom. 
  But 
  if 
  the 
  beak 
  of 
  T. 
  grandis 
  

   were 
  cut 
  across, 
  it 
  would 
  show 
  only 
  two 
  orifices, 
  and 
  they 
  would 
  

   be 
  the 
  homologues 
  of 
  the 
  two 
  lateral 
  perforations 
  [our 
  umbonal 
  

   chambers'] 
  in 
  the 
  section 
  of 
  T. 
  aciiminata, 
  because 
  in 
  T. 
  grandis 
  the 
  

   two 
  central 
  tubes 
  [our 
  platform-vaults'] 
  do 
  not 
  extend 
  into 
  the 
  

   beak, 
  but 
  terminate 
  before 
  they 
  reach 
  it." 
  

  

  According 
  to 
  our 
  mode 
  of 
  describing 
  its 
  interior, 
  the 
  species 
  

   under 
  description 
  shows 
  six 
  "perforations" 
  — 
  four 
  in 
  the 
  pedicle-, 
  

   and 
  two 
  in 
  the 
  brachial 
  valve. 
  It 
  is 
  extremely 
  difficult 
  to 
  give 
  all 
  

   the 
  " 
  perforations 
  " 
  in 
  one 
  view 
  : 
  our 
  figure 
  4 
  d 
  , 
  PI. 
  XV. 
  only 
  shows 
  

   those 
  (vaults) 
  belonging 
  to 
  the 
  platforms 
  : 
  there 
  are, 
  however, 
  in 
  the 
  

   pedicle-valve 
  of 
  the 
  specimen 
  represented 
  other 
  two 
  (umbonal 
  

   chambers), 
  one 
  on 
  each 
  side 
  of 
  the 
  umbo. 
  

  

  The 
  deltidium 
  is 
  usually 
  wide 
  and 
  concave, 
  giving 
  the 
  area 
  a 
  

   singular 
  appearance. 
  The 
  hinge 
  is 
  remarkable 
  in 
  having 
  its 
  

   middle 
  in 
  the 
  pedicle-valve 
  concave, 
  and 
  the 
  corresponding 
  part 
  in 
  

   the 
  opposite 
  valve 
  convex 
  : 
  the 
  convexity 
  of 
  the 
  latter 
  part 
  is 
  in- 
  

   creased 
  by 
  the 
  lamelliform 
  cardinal 
  promioence 
  (PL 
  XV. 
  fig. 
  4 
  d 
  ). 
  The 
  

   cardinal 
  facet 
  exhibits 
  the 
  lozenge 
  faintly, 
  though 
  with 
  close 
  inspec- 
  

   tion 
  its 
  bordering 
  lines 
  may 
  be 
  made 
  out. 
  There 
  are 
  two 
  small 
  

   " 
  umbo-lateral 
  scars," 
  one 
  at 
  the 
  entrance, 
  and 
  on 
  the 
  outer 
  side, 
  

   of 
  each 
  umbonal 
  chamber 
  (fig. 
  1, 
  x, 
  PI. 
  XII.). 
  The 
  scars 
  on 
  

   the 
  platform 
  and 
  crescent, 
  though 
  far 
  from 
  satisfactorily 
  defined 
  

   (PI. 
  XV. 
  fig. 
  4"), 
  appear 
  to 
  be 
  formed 
  and 
  arranged 
  as 
  represented 
  in 
  

   PI. 
  XII. 
  fig. 
  1. 
  

  

  T. 
  acuminata 
  occurs 
  chiefly 
  in 
  the 
  state 
  of 
  casts 
  *, 
  in 
  the 
  Guelph 
  

  

  * 
  We 
  have 
  succeeded 
  in 
  making 
  some 
  very 
  good 
  gutta-percha 
  moulds 
  from 
  

   some 
  of 
  these 
  casts. 
  

  

  