﻿130 
  T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLIDJE. 
  

  

  in 
  fig. 
  7, 
  PI. 
  XIV. 
  Nothing 
  is 
  more 
  singular 
  than 
  the 
  aspect 
  of 
  the 
  

   interior 
  of 
  the 
  typical 
  Trimerellids 
  — 
  some, 
  as 
  will 
  now 
  be 
  under- 
  

   stood, 
  having 
  two 
  specialized 
  cavities 
  (Monomerella) 
  , 
  others 
  four 
  

   (PI. 
  XIY. 
  fig. 
  6, 
  PI. 
  XV. 
  fig. 
  4 
  d 
  &c), 
  and 
  others 
  six 
  (T. 
  Lindstromi, 
  

   T. 
  Dalli, 
  &c). 
  

  

  It 
  may 
  be 
  stated, 
  subject 
  to 
  correction, 
  that 
  tbe 
  platform 
  in 
  each 
  

   valve 
  is 
  marked 
  by 
  four 
  sets 
  of 
  scars, 
  which, 
  for 
  the 
  present, 
  we 
  

   shall 
  merely 
  distinguish 
  by 
  the 
  names 
  medians 
  (m, 
  m), 
  anteriors 
  

   (n, 
  n), 
  latercds 
  (o, 
  o), 
  and 
  postmedians 
  (p,p)- 
  The 
  postmedians, 
  we 
  

   believe, 
  are 
  a 
  set 
  having 
  no 
  relation 
  to 
  the 
  others. 
  

  

  In 
  addition 
  to 
  the 
  foregoing, 
  the 
  entire 
  family 
  is 
  characterized 
  by 
  

   a 
  submarginal 
  acescent 
  (q, 
  r, 
  s, 
  q, 
  r, 
  s) 
  confined 
  to 
  the 
  posterior 
  half 
  

   of 
  each 
  valve. 
  It 
  varies 
  to 
  some 
  extent 
  in 
  each 
  genus, 
  also 
  in 
  dif- 
  

   ferent 
  species 
  : 
  even 
  different 
  individuals 
  of 
  the 
  same 
  species 
  do 
  not 
  

   unfrequently 
  present 
  variations 
  that 
  seem 
  to 
  depend 
  on 
  differences 
  

   of 
  age, 
  thickness 
  of 
  the 
  valves, 
  and 
  other 
  peculiarities. 
  As 
  a 
  conse- 
  

   quence, 
  this 
  part, 
  or 
  merely 
  a 
  portion 
  of 
  it, 
  may 
  present 
  itself 
  in 
  relief, 
  

   excavated, 
  obscurely, 
  often 
  partially 
  in 
  any 
  of 
  these 
  conditions, 
  or 
  

   complicated 
  by 
  successive 
  enlargements 
  of 
  its 
  area 
  ; 
  so 
  that 
  we 
  feel 
  a 
  

   difficulty 
  in 
  representing 
  it 
  otherwise 
  than 
  diagrammatically, 
  or 
  in 
  

   giving 
  a 
  description 
  that 
  is 
  strictly 
  accurate 
  when 
  applied 
  to 
  any 
  

   single 
  case. 
  Closing 
  these 
  preliminary 
  remarks, 
  the 
  crescent 
  may 
  

   be 
  described 
  as 
  formed 
  of 
  three 
  portions 
  — 
  the 
  crown 
  (q, 
  q), 
  sides 
  

   (r, 
  r), 
  and 
  ends 
  (s, 
  s). 
  The 
  crown 
  consists 
  of 
  a 
  line 
  running 
  along 
  

   the 
  hinge, 
  from 
  each 
  side 
  of 
  which 
  it 
  descends 
  into 
  the 
  cavity 
  of 
  the 
  

   valves 
  at 
  the 
  origin 
  of 
  their 
  postlateral 
  margins 
  : 
  this 
  portion, 
  in 
  

   the 
  brachial 
  valve, 
  is 
  pointed 
  in 
  the 
  middle 
  of 
  the 
  hinge 
  (PI 
  XII, 
  

   fig. 
  6, 
  q) 
  ; 
  while, 
  in 
  the 
  pedicle-valve, 
  it 
  passes 
  along 
  and 
  close 
  to 
  

   the 
  outer 
  edge 
  of 
  the 
  cardinal 
  facet. 
  The 
  sides, 
  which 
  are 
  pointed 
  

   oval 
  in 
  shape 
  in 
  the 
  typical 
  genus, 
  pass 
  forward 
  along 
  and 
  a 
  little 
  

   within 
  the 
  postlateral 
  margins 
  of 
  the 
  valves. 
  The 
  ends, 
  for 
  the 
  

   most 
  part 
  obscurely 
  defined, 
  occupy 
  the 
  space 
  between 
  the 
  widest 
  

   portion 
  of 
  the 
  platform 
  and 
  the 
  adjacent 
  margin 
  of 
  the 
  valve. 
  

  

  Somewhat 
  similar 
  in 
  outline 
  to 
  the 
  crown 
  is 
  a 
  submarginal 
  impress 
  

   sion 
  occurring 
  in 
  the 
  anterior 
  half 
  of 
  both 
  valves 
  : 
  it 
  resembles 
  a 
  small 
  

   arch 
  or 
  archlet 
  (u, 
  u), 
  is 
  simple 
  in 
  form, 
  consists 
  of 
  two 
  halves, 
  has 
  its 
  

   two 
  bases 
  attached 
  one 
  to 
  each 
  end 
  of 
  the 
  crescent, 
  and 
  throws 
  itself 
  

   forward 
  to 
  within 
  a 
  short 
  distance 
  of 
  the 
  anterior 
  margin 
  of 
  the 
  valves, 
  

   apparently 
  to 
  a 
  less 
  extent 
  in 
  the 
  pedicle 
  than 
  in 
  the 
  brachial 
  one. 
  

  

  On 
  each 
  side 
  of 
  both 
  valves 
  one 
  member 
  of 
  a 
  pair 
  of 
  linear 
  trans-? 
  

   verse 
  sears 
  (t, 
  t) 
  strikes 
  off 
  from 
  near 
  the 
  point 
  of 
  junction 
  of 
  the 
  cres-^ 
  

   cent 
  and 
  archlet 
  to 
  the 
  widest 
  part 
  of 
  the 
  platform, 
  thence 
  in 
  front 
  of 
  

   the 
  latter 
  part 
  to 
  the 
  median 
  plate 
  : 
  the 
  other 
  member 
  makes 
  a 
  similar 
  

   traverse 
  on 
  the 
  opposite 
  side, 
  and 
  terminates 
  at 
  the 
  same 
  place. 
  

  

  Reverting 
  to 
  the 
  hinge 
  of 
  the 
  pedicle-valve, 
  there 
  is 
  situated 
  on 
  

   the 
  cardinal 
  facet 
  an 
  oval-shaped 
  scar 
  or 
  lozenge 
  (g), 
  having 
  its 
  long 
  

   axis 
  in 
  the 
  transverse 
  direction 
  of 
  the 
  valve. 
  It 
  is 
  formed 
  poste- 
  

   riorly 
  by 
  a 
  line 
  that 
  runs 
  parallel 
  and 
  near 
  to 
  the 
  deltidial 
  slope, 
  and 
  

   anteriorly 
  by 
  what 
  may 
  be 
  supposed 
  to 
  be 
  another 
  line 
  running 
  close 
  

   to 
  the 
  crown 
  of 
  the 
  crescent. 
  There 
  is 
  much 
  obscurity 
  in 
  the 
  part 
  

  

  