﻿T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TEIMEKELLID^. 
  149 
  

  

  elevated. 
  In 
  the 
  latter 
  state 
  it 
  appears 
  to 
  have 
  fitted 
  like 
  a 
  tooth 
  

   into 
  a 
  deep 
  excavation 
  in 
  the 
  hinge 
  or 
  near 
  the 
  origin 
  of 
  the 
  plat- 
  

   form 
  (PI. 
  XIV. 
  fig. 
  5), 
  thereby 
  apparently 
  effacing 
  both 
  the 
  cardinal 
  

   facet 
  and 
  buttress 
  : 
  this 
  excavation 
  is 
  bounded 
  posteriorly 
  by 
  a 
  

   perpendicular 
  wall. 
  "Whether 
  the 
  facet 
  was 
  situated 
  on 
  the 
  wall 
  is 
  

   a 
  point 
  on 
  which 
  it 
  is 
  difficult 
  to 
  offer 
  any 
  more 
  than 
  a 
  suspicion. 
  

   A 
  similar 
  difficulty 
  attaches 
  to 
  the 
  excavation 
  ; 
  was 
  it 
  produced 
  by 
  

   the 
  attachment 
  of 
  the 
  umbonal 
  muscle 
  ? 
  A 
  variation 
  in 
  the 
  opposite 
  

   extreme 
  is 
  represented 
  in 
  fig. 
  2, 
  PI. 
  XIV., 
  which 
  shows 
  the 
  facet, 
  with 
  

   the 
  lozenge, 
  occupying 
  a 
  place 
  seemingly 
  corresponding 
  to 
  the 
  bottom 
  

   of 
  the 
  excavation 
  seen 
  in 
  fig. 
  5. 
  But 
  certain 
  appearances, 
  presented 
  

   to 
  us, 
  favour 
  the 
  suspicion 
  that 
  the 
  facet 
  and 
  lozenge 
  were 
  situated 
  

   on 
  the 
  posterior 
  wall, 
  rather 
  than 
  at 
  the 
  bottom 
  of 
  the 
  excavation. 
  

   In 
  most 
  of 
  the 
  Trimerellids 
  the 
  facet 
  dips 
  forward 
  from 
  the 
  delti- 
  

   dium, 
  — 
  suddenly, 
  we 
  should 
  say, 
  in 
  the 
  present 
  species, 
  if 
  represented 
  

   only 
  by 
  such 
  individuals 
  as 
  that 
  in 
  fig. 
  5 
  ; 
  but 
  other 
  specimens 
  

   exhibit 
  it 
  with 
  a 
  moderate 
  inclination, 
  as 
  in 
  T. 
  acuminata 
  (PI. 
  XV. 
  

   fig. 
  4 
  a) 
  ; 
  while 
  in 
  the 
  one 
  represented 
  in 
  fig. 
  2, 
  PI. 
  XIV., 
  it 
  actually 
  

   inclines 
  backwards 
  or 
  in 
  the 
  opposite 
  direction. 
  The 
  umbonal 
  

   muscle 
  may 
  or 
  may 
  not 
  have 
  been 
  attached 
  to 
  the 
  bottom 
  of 
  the 
  

   excavation 
  in 
  fig. 
  5 
  specimen 
  ; 
  but 
  it 
  is 
  difficult 
  to 
  point 
  out 
  its 
  

   locality 
  of 
  attachment 
  in 
  fig. 
  2 
  specimen 
  : 
  possibly 
  it 
  was 
  attached 
  at 
  

   the 
  origin 
  of 
  the 
  platform, 
  where 
  occasionally 
  there 
  is 
  something 
  like 
  

   a 
  scar. 
  

  

  The 
  dentary 
  system, 
  taking 
  it 
  to 
  be 
  represented 
  by 
  the 
  deltidial 
  

   callosities 
  in 
  the 
  pedicle-valve, 
  and 
  the 
  cardinal 
  sockets 
  in 
  the 
  oppo- 
  

   site 
  one, 
  is 
  equally 
  variable 
  ; 
  though, 
  possibly, 
  some 
  of 
  the 
  appear- 
  

   ances 
  of 
  variation 
  may 
  be 
  due 
  to 
  erosion. 
  The 
  callosities 
  are 
  strong 
  

   bosses 
  in 
  certain 
  specimens, 
  and 
  thick 
  plates 
  in 
  others 
  ; 
  it 
  is 
  therefore 
  

   possible 
  that 
  the 
  appearances 
  which 
  led 
  Lindstrom 
  to 
  believe 
  in 
  the 
  

   groove-and-ridge 
  mode 
  of 
  articulation 
  were 
  presented 
  by 
  an 
  extreme 
  

   case 
  of 
  lamelliformity 
  *. 
  

  

  The 
  umbonal 
  chambers 
  are 
  of 
  small 
  calibre, 
  being 
  little 
  more 
  than 
  

   tubular 
  perforations. 
  Lindstrom 
  has 
  given 
  a 
  figure 
  f 
  showing 
  their 
  

   presence 
  in 
  an 
  early 
  stage 
  of 
  growth, 
  also 
  that 
  their 
  tubular 
  form 
  

   was 
  persistent. 
  Such 
  a 
  case 
  strongly 
  favours 
  the 
  idea 
  that 
  the 
  

   chambers 
  were 
  for 
  some 
  particular 
  purpose 
  ; 
  otherwise 
  it 
  is 
  difficult 
  

   to 
  understand 
  why 
  they 
  have 
  not 
  been 
  filled 
  up 
  with 
  organic 
  accre- 
  

   tions. 
  The 
  umbonal 
  cavity 
  of 
  the 
  brachial 
  valve 
  in 
  the 
  specimen 
  

   represented 
  in 
  our 
  fig. 
  7, 
  PI. 
  XIV. 
  is 
  doubly 
  chambered; 
  the 
  large 
  size 
  

   of 
  the 
  conical 
  prominences 
  (casts), 
  on 
  each 
  side, 
  shows 
  that 
  the 
  

   chambers 
  have 
  considerable 
  depth 
  and 
  width. 
  

  

  The 
  platforms 
  do 
  not 
  appear 
  to 
  be 
  so 
  large 
  as 
  in 
  the 
  last 
  species 
  : 
  

   they 
  resemble 
  more 
  closely 
  those 
  of 
  T. 
  BiUingsii, 
  particularly 
  in 
  

   having 
  short 
  vaults. 
  The 
  scars 
  are 
  represented 
  with 
  approximate 
  

   correctness 
  in 
  fig. 
  2, 
  PI. 
  XIV. 
  The 
  submarginal 
  impressions, 
  generally 
  

   indistinct, 
  are 
  resolvable 
  into 
  the 
  typical 
  crescent. 
  

  

  * 
  The 
  specimen 
  in 
  question, 
  which 
  we 
  have 
  not 
  seen, 
  is 
  preserved 
  in 
  the 
  Mu- 
  

   seum 
  of 
  Stockholm. 
  

  

  f 
  Geol. 
  Mag. 
  vol. 
  v. 
  pi. 
  xx. 
  fig. 
  9. 
  

  

  