﻿128 
  T. 
  DAVIDSON 
  AND 
  W. 
  XING 
  ON 
  THE 
  TRIMEEELLID^. 
  

  

  dial 
  ridges 
  " 
  (c), 
  prominent 
  in 
  some 
  species, 
  more 
  or 
  less 
  flattened 
  

   in 
  others, 
  and 
  marked 
  with 
  arching 
  laminae 
  : 
  they 
  are 
  each 
  defined 
  

   on 
  its 
  outer 
  side 
  by 
  a 
  well-marked 
  incised 
  line. 
  The 
  deltidial 
  ridges 
  

   terminate 
  near 
  the 
  hinge-margin, 
  each 
  as 
  a 
  cardinal 
  callosity 
  (e), 
  

   which 
  is 
  extremely 
  variable 
  in 
  form 
  and 
  size. 
  The 
  areal 
  borders 
  

   (d), 
  usually 
  narrow 
  and 
  transversely 
  scored, 
  differ 
  in 
  nothing 
  from 
  

   those 
  in 
  other 
  Palliobranchs. 
  

  

  In 
  front 
  of 
  the 
  deltidial 
  slope 
  is 
  situated 
  a 
  tolerably 
  well-defined 
  

   cardinal 
  facet 
  (f), 
  the 
  long 
  axis 
  of 
  which 
  corresponds 
  to 
  the 
  trans- 
  

   verse 
  direction 
  of 
  the 
  hinge-line 
  : 
  it 
  varies 
  much 
  in 
  size 
  according 
  

   to 
  species. 
  This 
  part 
  generally 
  inclines 
  or 
  falls 
  towards 
  the 
  cavity 
  

   of 
  the 
  shell 
  : 
  in 
  one 
  species, 
  however, 
  as 
  will 
  be 
  noticed 
  hereafter, 
  

   the 
  inclination 
  is 
  in 
  the 
  opposite 
  direction. 
  Although 
  in 
  some 
  

   cases 
  the 
  cardinal 
  facet 
  passes 
  without 
  any 
  marked 
  break 
  into 
  the 
  

   inner 
  surface 
  of 
  the 
  valve, 
  it 
  is 
  often 
  raised 
  by 
  the 
  hinge 
  underneath 
  

   becoming 
  thickened. 
  

  

  The 
  part 
  beneath 
  the 
  cardinal 
  facet 
  thus 
  raised, 
  while 
  in 
  many 
  

   species 
  its 
  thickness 
  is 
  in 
  no 
  way 
  diminished 
  vertically, 
  is 
  in 
  some 
  

   others 
  materially 
  reduced 
  in 
  lateral 
  dimensions, 
  thereby 
  presenting 
  

   itself 
  under 
  the 
  form 
  of 
  a 
  thin 
  vertical 
  hinge-supporting 
  plate 
  or 
  

   cardinal 
  buttress 
  (h). 
  

  

  The 
  parts 
  belonging 
  to 
  the 
  hinge 
  of 
  the 
  brachial 
  valve 
  are 
  rude 
  

   and 
  extremely 
  variable 
  in 
  form 
  : 
  one 
  of 
  them 
  may 
  be 
  variable 
  in 
  

   position. 
  The 
  central 
  portion 
  of 
  the 
  hinge 
  in 
  certain 
  species 
  shows 
  

   nothing 
  remarkable, 
  no 
  marks 
  or 
  prominences 
  — 
  due 
  possibly 
  to 
  ero- 
  

   sion 
  ; 
  but 
  in 
  others 
  it 
  distinctly 
  displays 
  an 
  excavation 
  or 
  cardinal 
  

   scar 
  (PL 
  XII. 
  fig. 
  2v), 
  as 
  in 
  T. 
  ohioensis, 
  also 
  apparently 
  in 
  T. 
  Dalli 
  

   (PI. 
  XV. 
  fig. 
  2). 
  The 
  excavation 
  is 
  bicupped 
  in 
  T. 
  ohioensis 
  (PI. 
  XVI. 
  

   fig. 
  7) 
  ; 
  but 
  the 
  same 
  species 
  also 
  presents 
  it 
  transversely 
  grooved 
  

   (PI. 
  XVI. 
  figs. 
  4, 
  5, 
  6). 
  There 
  is 
  no 
  prominence 
  in 
  these 
  cases. 
  

   In 
  some 
  other 
  species, 
  however, 
  the 
  cardinal 
  scar, 
  though 
  in 
  general 
  

   badly 
  seen, 
  appears 
  to 
  be 
  situated 
  on 
  an 
  elevation 
  — 
  -rounded, 
  or 
  

   squared, 
  and 
  more 
  or 
  less 
  developed, 
  according 
  to 
  species. 
  Nay, 
  in 
  

   the 
  same 
  species, 
  as 
  in 
  T. 
  Lindstromi, 
  the 
  elevation 
  varies 
  much 
  iD 
  

   size, 
  certain 
  individuals 
  having 
  it 
  very 
  small, 
  others 
  very 
  large 
  and 
  

   standing 
  out 
  like 
  a 
  great 
  tooth 
  — 
  so 
  large 
  that 
  it 
  must 
  have 
  dipped 
  

   deeply 
  into 
  the 
  umbonal 
  cavity 
  of 
  the 
  pedicle-valve 
  : 
  fig. 
  3, 
  PI. 
  XIV., 
  

   represents 
  the 
  elevation 
  in 
  its 
  median 
  size. 
  In 
  short, 
  the 
  cardinal 
  

   scar 
  is 
  so 
  variable 
  that 
  to 
  give 
  a 
  description 
  of 
  it 
  applicable 
  to 
  any 
  

   one 
  species 
  would 
  be 
  a 
  somewhat 
  difficult 
  task. 
  

  

  The 
  hinge 
  of 
  the 
  brachial 
  valve 
  is 
  further 
  characterized 
  by 
  another 
  

   variable 
  part. 
  Though 
  absent 
  in 
  several 
  species, 
  there 
  exist 
  in 
  others 
  

   two 
  depressions 
  or 
  cardinal 
  sockets 
  (e) 
  on 
  the 
  hinge, 
  one 
  on 
  each 
  side 
  : 
  

   when 
  well 
  defined 
  (which, 
  however, 
  is 
  seldom 
  the 
  case) 
  by 
  a 
  promi- 
  

   nent 
  border 
  on 
  the 
  inner 
  side, 
  the 
  cardinal 
  sockets 
  appear 
  to 
  have 
  

   received 
  the 
  cardinal 
  callosities 
  belonging 
  to 
  the 
  pedicle-valve*. 
  

  

  * 
  Lindstrorn, 
  who 
  has 
  represented 
  the 
  lateral 
  margins 
  of- 
  a 
  species 
  of 
  Trime- 
  

   retta 
  "grooved 
  by 
  a 
  furrow 
  which 
  helps 
  to 
  close 
  the 
  valves 
  more 
  tightly" 
  (G-eol. 
  

   Mag. 
  vol. 
  v. 
  p. 
  14), 
  seems 
  to 
  think 
  that 
  this 
  was 
  the 
  only 
  provision 
  the 
  genus 
  

   had 
  for 
  a 
  dentary 
  apparatus. 
  We 
  cannot 
  help 
  thinking 
  that 
  he 
  has 
  taken 
  too 
  

  

  