﻿144 
  T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLID2E. 
  

  

  Median 
  plate 
  occurring 
  generally 
  in 
  both 
  valves 
  ; 
  longest 
  in 
  the 
  

   brachial 
  one. 
  

  

  Type 
  species 
  Trimerella 
  grandis, 
  Billings. 
  

  

  The 
  genus 
  varies 
  much 
  in 
  some 
  of 
  its 
  chief 
  characteristics. 
  The 
  

   umbo 
  of 
  the 
  pedicle-valve, 
  usually 
  large, 
  subconical, 
  and 
  occasionally 
  

   double-chambered, 
  is 
  exceptionally 
  flattened 
  in 
  T. 
  Billingsii 
  and 
  T. 
  

   galtensis 
  ; 
  and 
  the 
  hinge 
  of 
  these 
  species 
  is 
  consequently 
  very 
  slightly 
  

   elevated 
  : 
  on 
  the 
  contrary, 
  in 
  T. 
  Dalli 
  the 
  umbo 
  is 
  prominently 
  

   double-chambered, 
  as 
  in 
  the 
  next 
  genus. 
  The 
  hinge 
  has 
  a 
  central 
  

   part, 
  projecting 
  in 
  certain 
  species, 
  and 
  excavated 
  in 
  others. 
  In 
  T. 
  

   Billingsii, 
  and 
  T. 
  galtensis, 
  the 
  platforms 
  are 
  slightly 
  raised 
  or 
  

   depressed, 
  a 
  peculiarity 
  which 
  remarkably 
  differentiates 
  them 
  from 
  

   the 
  typical 
  species. 
  

  

  The 
  genus 
  Ehynobolus, 
  proposed 
  by 
  Hall 
  for 
  Obolus 
  galtensis, 
  

   appears 
  at 
  first 
  sight 
  to 
  possess 
  sufficient 
  distinctive 
  characters 
  ; 
  

   but, 
  comparing 
  its 
  type 
  with 
  T. 
  Billingsii, 
  we 
  cannot 
  see 
  how 
  

   it 
  can 
  be 
  separated 
  from 
  the 
  latter 
  generically 
  : 
  both 
  species 
  are 
  

   similarly 
  characterized 
  by 
  flat 
  longitudinally-elongated 
  valves, 
  a 
  

   flattened 
  umbo 
  in 
  the 
  pedicle-valve, 
  a 
  large 
  area 
  and 
  deltidium, 
  and 
  

   solid 
  or 
  slightly 
  elevated 
  platforms. 
  The 
  distinctions 
  made 
  out 
  by 
  

   Hall 
  for 
  T. 
  galtensis 
  appear 
  to 
  us 
  to 
  be 
  insufficient 
  ; 
  as 
  through 
  the 
  

   depression 
  of 
  its 
  platforms 
  the 
  anterior 
  portion 
  of 
  these 
  parts, 
  in- 
  

   cluding 
  their 
  scars, 
  show 
  nothing 
  more 
  than 
  what 
  is 
  seen 
  in 
  T. 
  

   Billingsii. 
  Both 
  species 
  may 
  be 
  considered 
  aberrant 
  forms 
  of 
  the 
  

   genus 
  ; 
  the 
  former 
  departing 
  somewhat 
  further 
  from 
  its 
  type 
  than 
  

   the 
  latter. 
  

  

  Trimerella 
  grandis, 
  Bill. 
  PL 
  XIII. 
  figs. 
  2, 
  3. 
  

  

  Trimerella 
  grandis, 
  Bill. 
  Geol. 
  Survey 
  of 
  Canada, 
  Palaeontology, 
  

   p. 
  166, 
  fig. 
  151, 
  21 
  Jan. 
  1862. 
  Dall, 
  American 
  Journal 
  of 
  

   Conch, 
  vol. 
  ii. 
  part 
  2, 
  p. 
  160, 
  1870, 
  vol. 
  vii. 
  p. 
  82, 
  1871. 
  

   Quenstedt, 
  Petrefactenkunde, 
  p. 
  172, 
  pi. 
  lxi. 
  fig. 
  42, 
  1871. 
  

   Hall, 
  Explanation 
  of 
  figures 
  in 
  " 
  Notes 
  on 
  some 
  new 
  or 
  

   imperfectly 
  known 
  forms 
  among 
  the 
  Brachiopoda," 
  pi. 
  xiii. 
  

   figs. 
  11-16, 
  1872. 
  Dav. 
  and 
  King, 
  Report 
  of 
  Brighton 
  Meet- 
  

   ing 
  of 
  Br. 
  Assoc. 
  20 
  August, 
  1872. 
  

   Shell 
  much 
  longer 
  than 
  wide 
  ; 
  broadly 
  rounded 
  anteriorly 
  ; 
  taper- 
  

   ing 
  posteriorly 
  to 
  an 
  almost 
  pointed 
  beak; 
  greatest 
  breadth 
  at 
  about 
  

   two 
  thirds 
  from 
  the 
  beak 
  ; 
  surface 
  smooth. 
  Pedicle-valve 
  — 
  beak 
  

   slightly 
  curved 
  : 
  area 
  a 
  little 
  more 
  than 
  a 
  quarter 
  of 
  the 
  length 
  of 
  

   the 
  shell 
  : 
  deltidium 
  rather 
  raised, 
  but 
  longitudinally 
  depressed 
  along 
  

   the 
  middle 
  : 
  deltidial 
  ridges 
  well 
  defined 
  : 
  platform 
  rather 
  wide 
  ; 
  oc- 
  

   cupying 
  nearly 
  the 
  entire 
  posterior 
  half 
  of 
  the 
  shell 
  ; 
  biconvex 
  ; 
  

   and 
  most 
  elevated 
  at 
  its 
  anterior 
  margin 
  ; 
  its 
  vaults 
  of 
  greater, 
  

   or 
  lesser 
  depth 
  : 
  median 
  plate 
  extending 
  to 
  a 
  short 
  distance 
  forward 
  

   along 
  the 
  bottom 
  of 
  the 
  shell 
  : 
  crescent 
  of 
  large 
  size 
  : 
  archlet 
  about 
  

   the 
  width 
  of 
  the 
  platform 
  ; 
  and 
  passing 
  a 
  little 
  way 
  in 
  advance 
  of 
  

   the 
  median 
  plate. 
  Brachial 
  valve 
  — 
  hinge 
  with 
  a 
  slightly 
  projecting 
  

   cardinal 
  process 
  : 
  crescent 
  with 
  sides 
  and 
  ends 
  of 
  considerable 
  width, 
  

   and 
  well 
  outlined 
  : 
  platform 
  comparatively 
  narrow, 
  occupying 
  half 
  

  

  