﻿152 
  T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLID^. 
  

  

  Mr. 
  Dall 
  also 
  published 
  a 
  very 
  brief 
  description 
  of 
  the 
  ventral 
  valve 
  

   under 
  the 
  designation 
  of 
  Trimerella 
  minor, 
  n. 
  sp. 
  ? 
  * 
  Later, 
  in 
  

   March 
  1872, 
  Prof. 
  Hall 
  noticed 
  this 
  shell, 
  calling 
  it 
  Ehynobolus 
  ; 
  

   and 
  in 
  December 
  of 
  the 
  same 
  year 
  he 
  gave 
  a 
  fuller 
  account 
  of 
  it. 
  

  

  Only 
  slight 
  traces 
  of 
  the 
  crescent 
  have 
  been 
  observed 
  in 
  the 
  

   pedicle-valve. 
  The 
  hinge 
  and 
  beak 
  bear 
  considerable 
  resemblance 
  

   to 
  those 
  of 
  T. 
  Billingsii. 
  We 
  suspect 
  that 
  Dall, 
  Hall, 
  and 
  others 
  

   view 
  what 
  we 
  call 
  the 
  deltidium 
  in 
  a 
  different 
  light 
  from 
  ourselves-^ 
  

   they 
  taking 
  the 
  median 
  furrow 
  to 
  represent 
  this 
  part 
  ; 
  we, 
  on 
  the 
  

   contrary, 
  look 
  upon 
  the 
  whole 
  as 
  forming 
  the 
  deltidium. 
  There 
  are 
  

   only 
  slight 
  traces 
  of 
  the 
  deltidial 
  ridges 
  : 
  Hall, 
  however, 
  has 
  repre- 
  

   sented 
  one 
  of 
  them 
  prominently 
  on 
  the 
  left 
  side 
  of 
  the 
  area. 
  The 
  scars 
  

   on 
  the 
  platform 
  of 
  the 
  brachial 
  valve 
  form 
  three 
  sets 
  : 
  — 
  one 
  on 
  the 
  

   ante-median 
  portion, 
  which 
  is 
  circular 
  or 
  slightly 
  lozenge-shaped 
  ; 
  

   the 
  second 
  or 
  median 
  set 
  immediately 
  behind 
  (partly 
  represented 
  in 
  

   fig. 
  4% 
  Plate 
  XIX.) 
  ; 
  the 
  third 
  with 
  its 
  two 
  members 
  separated 
  by 
  

   the 
  last 
  set, 
  and 
  diverging 
  backwards. 
  

  

  Trimerella 
  galtensis 
  has 
  been 
  placed 
  in 
  different 
  genera. 
  Billings 
  at 
  

   first 
  considered 
  it 
  to 
  belong 
  to 
  Obolus, 
  but 
  afterwards 
  included 
  it 
  in 
  his 
  

   genus 
  Obolellina 
  : 
  Dall 
  placed 
  it 
  in 
  Trimerella 
  : 
  Hall 
  constructed 
  his 
  

   genus 
  Rhynobolus 
  for 
  it: 
  and 
  the 
  present 
  writers, 
  on 
  a 
  former 
  occasion, 
  

   allocated 
  it 
  in 
  Dinobolus 
  ! 
  It 
  certainly 
  differs 
  in 
  some 
  important 
  fea- 
  

   tures 
  from 
  the 
  typical 
  forms 
  of 
  Trimerella, 
  having 
  partly 
  sunk 
  plat- 
  

   forms 
  and 
  no 
  vaults 
  ; 
  but 
  so 
  does 
  T. 
  Billingsii, 
  though 
  not 
  to 
  the 
  same 
  

   extent. 
  In 
  the 
  last 
  species, 
  as 
  already 
  noticed, 
  the 
  platforms 
  are 
  

   considerably 
  reduced 
  (at 
  least 
  the 
  one 
  belonging 
  to 
  the 
  pedicle- 
  valve), 
  

   nearly 
  to 
  the 
  exclusion 
  of 
  the 
  vaults. 
  

  

  The 
  absence 
  of 
  vaults 
  in 
  Monomerella, 
  it 
  may 
  be 
  readily 
  seen, 
  

   causes 
  the 
  platform 
  in 
  the 
  pedicle-valve 
  to 
  be 
  considerably 
  modified 
  ; 
  

   and 
  evidently 
  their 
  absence 
  in 
  the 
  present 
  species 
  has 
  given 
  the 
  

   platform 
  of 
  the 
  same 
  valve 
  its 
  peculiar 
  character. 
  We 
  are 
  quite 
  

   ready 
  to 
  admit 
  that 
  in 
  its 
  apophysary 
  system 
  T. 
  galtensis 
  offers 
  a 
  

   close 
  approach 
  to 
  Monomerella 
  ; 
  also, 
  looking 
  at 
  the 
  scars 
  on 
  the 
  

   platform 
  of 
  the 
  brachial 
  valve, 
  that 
  it 
  very 
  closely 
  agrees 
  in 
  this 
  

   respect 
  with 
  Dinobolus 
  : 
  but, 
  with 
  the 
  exception 
  of 
  these 
  scars, 
  we 
  

   see 
  very 
  little 
  resemblance 
  between 
  this 
  and 
  any 
  of 
  the 
  known 
  

   species 
  of 
  the 
  latter 
  genus. 
  Were 
  it 
  not 
  that 
  its 
  umbonal 
  characters 
  

   are 
  characteristic 
  of 
  Trimerella, 
  we 
  might 
  have 
  been 
  induced 
  to 
  give 
  

   the 
  present 
  species 
  a 
  different 
  generic 
  allocation 
  : 
  our 
  views 
  in 
  this 
  

   matter 
  are 
  generally 
  influenced 
  by 
  the 
  peculiar 
  characters 
  of 
  

   Trimerella 
  Billingsii 
  : 
  besides, 
  excepting 
  the 
  form 
  of 
  the 
  scars 
  on 
  the 
  

   platform, 
  we 
  see 
  very 
  little 
  resemblance 
  between 
  T. 
  galtensis, 
  and' 
  

   any 
  of 
  the 
  known 
  species 
  of 
  Dinobolus. 
  Another 
  most 
  important 
  

   difference 
  prevails 
  : 
  in 
  the 
  shell 
  under 
  description 
  the 
  crescent 
  shows 
  

   no 
  appearance 
  of 
  the 
  composite 
  structure 
  (the 
  strong 
  indentations) 
  

   which 
  characterizes 
  the 
  inner 
  border 
  of 
  the 
  sides 
  of 
  the 
  same 
  part 
  

  

  * 
  Both 
  Mr. 
  Billings 
  and 
  the 
  authors 
  of 
  this 
  paper, 
  who 
  have 
  seen 
  the 
  speci- 
  

   men 
  upon 
  which 
  it 
  was 
  established, 
  are 
  able 
  to 
  assert 
  that 
  Mr. 
  Dall's 
  Trime- 
  

   rella 
  minor 
  was 
  founded 
  on 
  one 
  of 
  the 
  typical 
  forms 
  upon 
  which 
  Mr. 
  Billings 
  

   had 
  originally 
  instituted 
  his 
  Obolus 
  galtensis. 
  

  

  