﻿T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLID.2E. 
  151 
  

  

  deltidial 
  ridges, 
  and 
  in 
  front 
  by 
  a 
  tolerably 
  well-defined 
  deltidial 
  slope. 
  

   The 
  cardinal 
  facet 
  appears 
  to 
  be 
  small. 
  The 
  platform 
  is 
  solid, 
  long, 
  

   and 
  large, 
  but 
  of 
  slight 
  elevation 
  : 
  it 
  has 
  a 
  raised 
  ^-shaped 
  anterior 
  

   margin 
  the 
  side 
  divisions 
  answering 
  to 
  the 
  openings 
  of 
  rudimentary 
  

   vaults. 
  The 
  median 
  plate 
  scarcely 
  exists, 
  and 
  extends 
  to 
  no 
  more 
  

   than 
  a 
  line 
  in 
  advance 
  of 
  the 
  anterior 
  termination 
  of 
  the 
  platform. 
  

   Scars 
  occupy 
  the 
  anterior 
  half 
  of 
  the 
  platform 
  : 
  some 
  strongly 
  marked 
  

   are 
  situated 
  near 
  its 
  front 
  border. 
  The 
  crescents 
  are 
  faintly 
  indicated. 
  

  

  This 
  remarkably 
  compressed 
  species 
  is 
  one 
  of 
  those 
  aberrant 
  

   forms 
  which 
  render 
  it 
  difficult 
  to 
  diagnose 
  a 
  generic 
  group, 
  or 
  to 
  

   determine 
  what 
  genera 
  they 
  should 
  be 
  placed 
  in. 
  It 
  has 
  much 
  the 
  

   appearance 
  of 
  Trimerella 
  grandis; 
  and 
  therefore 
  seems 
  naturally 
  to 
  fall 
  

   into 
  the 
  same 
  genus. 
  Another 
  species, 
  next 
  to 
  be 
  described, 
  stands 
  out 
  

   prominently 
  as 
  a 
  still 
  more 
  aberrant 
  form. 
  T. 
  Billingsii 
  does 
  not 
  

   possess 
  any 
  thing 
  like 
  a 
  dentary 
  system 
  : 
  its 
  pedicle-valve 
  has 
  no 
  

   median 
  plate 
  ; 
  but 
  the 
  centre 
  of 
  the 
  anterior 
  margin 
  of 
  the 
  platform 
  

   is 
  somewhat 
  prolonged. 
  Besides 
  the 
  scars 
  on 
  the 
  anterior 
  half 
  of 
  

   the 
  platform, 
  there 
  are 
  linear 
  impressions 
  (answering 
  to 
  the 
  trans- 
  

   verse 
  scars, 
  t 
  in 
  fig. 
  1, 
  and 
  t 
  in 
  fig. 
  2 
  of 
  PI. 
  XII.) 
  passing 
  in 
  

   front 
  of 
  the 
  rudimentary 
  vaults. 
  The 
  platform 
  and 
  the 
  openings 
  

   of 
  its 
  vaults 
  have 
  much 
  the 
  appearance 
  of 
  a 
  flattened 
  nose, 
  with 
  

   the 
  nostrils 
  and 
  their 
  rim 
  prominently 
  exposed. 
  

  

  All 
  we 
  have 
  seen 
  of 
  this 
  species 
  is 
  the 
  internal 
  cast, 
  measuring 
  

   two 
  inches 
  three 
  lines 
  in 
  length 
  by 
  one 
  inch 
  five 
  lines 
  in 
  width, 
  of 
  

   a 
  single 
  pedicle-valve, 
  found 
  by 
  Mr. 
  Billings 
  in 
  the 
  Guelph 
  Limestone 
  

   at 
  New 
  Hope, 
  West 
  Canada. 
  

  

  Trimerella 
  (?) 
  galtensis, 
  Billings. 
  PI. 
  XVIII. 
  fig. 
  13 
  ; 
  and 
  

   PL 
  XIX. 
  figs. 
  4 
  & 
  4\ 
  

   Obolus 
  galtensis, 
  Billings, 
  Geol. 
  Survey 
  of 
  Canada, 
  Pal. 
  p. 
  168, 
  

   fig. 
  151, 
  21 
  Jan. 
  1862. 
  Trimerella 
  minor, 
  Dall, 
  Am. 
  Journ. 
  of 
  

   Conch, 
  vol. 
  vii. 
  p. 
  83, 
  1871. 
  Rhynobolus 
  galtensis, 
  Hall, 
  Re- 
  

   port 
  on 
  the 
  State 
  Cab. 
  of 
  Nat. 
  Hist, 
  preparations 
  of 
  Pal. 
  New 
  

   York, 
  p. 
  5, 
  March 
  1871. 
  Obolellina, 
  Bill. 
  Can. 
  Nat. 
  vol. 
  vi. 
  

   p. 
  222, 
  29 
  Dec. 
  1871 
  ; 
  also 
  April 
  1872. 
  Dinobolus, 
  Dav. 
  and 
  

   King, 
  Report 
  of 
  Brighton 
  Meeting 
  of 
  Br. 
  Assoc. 
  20 
  August, 
  1872. 
  

  

  Shell 
  compressed 
  or 
  moderately 
  convex 
  ; 
  tapering 
  rather 
  sharply 
  

   behind; 
  broadest, 
  and 
  rounded 
  anteriorly. 
  Pedicle-valve 
  flatly 
  convex: 
  

   area 
  large 
  ; 
  long 
  ; 
  nearly 
  an 
  equilateral 
  triangle 
  : 
  deltidium 
  raised 
  ; 
  

   occupying 
  a 
  considerable 
  portion 
  of 
  the 
  area 
  ; 
  crossed 
  by 
  strong 
  

   imbricated 
  lines 
  ; 
  with 
  a 
  broad 
  deepish 
  furrow 
  along 
  its 
  entire 
  length 
  : 
  

   areal 
  borders 
  transversely 
  striated 
  : 
  platform 
  wide 
  ; 
  very 
  slightly 
  

   raised 
  ; 
  its 
  anterior 
  very 
  obtusely 
  V-shaped. 
  Brachial 
  valve 
  slightly 
  

   convex 
  : 
  platform 
  V-shaped 
  ; 
  scarcely 
  raised, 
  or 
  an 
  excavation 
  ; 
  

   deeply 
  marked 
  with 
  scars, 
  which 
  give 
  it 
  a 
  tripartite 
  appearance 
  : 
  

   crescent 
  well 
  pronounced, 
  especially 
  at 
  the 
  crown, 
  the 
  middle 
  of 
  

   which 
  is 
  pointed, 
  the 
  point 
  directed 
  backward. 
  

  

  Under 
  the 
  name 
  of 
  Obolus 
  galtensis 
  this 
  shell 
  was 
  very 
  briefly 
  

   described 
  in 
  1862 
  by 
  Mr. 
  Billings, 
  who 
  at 
  the 
  same 
  time 
  gave 
  an 
  

   incomplete 
  figure 
  of 
  an 
  internal 
  cast 
  of 
  the 
  dorsal 
  valve. 
  In 
  1871 
  

  

  