﻿T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIHERELLID^I. 
  157 
  

  

  the 
  hindermost 
  or 
  posterior 
  lying 
  close 
  against 
  the 
  cardinal 
  buttress 
  ; 
  

   the 
  anterior 
  a 
  strongly 
  striated 
  pair. 
  There 
  are 
  also 
  faint 
  indica- 
  

   tions 
  of 
  a 
  third 
  pair 
  situated 
  at 
  the 
  ante-median 
  point 
  of 
  the 
  platform. 
  

   A 
  pair 
  of 
  small 
  umbo-lateral 
  scars, 
  x, 
  occurs 
  on 
  the 
  outer 
  side, 
  a 
  mem- 
  

   ber 
  in 
  each, 
  of 
  the 
  umbonal 
  cavities, 
  corresponding 
  to 
  a 
  pair 
  similarly 
  

   situated 
  in 
  Trimerella. 
  The 
  deeply 
  excavated 
  hinge 
  gives 
  a 
  promi- 
  

   nent 
  appearance 
  to 
  the 
  cardinal 
  facet 
  : 
  faint 
  but 
  determinable 
  traces 
  

   of 
  the 
  bordering 
  lines 
  of 
  the 
  lozenge 
  occur 
  on 
  the 
  last 
  part. 
  

  

  The 
  largest 
  example 
  that 
  has 
  come 
  under 
  our 
  notice 
  measures 
  

   2 
  inches 
  in 
  length 
  by 
  1 
  inch 
  and 
  10 
  lines 
  in 
  breadth. 
  

  

  Internal 
  casts 
  of 
  this 
  species, 
  discovered 
  by 
  Mr. 
  T. 
  C. 
  "Weston, 
  of 
  

   the 
  Geological 
  Survey 
  of 
  Canada, 
  do 
  not 
  appear 
  to 
  be 
  rare 
  in 
  the 
  

   Guelph 
  Limestone 
  (Upper 
  Silurian) 
  at 
  Hespelar, 
  Ontario, 
  in 
  Canada. 
  

   Specimens 
  showing 
  the 
  exterior 
  of 
  the 
  shell 
  have 
  not 
  yet 
  been 
  dis- 
  

   covered 
  ; 
  but 
  it 
  was 
  in 
  all 
  probability 
  smooth, 
  or 
  nearly 
  so, 
  as 
  ap- 
  

   pears 
  to 
  be 
  the 
  ease 
  with 
  all 
  the 
  species 
  of 
  Trimerellids 
  hitherto 
  

   discovered. 
  

  

  Monomerella 
  Walmstedti, 
  Dav. 
  & 
  King. 
  PI. 
  XVII. 
  figs. 
  1-4, 
  and 
  

   PI. 
  XIX. 
  fig. 
  3, 
  3 
  a 
  ? 
  

  

  Monomerella 
  Walmstedti, 
  Dav. 
  & 
  King, 
  Remarks 
  on 
  the 
  Genera 
  

   Trimerella, 
  Dinobolus, 
  and 
  Monomerella, 
  Brighton 
  Meeting 
  of 
  

   Brit. 
  Assoc, 
  20th 
  August 
  1872 
  ; 
  Annals 
  & 
  Mag. 
  Nat. 
  Hist. 
  

   4th 
  ser. 
  Oct. 
  1872 
  ; 
  and 
  Geol. 
  Mag. 
  vol. 
  ix. 
  Oct. 
  1872. 
  

  

  Shell 
  somewhat 
  triangular 
  ; 
  with 
  rounded 
  corners 
  ; 
  very 
  thick 
  ; 
  

   broadest 
  anteriorly; 
  tapering 
  posteriorly; 
  front 
  flatly 
  rounded. 
  

   Valves 
  almost 
  equally 
  deep 
  ; 
  most 
  convex 
  in 
  the 
  umbonal 
  region 
  : 
  

   surface 
  smooth 
  or 
  marked 
  by 
  fine 
  concentric 
  lines 
  of 
  growth. 
  

   Pedicle-valve 
  flattened 
  anteriorly 
  : 
  beak 
  large 
  ; 
  massive 
  ; 
  slightly 
  

   incurved 
  : 
  area 
  wide 
  ; 
  equilatero-triangular 
  : 
  deltidium 
  broad 
  ; 
  flat 
  ; 
  

   strongly 
  laminated 
  : 
  deltidial 
  slope 
  narrow, 
  but 
  well 
  defined 
  : 
  del- 
  

   tidial 
  ridges 
  wide 
  ; 
  flattened 
  : 
  hinge 
  deeply 
  depressed 
  or 
  excavated 
  

   in 
  its 
  middle 
  ; 
  projecting 
  to 
  a 
  point 
  in 
  its 
  centre 
  ; 
  and 
  backed 
  by 
  a 
  

   nearly 
  perpendicular 
  wall: 
  cardinal 
  facet 
  situated 
  on 
  the 
  back- 
  wall 
  

   of 
  the 
  hinge 
  : 
  umbonal 
  chambers 
  deep 
  ; 
  and 
  wide-mouthed 
  : 
  cardinal 
  

   buttress 
  stout 
  ; 
  extending 
  to 
  some 
  little 
  distance 
  along 
  the 
  bottom 
  of 
  

   the 
  valve 
  : 
  platform 
  obtusely 
  angular 
  in 
  front 
  ; 
  more 
  or 
  less 
  excavated 
  

   laterally 
  by 
  three 
  pairs 
  of 
  scars. 
  Brachial 
  valve 
  evenly 
  convex 
  in 
  

   its 
  posterior 
  half 
  : 
  umbo 
  much 
  incurved 
  : 
  hinge 
  transversely 
  ridged, 
  

   and 
  raised 
  in 
  the 
  middle 
  portion 
  ; 
  somewhat 
  excavated 
  at 
  the 
  sides 
  : 
  

   platform 
  with 
  a 
  V-shaped 
  anterior 
  outline 
  ; 
  sharp, 
  and 
  raised 
  at 
  the 
  

   edges 
  ; 
  excavated 
  on 
  its 
  surface 
  by 
  three 
  sets 
  of 
  sears. 
  

  

  The 
  proportions 
  of 
  this 
  species 
  are 
  variable 
  : 
  one 
  example, 
  appa- 
  

   rently 
  of 
  ordinary 
  size, 
  measured 
  1 
  inch 
  8 
  lines 
  in 
  length 
  by 
  1 
  inch 
  

   5 
  lines 
  in 
  breadth 
  and 
  10 
  lines 
  in 
  depth. 
  

  

  Monomerella 
  Walmstedti, 
  although 
  agreeing 
  very 
  closely 
  with 
  the 
  

   last 
  species, 
  is 
  characterized 
  by 
  some 
  well-marked 
  differences. 
  The 
  

   umbo 
  of 
  the 
  pedicle-valve 
  is 
  more 
  elevated 
  ; 
  while 
  that 
  of 
  the 
  opposite 
  

   valve 
  is 
  more 
  rounded 
  or 
  incurved. 
  The 
  hinge 
  of 
  the 
  brachial 
  valve 
  

   is 
  thicker 
  ; 
  has 
  its 
  middle 
  portion 
  more 
  elevated, 
  becoming, 
  it 
  would 
  

  

  Q.J. 
  G/S. 
  No. 
  118. 
  n 
  

  

  1 
  

  

  