﻿148 
  T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLID^E. 
  

  

  limestone 
  immediately 
  overlying 
  the 
  Niagara 
  limestone 
  (a 
  rock 
  

   equal 
  in 
  age 
  to 
  the 
  Aymestry 
  limestone 
  of 
  England, 
  or 
  about 
  that 
  

   level, 
  according 
  to 
  Mr. 
  Billings) 
  at 
  Hespelar, 
  Gait, 
  Elora 
  — 
  villages 
  

   situated 
  near 
  each 
  other 
  in 
  Ontario 
  (Canada 
  West). 
  Mr. 
  Weston, 
  of 
  

   the 
  Geological 
  Survey 
  of 
  Canada, 
  has 
  discovered 
  several 
  new 
  specimens 
  

   in 
  the 
  localities, 
  already 
  mentioned 
  in 
  our 
  description, 
  which 
  have 
  

   yielded 
  T. 
  grandis 
  : 
  among 
  these 
  are 
  two 
  exhibiting 
  casts 
  of 
  both 
  

   valves 
  in 
  connexion. 
  Mr. 
  Weston 
  discovered 
  also 
  at 
  Hespelar 
  an 
  

   incomplete 
  specimen, 
  measuring 
  three 
  and 
  a 
  half 
  inches 
  in 
  length, 
  

   which 
  showed 
  that 
  the 
  shell 
  was 
  smooth, 
  or 
  marked 
  only 
  by 
  con- 
  

   centric 
  lines 
  of 
  growth. 
  Well-characterized 
  internal 
  casts 
  of 
  T. 
  

   acuminata 
  were 
  likewise 
  found 
  by 
  Dr. 
  Lindstrom 
  in 
  several 
  parts 
  of 
  

   the 
  Island 
  of 
  Gotland, 
  as 
  far 
  back 
  as 
  1859, 
  in 
  rocks 
  about 
  equivalent 
  

   in 
  age 
  to 
  those 
  of 
  Canada. 
  

  

  The 
  species 
  appears 
  to 
  have 
  had 
  a 
  very 
  extended 
  horizontal 
  

   range 
  ; 
  for 
  it 
  occurs 
  in 
  the 
  same 
  horizon 
  in 
  Canada, 
  the 
  United 
  

   States, 
  and 
  in 
  Sweden. 
  

  

  Trimerella 
  LlNDSTRffiMT, 
  Dall. 
  PI. 
  XIV. 
  figs. 
  1-7. 
  

  

  Trimerella 
  ? 
  Lindstrom, 
  Ofversigt 
  af 
  Kongl. 
  Vetenskaps-Aka- 
  

  

  demiens 
  Forhandlingar, 
  p. 
  253, 
  pi. 
  xxi. 
  figs. 
  1 
  & 
  5 
  to 
  9, 
  1867. 
  

  

  Gotlandia 
  Lindstromi, 
  Dall, 
  American 
  Journal 
  of 
  Conch, 
  vol. 
  ii. 
  

  

  part 
  2, 
  p. 
  160, 
  1870. 
  ? 
  Trimerella, 
  Dall, 
  vol. 
  vii. 
  part 
  2, 
  p. 
  84, 
  

   . 
  1871. 
  Dav. 
  and 
  King, 
  Eeport 
  of 
  Brighton 
  Meeting 
  of 
  Br. 
  

  

  Assoc. 
  20 
  August, 
  1872. 
  

   Shell 
  very 
  massive 
  ; 
  longitudinally 
  oval 
  ; 
  anterior 
  half 
  broadly 
  

   rounded 
  ; 
  posterior 
  half 
  tapering 
  rather 
  rapidly 
  to 
  the 
  extremity 
  of 
  

   the 
  beak, 
  which 
  is 
  pointed 
  ; 
  greatest 
  breadth 
  at 
  about 
  one 
  third 
  of 
  

   the 
  length 
  of 
  the 
  shell 
  from 
  the 
  anterior 
  margin 
  : 
  surface 
  smooth 
  

   or 
  marked 
  with 
  concentric 
  lines 
  of 
  growth. 
  Pedicle-valve 
  very 
  

   thick 
  ; 
  rather 
  less 
  deep 
  than 
  the 
  opposite 
  one 
  ; 
  sloping 
  from 
  the 
  

   point 
  of 
  the 
  umbo 
  to 
  the 
  anterior 
  margin 
  : 
  area 
  equilateral 
  ; 
  trian- 
  

   gular 
  ; 
  rather 
  longer 
  than 
  one 
  fifth 
  of 
  the 
  length 
  of 
  the 
  shell 
  : 
  cereal 
  

   borders 
  rather 
  wide 
  : 
  deltidium 
  flat, 
  or 
  raised 
  ; 
  wide; 
  triangular; 
  with 
  

   a 
  more 
  or 
  less 
  marked 
  longitudinal 
  depression 
  along 
  the 
  middle 
  : 
  

   deltidial 
  ridges 
  and 
  callosities 
  prominent; 
  crossed 
  by 
  strongly 
  in- 
  

   dented 
  lines 
  or 
  ridges 
  : 
  deltidial 
  slope 
  variable 
  ; 
  occasionally 
  its 
  lines 
  

   appear 
  to 
  be 
  coarse 
  : 
  cardinal 
  facet 
  large 
  ; 
  variable 
  in 
  its 
  inclination: 
  

   umbonal 
  chambers 
  resembling 
  tubular 
  perforations 
  : 
  platform 
  and 
  

   vaults 
  nearly 
  as 
  in 
  T. 
  grandis. 
  Brachial 
  valve 
  massive 
  ; 
  moderately 
  

   and 
  uniformly 
  convex, 
  especially 
  about 
  the 
  umbo 
  : 
  hinge 
  generally 
  

   with 
  a 
  large, 
  massive, 
  projecting 
  cardinal 
  process 
  in 
  the 
  centre 
  ; 
  

   and 
  two 
  depressions, 
  one 
  on 
  either 
  side 
  : 
  platform 
  and 
  vaults 
  agreeing 
  

   pretty 
  nearly 
  with 
  those 
  in 
  T. 
  grandis. 
  

  

  Trimerella 
  Lindstroemi 
  varies 
  remarkably 
  in 
  its 
  cardinal 
  features 
  ; 
  

   the 
  variations 
  in 
  some 
  cases 
  being 
  highly 
  puzzling. 
  The 
  cardinal 
  

   process 
  is 
  rude 
  and 
  inconstant 
  in 
  shape 
  : 
  the 
  variety 
  given 
  in 
  

   PI. 
  XIV. 
  fig. 
  3 
  (the 
  part 
  answering 
  to 
  the 
  cardinal 
  scar 
  v 
  in 
  fig. 
  2, 
  

   PI. 
  XII.) 
  may 
  be 
  regarded 
  as 
  showing 
  its 
  median 
  state 
  of 
  deve- 
  

   lopment 
  — 
  some 
  specimens 
  exhibiting 
  it 
  lower, 
  others 
  even 
  more 
  

  

  