﻿T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLIDJE. 
  153 
  

  

  in 
  the 
  latter 
  genus. 
  The 
  present 
  species 
  appears 
  to 
  us 
  to 
  be 
  an 
  ex- 
  

   treme 
  aberrant 
  form 
  linked 
  to 
  Trimerella 
  by 
  means 
  of 
  T, 
  Billingsii. 
  

   Some 
  well-preserved 
  internal 
  casts 
  of 
  both 
  valves 
  have 
  been 
  pro- 
  

   cured 
  by 
  Mr. 
  Billings 
  and 
  Prof. 
  Hall 
  from 
  a 
  light-yellow 
  rock 
  of 
  

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

   The 
  largest 
  examples, 
  we 
  have 
  seen, 
  measured 
  — 
  length 
  one 
  inch 
  eight 
  

   lines, 
  breadth 
  one 
  inch 
  one 
  line. 
  

  

  Trimerella 
  ohioensis, 
  Meek. 
  PI. 
  XVI. 
  figs. 
  3-7; 
  PI. 
  XIX. 
  figs. 
  1, 
  2. 
  

  

  Trimerella 
  ohioensis, 
  Meek, 
  Sill. 
  Am. 
  Journ. 
  Sci. 
  & 
  Art, 
  p. 
  305, 
  

   April 
  1871. 
  Dall, 
  Am. 
  Journ. 
  of 
  Conch, 
  vol. 
  vii. 
  part 
  2, 
  p. 
  83, 
  

   pi. 
  ii. 
  figs. 
  3, 
  4, 
  1871. 
  Dav. 
  & 
  King, 
  Report 
  of 
  Brighton 
  Meet- 
  

   ing 
  of 
  Br. 
  Assoc. 
  20 
  August, 
  1872. 
  

   Shell 
  large 
  ; 
  massive 
  ; 
  globose, 
  or 
  longitudinally 
  oval 
  ; 
  anteriorly 
  

   broadly 
  rounded 
  ; 
  greatest 
  breadth 
  towards 
  the 
  middle 
  : 
  beak 
  

   moderately 
  produced 
  ; 
  occasionally 
  much 
  incurved 
  : 
  exterior 
  smooth. 
  

   Pedicle-valve 
  moderately 
  convex, 
  or 
  flattened 
  : 
  deltidium 
  large 
  ; 
  wide 
  ; 
  

   triangular 
  ; 
  deeply 
  concave 
  : 
  platform 
  commencing 
  almost 
  imme- 
  

   diately 
  under 
  the 
  hinge, 
  and 
  extending 
  to 
  about 
  two 
  thirds 
  of 
  the 
  

   length 
  of 
  the 
  valve 
  : 
  vaults 
  very 
  large 
  and 
  deep 
  : 
  median-plate 
  

   tolerably 
  long. 
  Brachial 
  valve 
  with 
  an 
  incurved 
  umbo 
  : 
  cardinal 
  

   process 
  variable 
  ; 
  well 
  developed 
  : 
  platform 
  commencing 
  under 
  the 
  

   hinge 
  ; 
  deeply 
  vaulted 
  : 
  median-plate 
  considerably 
  elevated 
  ; 
  extend- 
  

   ing 
  to 
  within 
  a 
  very 
  short 
  distance 
  of 
  the 
  margin 
  of 
  the 
  valve. 
  

  

  This, 
  which 
  may 
  be 
  considered 
  a 
  typical 
  form, 
  is 
  more 
  tumid 
  

   than 
  any 
  of 
  the 
  preceding 
  species. 
  Mr. 
  Meek 
  states 
  that 
  on 
  com- 
  

   paring 
  specimens 
  of 
  T. 
  ohioensis 
  with 
  T. 
  grandis 
  he 
  found 
  them 
  

   quite 
  different 
  and 
  distinct 
  — 
  that 
  proportionally 
  T. 
  ohioensis 
  is 
  a 
  

   much 
  broader 
  shell, 
  and 
  differs 
  in 
  having 
  the 
  beak 
  of 
  the 
  dorsal 
  valve 
  

   strongly 
  incurved 
  beyond 
  the 
  plane 
  of 
  the 
  connecting 
  edges 
  of 
  the 
  two 
  

   valves 
  — 
  that 
  it 
  is 
  also 
  more 
  convex, 
  its 
  ventral 
  valve 
  more 
  arched, 
  and 
  

   the 
  rostral 
  cavity 
  of 
  its 
  dorsal 
  valve 
  deeper. 
  The 
  platforms 
  and 
  

   vaults, 
  in 
  their 
  length, 
  resemble 
  those 
  of 
  T. 
  acuminata. 
  We 
  

   have 
  not 
  become 
  acquainted 
  with 
  its 
  platform-scars 
  and 
  crescents 
  ; 
  but 
  

   certain 
  obscure 
  indications 
  of 
  the 
  former 
  are 
  observable 
  close 
  to 
  the 
  

   almost 
  perpendicular 
  hinge-wall 
  and 
  below 
  the 
  cardinal 
  process 
  : 
  

   they 
  seem 
  to 
  have 
  been 
  produced 
  by 
  the 
  attachment 
  of 
  a 
  valvular 
  

   muscle, 
  possibly 
  the 
  umbonal. 
  The 
  hinge, 
  much 
  elevated 
  in 
  the 
  

   middle 
  of 
  the 
  brachial 
  valve, 
  presents 
  different 
  appearances 
  through 
  

   modifications 
  in 
  the 
  mode 
  of 
  the 
  attachment 
  of 
  the 
  muscles 
  belong- 
  

   ing 
  to 
  the 
  cardinal 
  scar, 
  v 
  ; 
  which 
  in 
  certain 
  individuals 
  appears 
  as 
  

   a 
  ridge 
  running 
  along 
  the 
  hinge 
  (figs. 
  5 
  & 
  6, 
  PL 
  XVI.) 
  ; 
  while 
  in 
  others 
  

   it 
  is 
  double 
  cup-shaped 
  (fig. 
  7, 
  PI. 
  XVI.). 
  Meek 
  mentions 
  that 
  " 
  the 
  

   centre 
  of 
  the 
  hinge 
  appears 
  to 
  have 
  been 
  furnished 
  with 
  a 
  massive 
  

   gently 
  rounded 
  cardinal 
  process." 
  

  

  Through 
  the 
  liberality 
  of 
  Mr. 
  Meek, 
  Mr. 
  Joseph 
  Henry, 
  Prof. 
  

   Hall, 
  and 
  Mr. 
  Whitfield, 
  we 
  have 
  been 
  able 
  to 
  examine 
  a 
  number 
  of 
  

   well-preserved 
  internal 
  casts. 
  These 
  vary 
  considerably 
  ; 
  for 
  while 
  

   some 
  examples 
  are 
  almost 
  spherical, 
  or 
  as 
  wide 
  as 
  long, 
  others 
  are 
  a 
  

   good 
  deal 
  longer 
  than 
  wide. 
  Mr. 
  Meek 
  assures 
  us 
  that 
  some 
  speci- 
  

  

  