﻿154 
  T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLID.^, 
  

  

  mens 
  are 
  nearly 
  four 
  inches 
  in 
  length, 
  two 
  and 
  a 
  half 
  in 
  breadth, 
  

   and 
  one 
  inch 
  and 
  a 
  half 
  in 
  depth. 
  The 
  breadth 
  is 
  also 
  greater 
  

   in 
  some 
  specimens, 
  as 
  we 
  possess 
  an 
  internal 
  cast 
  in 
  which 
  it 
  

   measures 
  three 
  inches. 
  

  

  T. 
  ohioensis 
  abounds 
  in 
  the 
  Niagara 
  Limestone 
  (Upper 
  Silurian) 
  at 
  

   Genoa, 
  Ottawa 
  Co., 
  Ohio 
  ; 
  Sinking 
  Springs, 
  Ohio 
  (I. 
  S. 
  Newberry). 
  

   According 
  to 
  Mr. 
  Dall 
  one 
  valve 
  was 
  found 
  in 
  the 
  Guelph 
  Limestone 
  

   of 
  Canada. 
  

  

  Trimerella 
  Dalli, 
  Dav. 
  & 
  King. 
  PI. 
  XY. 
  figs. 
  1-3. 
  

  

  Trimerella 
  Dalli, 
  Dav. 
  & 
  King, 
  Eeport 
  of 
  Brighton 
  Meeting 
  of 
  

   Brit. 
  Assoc. 
  20th 
  August, 
  1872. 
  

  

  Shell 
  ovate, 
  or 
  longitudinally 
  oval 
  : 
  surface 
  smooth. 
  Pedicle-valve 
  

   flatly 
  convex, 
  especially 
  along 
  the 
  middle 
  : 
  beak 
  prominent 
  ; 
  nearly 
  

   straight 
  ; 
  about 
  one 
  fifth 
  the 
  length 
  of 
  the 
  shell 
  ; 
  with 
  two 
  umbonal 
  

   chambers 
  rather 
  deep, 
  conical, 
  separated 
  by 
  a 
  strong 
  cardinal 
  but- 
  

   tress: 
  crescent 
  tolerably 
  well 
  pronounced: 
  platform 
  divided 
  at 
  its 
  

   posterior 
  end 
  by 
  an 
  extension 
  of 
  the 
  buttress 
  into 
  two 
  lateral 
  por- 
  

   tions 
  ; 
  wide 
  and 
  short 
  : 
  vaults 
  not 
  of 
  much 
  depth 
  : 
  median 
  plate 
  of 
  

   small 
  extent. 
  Brachial 
  valve 
  uniformly 
  convex 
  : 
  cardinal 
  process 
  

   an 
  excavated 
  projection 
  : 
  platform 
  long 
  ; 
  extending 
  to 
  the 
  middle 
  

   half 
  of 
  the 
  valve 
  : 
  vaults 
  rather 
  short 
  : 
  median 
  plate 
  much 
  elon- 
  

   gated. 
  

  

  Several 
  internal 
  casts 
  of 
  the 
  present 
  fossil 
  were 
  sent 
  us 
  for 
  exami- 
  

   nation 
  by 
  Mr. 
  Billings. 
  After 
  taking 
  gutta-percha 
  squeezes 
  from 
  

   them 
  we 
  arrived 
  at 
  the 
  conclusion 
  that 
  they 
  are 
  distinguisbable 
  from 
  

   all 
  other 
  species 
  of 
  the 
  genus 
  chiefly 
  by 
  the 
  presence 
  of 
  deep 
  um- 
  

   " 
  bonal 
  chambers 
  separated 
  by 
  a 
  lamelliform 
  cardinal 
  buttress. 
  These 
  

   characters 
  approximate 
  the 
  species 
  to 
  the 
  genus 
  Monomerella. 
  

  

  The 
  largest 
  example 
  that 
  has 
  come 
  under 
  our 
  notice 
  measured 
  

   1 
  inch 
  4 
  lines 
  in 
  length, 
  1 
  inch 
  1 
  line 
  in 
  breadth, 
  and 
  8 
  lines 
  in 
  

   depth. 
  

  

  T. 
  Dalli 
  occurs 
  in 
  the 
  Guelph 
  Limestone 
  at 
  Hespelar, 
  Canada 
  "West; 
  

   where 
  it 
  was 
  found 
  by 
  Mr. 
  Weston. 
  

  

  Trimekella 
  wisbyensis, 
  Dav. 
  & 
  King. 
  PI. 
  XIII. 
  fig. 
  1. 
  

  

  Trimerella 
  wisbyensis, 
  Dav. 
  & 
  King, 
  Beport 
  of 
  Brighton 
  Meeting 
  

   of 
  Brit. 
  Assoc. 
  20th 
  August, 
  1872. 
  

  

  Brachial 
  valve 
  nearly 
  circular, 
  or 
  a 
  little 
  wider 
  than 
  long 
  : 
  platform 
  

   moderately 
  long 
  ; 
  depressed 
  along 
  its 
  middle 
  : 
  vaults 
  extending 
  

   underneath 
  the 
  platform 
  to 
  apparently 
  half 
  its 
  length 
  : 
  median 
  plate 
  

   long 
  *. 
  

  

  Of 
  this 
  species 
  all 
  we 
  know 
  is 
  the 
  internal 
  cast 
  of 
  a 
  single 
  dorsal 
  

   valve, 
  13 
  lines 
  in 
  length 
  by 
  11 
  in 
  breadth, 
  found 
  by 
  Dr. 
  Lindstrom 
  

   in 
  the 
  Upper 
  Silurian 
  (Wenlock) 
  near 
  Wisby, 
  in 
  Gotland. 
  

  

  * 
  Traces 
  of 
  the 
  crescent 
  are 
  visible 
  on 
  the 
  hinge. 
  Although 
  our 
  figure 
  repre- 
  

   sents 
  the 
  platform 
  as 
  if 
  complete, 
  we 
  suspect 
  that 
  it 
  was 
  larger 
  than 
  represented. 
  

   Its 
  surface 
  shows 
  median 
  and 
  lateral 
  scars. 
  

  

  