﻿T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLID^E. 
  155 
  

  

  IX. 
  MONOMERELLA 
  AND 
  ITS 
  SPECIES. 
  

  

  Genus 
  Monomerella*, 
  Billings, 
  1871. 
  

  

  Shell 
  thick 
  ; 
  circular, 
  or 
  transversely 
  oval, 
  in 
  its 
  marginal 
  outline. 
  

   Pedicle-valve 
  — 
  umbo 
  usually 
  large 
  ; 
  projecting; 
  double-chambered: 
  

   area 
  of 
  considerable 
  size 
  : 
  deltidium 
  large 
  : 
  hinge 
  generally 
  thick 
  and 
  

   elevated 
  ; 
  ledge- 
  shaped 
  ; 
  depressed 
  or 
  concave 
  in 
  the 
  middle 
  portion 
  : 
  

   cardinal 
  facet 
  a 
  wall-like 
  space 
  rising 
  out 
  of 
  or 
  behind 
  the 
  ledge 
  or 
  

   flat 
  of 
  the 
  hinge 
  : 
  cardinal 
  buttress 
  strong 
  ; 
  lamelliform 
  : 
  platform 
  

   flat 
  ; 
  slightly 
  elevated 
  ; 
  widest, 
  highest, 
  and 
  very 
  obtusely 
  angulated 
  

   in 
  front. 
  Brachial 
  valve 
  — 
  umbo 
  rounded 
  : 
  hinge 
  moderately 
  thick 
  : 
  

   platform 
  trilobed 
  ; 
  usually 
  with 
  a 
  thin 
  margin. 
  

  

  Type 
  species, 
  Monomerella 
  prisca, 
  Billings. 
  

  

  The 
  unvaulted 
  character 
  of 
  its 
  platforms, 
  large 
  umbonal 
  chambers, 
  

   and 
  certain 
  other 
  characters 
  differentiate 
  this 
  genus 
  remarkably 
  from 
  

   Trimerella. 
  The 
  platform 
  of 
  the 
  pedicle-valve, 
  which 
  widons 
  out 
  

   from 
  behind, 
  has 
  its 
  front 
  formed 
  by 
  two 
  edges 
  meeting 
  at 
  a 
  very 
  

   obtuse 
  angle. 
  The 
  platform 
  of 
  the 
  brachial 
  valve 
  is 
  widest 
  behind, 
  

   and 
  conspicuously 
  trilobed, 
  the 
  latter 
  character 
  deeply 
  incurving 
  its 
  

   sides 
  anteriorly 
  : 
  its 
  surface 
  is 
  divided, 
  answering 
  to 
  the 
  attachment 
  

   of 
  three 
  sets 
  or 
  pairs 
  of 
  muscles 
  — 
  a 
  lateral 
  pair, 
  with 
  a 
  member 
  be- 
  

   longing 
  to 
  each 
  of 
  the 
  lateral 
  lobes, 
  a 
  median 
  pair 
  belonging 
  to 
  the 
  

   space 
  between 
  the 
  lateral 
  lobes, 
  and 
  an 
  anterior 
  pair 
  belonging 
  to 
  

   the 
  anterior 
  lobe. 
  

  

  The 
  cardinal 
  facet 
  reminds 
  one 
  of 
  the 
  perpendicular 
  wall 
  behind 
  

   the 
  deep 
  excavation 
  that 
  characterizes 
  the 
  hinge 
  in 
  certain 
  varieties 
  

   of 
  Trimerella 
  Lindstroemi 
  (PI. 
  XIV. 
  fig. 
  5) 
  — 
  a 
  peculiarity 
  strongly 
  

   favouring 
  the 
  idea 
  that 
  in 
  such 
  varieties 
  the 
  cardinal 
  facet 
  was 
  situated 
  

   similarly 
  to 
  what 
  it 
  is 
  in 
  the 
  present 
  genus, 
  particularly 
  as 
  traces 
  of 
  

   the 
  lozenge 
  have 
  occurred 
  to 
  us 
  on 
  the 
  " 
  wall-like 
  space 
  " 
  belonging 
  

   to 
  the 
  hinge 
  of 
  its 
  typical 
  species. 
  The 
  dentary 
  system 
  of 
  Mono- 
  

   merella 
  does 
  not 
  form 
  a 
  prominent 
  feature 
  : 
  we 
  only 
  find 
  evidence 
  

  

  * 
  Mr. 
  Billings, 
  in 
  a 
  letter 
  bearing 
  the 
  date 
  5th 
  May, 
  1871, 
  notified 
  to 
  me 
  

   the 
  discovery 
  of 
  a 
  species 
  (which 
  he 
  forwarded 
  at 
  the 
  same 
  time 
  for 
  exa- 
  

   mination, 
  see 
  PI. 
  XVII. 
  fig. 
  5, 
  the 
  type 
  of 
  Monomerella) 
  that 
  appeared 
  to 
  

   him 
  to 
  " 
  look 
  like 
  a 
  cross 
  between 
  Obolus 
  and 
  a 
  Trimerella," 
  and 
  added 
  

   " 
  What 
  shall 
  I 
  call 
  it 
  ? 
  " 
  I 
  at 
  once 
  perceived 
  that 
  the 
  species 
  was 
  referable 
  

   to 
  the 
  same 
  genus 
  as 
  another 
  fossil 
  (herein 
  named 
  M. 
  Walmstedti), 
  occur- 
  

   ring 
  in 
  Sweden 
  ; 
  a 
  figure 
  of 
  the 
  pedicle-valve 
  of 
  which 
  had 
  likewise 
  been 
  

   communicated 
  to 
  me 
  by 
  Dr. 
  Lindstrom, 
  on 
  the 
  18th 
  of 
  December, 
  1869, 
  

   with 
  the 
  intimation 
  that 
  it 
  belonged 
  to 
  another 
  genus, 
  more 
  akin 
  to 
  Trime- 
  

   rella 
  and 
  Obolus 
  than 
  the 
  others 
  (alluding 
  to 
  some 
  other 
  genera 
  spoken 
  

   of 
  in 
  the 
  letter). 
  Inadvertently 
  overlooking 
  for 
  the 
  moment 
  the 
  fact 
  last 
  

   noticed 
  (for 
  which 
  I 
  feel 
  some 
  apology 
  is 
  due 
  to 
  both 
  Dr. 
  Lindstrom 
  and 
  the 
  

   public), 
  and 
  as 
  both 
  the 
  Swedish 
  and 
  the 
  American 
  fossils 
  required 
  a 
  new 
  generic 
  

   name, 
  I 
  left 
  it 
  to 
  Mr. 
  Billings 
  to 
  impose 
  one 
  ; 
  which 
  he 
  did 
  in 
  the 
  ' 
  Cana- 
  

   dian 
  Naturalist,' 
  published 
  December 
  1871. 
  It 
  has 
  been 
  considered 
  necessary 
  to 
  

   mention 
  these 
  circumstances, 
  that 
  Dr. 
  Lindstrom's 
  discovery 
  and 
  opinion 
  might 
  

   be 
  adequately 
  recognized. 
  

  

  The 
  figure 
  above 
  alluded 
  to 
  represented 
  the 
  valve 
  we 
  have 
  figured 
  in 
  PI. 
  XVII. 
  

   fi<* 
  . 
  2. 
  Dr. 
  Lindstrom 
  had 
  also 
  sent 
  me 
  a 
  figure 
  of 
  the 
  brachial 
  valve 
  (the 
  one 
  re- 
  

   presented 
  in 
  PI. 
  XVIII. 
  fig. 
  3), 
  in 
  a 
  letter 
  dated 
  so 
  far 
  back 
  as 
  the 
  18th 
  January, 
  

   I860.— 
  T. 
  D. 
  

  

  