﻿124 
  T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMEEELLID^. 
  

  

  16. 
  On 
  the 
  Trimerellids, 
  a 
  Paleozoic 
  Family 
  of 
  the 
  Pallio- 
  

   branchs 
  or 
  Brachiopoda. 
  By 
  Thomas 
  Davidson, 
  Esq., 
  F.R.S., 
  

   F.G.S., 
  &c, 
  and 
  Professor 
  William 
  King, 
  Sc.D. 
  Honoris 
  Causa, 
  

   Queen's 
  University 
  in 
  Ireland. 
  (Read 
  February 
  25, 
  1874.) 
  

  

  [Plates 
  XII.-XIX.] 
  

  

  Contents. 
  

  

  I. 
  Bibliography 
  of 
  the 
  Trimerellids. 
  

   II. 
  Chiefly 
  descriptive 
  of 
  their 
  internal 
  features. 
  

  

  III. 
  Myology 
  and 
  other 
  characters 
  of 
  the 
  family. 
  

  

  IV. 
  Affinities 
  of 
  the 
  family. 
  

  

  V. 
  Geological 
  range, 
  Chronogenesis, 
  and 
  Evolution 
  of 
  the 
  family. 
  

   VI. 
  Physiography 
  of 
  the 
  Seas 
  tenanted 
  by 
  the 
  Trimerellids, 
  as 
  compared 
  

   with 
  that 
  of 
  the 
  Cambrian 
  Seas. 
  

   VII. 
  Diagnosis 
  of 
  the 
  family. 
  

   VIII. 
  The 
  genus 
  Trimerella 
  and 
  its 
  species. 
  

   IX. 
  The 
  genus 
  Mononierella 
  and 
  its 
  species. 
  

   X, 
  The 
  genus 
  Dinobolus 
  and 
  its 
  species. 
  

  

  Appendix. 
  

   a. 
  Lingulops 
  Wldtfieldi. 
  b. 
  Ckelodes 
  Bergmani. 
  

  

  I. 
  Bibliography 
  of 
  the 
  Trimerellids. 
  

  

  The 
  important 
  group 
  of 
  shells 
  forming 
  the 
  subject-matter 
  of 
  this 
  

   memoir 
  is, 
  comparatively 
  speaking, 
  of 
  recent 
  acquisition 
  in 
  palaeon- 
  

   tology 
  ; 
  for, 
  though 
  two 
  of 
  its 
  species 
  were 
  made 
  known 
  in 
  1853, 
  under 
  

   the 
  designations 
  of 
  Obolus 
  Davidsoni 
  and 
  0. 
  transversus* 
  ', 
  it 
  is 
  only 
  

   of 
  late 
  years 
  that 
  a 
  beginning 
  was 
  made 
  towards 
  gaining 
  a 
  correct 
  

   view 
  of 
  its 
  remarkable 
  internal 
  features. 
  So 
  little 
  understood 
  were 
  

   tbe 
  interiors 
  of 
  these 
  species 
  at 
  first, 
  that 
  they 
  were 
  confidently, 
  but 
  

   erroneously, 
  referred 
  to 
  the 
  genus 
  Obolus 
  by 
  one 
  of 
  the 
  present 
  

   writers. 
  Billings 
  did 
  not 
  escape 
  the 
  same 
  error 
  when 
  he 
  described 
  

   his 
  Obohis 
  canadensis^ 
  '. 
  

  

  The 
  next 
  stage, 
  one 
  decidedly 
  in 
  advance, 
  was 
  gained 
  by 
  Billings, 
  

   who, 
  finding 
  some 
  Canadian 
  specimens 
  with 
  a 
  singular 
  interior, 
  was 
  

   led 
  to 
  institute 
  a 
  new 
  genus 
  for 
  them 
  under 
  the 
  name 
  Trimerella 
  — 
  

   observing, 
  at 
  the 
  same 
  time, 
  that 
  it 
  " 
  is 
  allied 
  to 
  Obolus, 
  but 
  from 
  

   which 
  it 
  differs 
  in 
  the 
  possession 
  of 
  longitudinal 
  septa 
  "J. 
  He 
  erred 
  

   again, 
  however, 
  in 
  describing 
  another 
  species 
  of 
  the 
  present 
  family 
  

   under 
  the 
  name 
  Obolus 
  galtensis§. 
  

  

  In 
  1867, 
  Lindstrom 
  published 
  an 
  important 
  paper, 
  describing 
  

   some 
  specimens 
  found 
  in 
  the 
  Island 
  of 
  Gotland 
  as 
  far 
  back 
  as 
  1859, 
  

   and 
  which 
  he 
  had 
  no 
  hesitation 
  in 
  referring 
  to 
  the 
  genus 
  Trimerella 
  

   of 
  Billings, 
  throwing 
  at 
  the 
  same 
  time 
  further 
  light 
  on 
  the 
  internal 
  

   structure 
  of 
  the 
  group. 
  He 
  also 
  referred 
  to 
  some 
  specimens 
  of 
  the 
  

  

  * 
  Davidson, 
  Introduction 
  Br. 
  Foss. 
  Brach. 
  p. 
  136, 
  1853. 
  

  

  t 
  Beport 
  on 
  the 
  Geological 
  Survey 
  of 
  Canada, 
  p. 
  28, 
  1857. 
  

  

  J 
  Pal. 
  Foss. 
  Geol. 
  Canada, 
  i. 
  p. 
  166, 
  June 
  1862. 
  § 
  Ibid. 
  p. 
  168. 
  

  

  