﻿I. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TIUMEREILID^. 
  13& 
  

  

  sentatives. 
  All 
  the 
  known 
  species 
  of 
  Trimerella 
  and 
  Monomerella, 
  

   hitherto 
  only 
  found 
  in 
  Sweden, 
  the 
  United 
  States, 
  Canada, 
  and 
  

   Russia, 
  are 
  from 
  beds 
  equivalent 
  in 
  age 
  to 
  the 
  Wenlock 
  and 
  Aymes- 
  

   try 
  formations 
  of 
  Great 
  Britain. 
  Dinobolus, 
  however, 
  has 
  obtained 
  

   a 
  more 
  extended 
  vertical 
  and 
  horizontal 
  range 
  : 
  of 
  the 
  seven 
  forms 
  

   at 
  present 
  known, 
  D. 
  Schmidti 
  (a 
  Russian 
  species), 
  D. 
  magnificus, 
  

   and 
  D. 
  canadensis 
  (both 
  from 
  Canada) 
  are 
  Cambro-Silurian 
  — 
  the 
  two 
  

   species 
  last-named 
  having 
  been 
  found 
  in 
  the 
  Trenton 
  or 
  Black-river 
  

   Limestone, 
  about 
  the 
  horizon 
  of 
  the 
  English 
  Upper 
  Llandeilo 
  or 
  the 
  base 
  

   of 
  the 
  Caradoc. 
  Dinobolus 
  Conradi, 
  confined 
  to 
  the 
  United 
  States, 
  be- 
  

   longs 
  to 
  the 
  Magara 
  Limestone 
  (Upper 
  Silurian). 
  D. 
  Davidsoni 
  appears 
  

   to 
  have 
  been 
  found 
  in 
  the 
  Llandovery 
  formation, 
  as 
  well 
  as 
  in 
  that 
  of 
  the 
  

   "Wenlock 
  in 
  Sweden 
  ; 
  but 
  it, 
  with 
  D. 
  Woodivardi 
  and 
  D. 
  transversus, 
  

   chiefly 
  characterizes 
  the 
  Wenlock 
  in 
  the 
  counties 
  of 
  Shropshire 
  and 
  

   Staffordshire 
  : 
  the 
  latter 
  three 
  are 
  the 
  only 
  British 
  species 
  known 
  

   belonging 
  to 
  the 
  family. 
  

  

  It 
  is 
  a 
  remarkable 
  peculiarity 
  in 
  Lingula 
  anatina 
  that 
  the 
  post- 
  

   lateral 
  parietals 
  of 
  the 
  splanchnoccele, 
  unlike 
  the 
  membranous 
  coun- 
  

   terparts 
  in 
  other 
  Palliobranchs, 
  are 
  highly 
  muscular 
  — 
  so 
  much 
  more 
  

   than 
  seems 
  to 
  be 
  needed 
  to 
  protect 
  the 
  viscera 
  as 
  to 
  give 
  rise 
  to 
  

   the 
  idea 
  that 
  they 
  also 
  served 
  to 
  some 
  extent 
  to 
  keep 
  the 
  valves 
  

   together. 
  It 
  is 
  therefore 
  not 
  improbable 
  that 
  in 
  some 
  forms 
  

   related 
  or 
  belonging 
  to 
  the 
  Lingulids 
  the 
  muscularity 
  of 
  the 
  

   walls 
  referred 
  to 
  was 
  still 
  more 
  developed, 
  thereby 
  causing 
  them 
  

   to 
  become 
  approximately 
  valvulars 
  or 
  adductors. 
  From 
  this 
  con- 
  

   dition 
  to 
  complete 
  specialization, 
  such 
  as 
  is 
  presented 
  by 
  the 
  

   pair 
  of 
  postadductor 
  muscles 
  that 
  characterize 
  Discina, 
  seems 
  to 
  

   be 
  an 
  easy 
  gradation. 
  Obolus 
  may 
  be 
  cited 
  as 
  an 
  additional 
  ex- 
  

   ample. 
  Although 
  a 
  considerable 
  number 
  of 
  Cambrian 
  Tretenterates, 
  

   usually 
  recognized 
  as 
  Lingulidce 
  or 
  Linguloid 
  shells, 
  are 
  known*, 
  it 
  

   nevertheless 
  happens 
  that 
  information 
  regarding 
  certain 
  of 
  their 
  in- 
  

   ternal 
  characters 
  is 
  too 
  limited 
  to 
  enable 
  palaeontologists 
  to 
  select 
  any 
  

   of 
  them 
  as 
  constituting 
  the 
  gradational 
  form 
  in 
  question. 
  A 
  better 
  

   source 
  seems 
  to 
  offer 
  itself 
  in 
  the 
  family 
  under 
  description. 
  

  

  Neither 
  Trimerella 
  nor 
  Monomerella 
  appears 
  likely 
  to 
  assist 
  us, 
  

   inasmuch 
  as 
  the 
  scars 
  (sides 
  of 
  the 
  crescent) 
  due 
  to 
  the 
  post-latero- 
  

   parietals 
  do 
  not 
  differ 
  notably 
  from 
  their 
  equivalents 
  in 
  Lingula. 
  

   In 
  Dinobolus, 
  however, 
  the 
  same 
  sears 
  are 
  strikingly 
  different 
  ; 
  each 
  

   one 
  has 
  a 
  composite 
  structure, 
  with 
  two, 
  or 
  more 
  separated 
  indenta- 
  

   tions 
  strongly 
  marked 
  — 
  showing 
  that 
  it 
  has 
  been 
  produced 
  by 
  a 
  muscle, 
  

   or 
  muscles, 
  unusually 
  large 
  and 
  powerful. 
  Though 
  evidently 
  belonging 
  

   to 
  the 
  postlateral 
  walls 
  of 
  the 
  splanchnoccele, 
  very 
  little 
  modification 
  

  

  or 
  Caradoc 
  formations 
  we 
  group 
  together 
  : 
  and 
  in 
  justice 
  to 
  the 
  labours 
  of 
  

   Sedgwick 
  and 
  Murchison, 
  also 
  in 
  accordance 
  with 
  the 
  compromise 
  proposed 
  by 
  

   the 
  first 
  cited 
  of 
  these 
  illustrious 
  men, 
  we 
  name 
  them 
  collectively, 
  as 
  others 
  have 
  

   already 
  done, 
  the 
  Cambro-Silurian 
  system. 
  

  

  * 
  See 
  paper 
  " 
  On 
  the 
  Earliest 
  Forms 
  of 
  Brachiopoda 
  hitherto 
  discovered 
  in 
  

   the 
  British 
  Paheozoic 
  Rocks," 
  Greol. 
  Mag. 
  vol. 
  v. 
  no. 
  7, 
  July 
  1868, 
  by 
  one 
  of 
  

   the 
  present 
  writers. 
  Some 
  minute 
  American 
  fossils, 
  described 
  by 
  Hall, 
  Bil- 
  

   lings, 
  and 
  others, 
  may 
  be 
  included 
  in 
  the 
  same 
  category. 
  

  

  