﻿142 
  T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLID^. 
  

  

  Now, 
  as 
  regards 
  the 
  silaeid 
  members 
  of 
  the 
  Archaeans, 
  there 
  is 
  no 
  

   difficulty 
  in 
  understanding 
  how 
  they 
  have 
  given 
  rise 
  to 
  the 
  argillites 
  

   and 
  sandstones 
  of 
  the 
  Cambrians. 
  But 
  where 
  are 
  the 
  limestones 
  

   that 
  we 
  ought 
  to 
  expect 
  would 
  have 
  been 
  formed 
  by 
  contemporary 
  

   reproduction 
  out 
  of 
  the 
  Silo-carbacid 
  rocks 
  ? 
  We 
  cannot 
  be 
  satisfied 
  

   with 
  the 
  answer 
  if 
  it 
  refer 
  to 
  the 
  American 
  cases 
  represented 
  by 
  the 
  

   primordial 
  limestones 
  of 
  Belle 
  Isle, 
  Troy 
  (N.T.), 
  and 
  other 
  places, 
  or 
  

   the 
  calciferous 
  sand-rock 
  usually 
  included 
  in 
  the 
  Potsdam 
  formation*. 
  . 
  

  

  Turning 
  next 
  to 
  the 
  British 
  Cambrians, 
  we 
  shall 
  be 
  met 
  by 
  a 
  

   still 
  less 
  satisfactory 
  answer. 
  The 
  Longmynds, 
  Menevians, 
  Ffes- 
  

   tiniogs 
  and 
  Tremadocs, 
  which 
  raDge 
  from 
  twenty 
  thousand 
  to 
  thirty 
  

   thousand 
  feet, 
  or 
  more, 
  in 
  aggregate 
  thickness, 
  consist 
  of 
  all 
  the 
  

   rock-forming 
  materials 
  except 
  the 
  one 
  we 
  are 
  searching 
  for. 
  The 
  

   lapse 
  of 
  time 
  during 
  which 
  the 
  argillites 
  and 
  sandstones 
  of 
  these 
  

   groups 
  were 
  in 
  process 
  of 
  formation 
  must 
  have 
  been 
  immense 
  ; 
  and 
  

   not 
  only 
  are 
  they 
  divested 
  of 
  limestone, 
  but 
  calcareous 
  fossils 
  are 
  

   exceedingly 
  rare 
  in 
  them. 
  

  

  The 
  cases 
  cited 
  form 
  one 
  of 
  the 
  most 
  difficult 
  problems 
  in 
  prim- 
  

   ordial 
  geology 
  ; 
  for 
  considering 
  the 
  " 
  masses 
  " 
  of 
  " 
  Crystalline 
  lime- 
  

   stone" 
  characterizing 
  the 
  American 
  Archasans, 
  the 
  totally 
  incom- 
  

   mensurate 
  amount 
  of 
  calcareous 
  rocks 
  amongst 
  the 
  Cambrians 
  seems 
  

   to 
  be 
  unintelligible 
  : 
  and 
  the 
  difficulty 
  is 
  further 
  increased 
  by 
  the 
  

   fact 
  that 
  contemporary 
  organisms 
  rarely 
  had 
  skeletons 
  in 
  which 
  lime 
  

   is 
  the 
  predominant 
  constituent. 
  

  

  We 
  shall 
  make 
  no 
  attempt 
  to 
  solve 
  the 
  problem 
  : 
  all 
  we 
  can 
  do 
  

   at 
  present 
  is 
  to 
  call 
  attention 
  to 
  the 
  subject. 
  

  

  Certainly, 
  it 
  may 
  be 
  said 
  in 
  conclusion, 
  there 
  are 
  some 
  grounds 
  

   for 
  believing 
  that 
  great 
  physiographical 
  differences 
  prevailed 
  between 
  

   the 
  Cambrian 
  and 
  the 
  succeeding 
  periods 
  — 
  that 
  great 
  changes 
  in 
  life 
  

   and 
  sedimentation, 
  of 
  a 
  persistent 
  nature, 
  began 
  during 
  the 
  early 
  

   portion 
  of 
  the 
  Cambro-Silurian, 
  and 
  have 
  been 
  maintained 
  ever 
  

   since. 
  The 
  Trimerellids 
  appear 
  to 
  have 
  played 
  no 
  unimportant 
  part 
  

   in 
  the 
  beginning 
  of 
  these 
  changes. 
  

  

  VII. 
  Diagnosis 
  oe 
  the 
  Family. 
  

  

  Class 
  Palliobeanchiata, 
  De 
  Blainvillef. 
  

  

  Subclass 
  Tketenterata, 
  King$. 
  

  

  Family 
  Trimerellidce, 
  Davidson 
  & 
  King§. 
  

  

  Shell-substance 
  of 
  the 
  valves 
  calcareous. 
  Species 
  generally 
  massive, 
  

   especially 
  in 
  the 
  umbonal 
  region. 
  Umbo 
  of 
  the 
  pedicle-valve 
  often 
  

  

  * 
  The 
  limestones 
  of 
  the 
  Quebec 
  group 
  are 
  excluded, 
  as 
  they 
  are 
  now 
  generally 
  

   considered 
  to 
  be 
  on 
  the 
  horizon 
  of 
  the 
  Lower 
  Llandeilo 
  formation 
  ; 
  and 
  it 
  is 
  

   even 
  possible 
  that 
  the 
  Calciferous 
  sand-rock 
  is 
  at 
  or 
  near 
  the 
  base 
  of 
  the 
  

   Cambro-Silurian 
  system, 
  and 
  equivalent 
  to 
  the 
  Arenig 
  group. 
  

  

  t 
  BracMopoda, 
  Cuvier 
  1805 
  ; 
  Durneril, 
  1806 
  ; 
  Lamarck, 
  1809 
  ; 
  and 
  of 
  the 
  

   generality 
  of 
  palaeontologists. 
  Pattiobranchiata, 
  Blainville, 
  1824. 
  

  

  t 
  Annals 
  and 
  Mag. 
  of 
  Nat. 
  Hist. 
  4th 
  Ser. 
  vol. 
  xii. 
  July 
  1873. 
  

  

  § 
  Cuvier's 
  name, 
  Brachiopoda, 
  is 
  a 
  misnomer, 
  as 
  it 
  is 
  now 
  generally 
  admitted 
  

  

  