﻿132 
  PROCEEDINGS 
  OF 
  THE 
  MALACOLOGICAL 
  SOCIETY. 
  

  

  North 
  America 
  to 
  California, 
  it 
  would 
  certainly 
  seem 
  highly 
  im- 
  

   probable, 
  and 
  any 
  other 
  theory 
  which 
  offers 
  a 
  reasonable 
  explanation 
  

   for 
  the 
  present 
  distribution 
  of 
  the 
  species 
  would 
  be 
  preferable. 
  

  

  If, 
  then, 
  the 
  American 
  origin 
  of 
  the 
  Margariiana 
  is 
  iaiprobable, 
  

   there 
  only 
  remains 
  the 
  possibility 
  that 
  it 
  originated 
  in 
  Asia. 
  If 
  such 
  

   an 
  origin 
  can 
  be 
  assumed, 
  it 
  certainly 
  gives 
  a 
  much 
  more 
  reasonable 
  

   theory 
  for 
  the 
  dispersal 
  of 
  the 
  species 
  than 
  that 
  of 
  an 
  American 
  centre 
  

   of 
  distribution. 
  It 
  is 
  practically 
  certain 
  the 
  Pacific 
  coast 
  race 
  of 
  the 
  

   species 
  was 
  derived 
  from 
  an 
  Asiatic 
  immigration. 
  So 
  that 
  portion 
  of 
  

   the 
  problem 
  is 
  satisfactorily 
  disposed 
  of. 
  An 
  Asiatic 
  centre 
  of 
  

   distribution 
  with 
  a 
  western 
  migration 
  into 
  Europe 
  would 
  be 
  quite 
  in 
  

   accord 
  with 
  the 
  recognized 
  theories 
  in 
  regard 
  to 
  a 
  very 
  large 
  part 
  of 
  

   the 
  present 
  European 
  fauna. 
  In 
  this 
  connexion 
  it 
  is 
  to 
  be 
  noted 
  

   that 
  both 
  Simpson 
  (1900) 
  and 
  Dall 
  (1905) 
  give 
  a 
  much 
  more 
  

   extensive 
  range 
  toward 
  the 
  west 
  in 
  northern 
  Asia 
  than 
  is 
  shown 
  by 
  

   Scharff 
  in 
  his 
  map 
  in 
  1907 
  (loc. 
  cit., 
  p. 
  35). 
  This 
  is 
  probably 
  due 
  to 
  

   the 
  inclusion 
  in 
  the 
  synonymy 
  of 
  margaritifera 
  of 
  several 
  North 
  Asian 
  

   forms 
  which 
  have 
  been 
  described 
  as 
  distinct 
  species. 
  This 
  granted, 
  

   a 
  western 
  extension 
  from 
  European 
  land 
  across 
  the 
  Greenland 
  bridge 
  

   into 
  eastern 
  North 
  America 
  would 
  be 
  quite 
  as 
  feasible 
  as 
  a 
  corre- 
  

   sponding 
  migration 
  at 
  the 
  same 
  time 
  in 
  the 
  other 
  direction. 
  There 
  

   is, 
  then, 
  no 
  insurmountable 
  objection 
  to 
  such 
  a 
  theory. 
  It 
  must 
  be 
  

   confessed, 
  however, 
  that 
  the 
  direct 
  evidence 
  in 
  favour 
  of 
  an 
  Asiatic 
  

   origin 
  of 
  Margaritana 
  is 
  not 
  as 
  satisfactory 
  as 
  it 
  perhaps 
  might 
  be, 
  if 
  

   we 
  knew 
  more 
  of 
  the 
  palaeontological 
  history 
  of 
  the 
  TJnionidse 
  in 
  that 
  

   country 
  and 
  of 
  the 
  exact 
  distribution 
  of 
  the 
  present 
  fauna. 
  

  

  Simpson 
  (1896, 
  pp. 
  334-6) 
  has 
  called 
  attention 
  to 
  the 
  remarkable 
  

   similarity 
  in 
  many 
  respects 
  existing 
  between 
  the 
  Unione 
  fauna 
  of 
  

   North 
  America, 
  and 
  the 
  present 
  fauna 
  of 
  south-eastern 
  Asia 
  and 
  the 
  

   Tertiary 
  fauna 
  of 
  both 
  Europe 
  and 
  Asia, 
  and 
  thus 
  states 
  his 
  con- 
  

   clusion 
  : 
  " 
  Whether 
  the 
  Naiades 
  originated 
  in 
  North 
  America 
  or 
  in 
  

   the 
  Old 
  World 
  is 
  not 
  known. 
  At 
  any 
  rate, 
  I 
  do 
  not 
  think 
  that 
  any 
  

   careful 
  student 
  can 
  examine 
  a 
  good 
  series 
  of 
  species 
  from 
  the 
  

   Oriental 
  region 
  without 
  being 
  convinced 
  that 
  the 
  Unione 
  fauna 
  of 
  

   that 
  area 
  is 
  somewhat 
  closely 
  related 
  to 
  that 
  of 
  the 
  Laramie 
  Beds 
  

   and 
  the 
  Mississippi 
  Yalley, 
  and 
  a 
  conclusion 
  seems 
  reasonable 
  that 
  

   a 
  migration 
  took 
  place 
  perhaps 
  during 
  or 
  shortly 
  after 
  the 
  Laramie 
  

   epoch 
  over 
  an 
  old, 
  now 
  submerged, 
  land-way 
  either 
  from 
  Asia 
  to 
  

   North 
  America, 
  or 
  vice 
  versa. 
  It 
  is, 
  I 
  believe, 
  more 
  probable 
  that 
  

   this 
  fauna 
  developed 
  in 
  the 
  western 
  continent 
  than 
  the 
  eastern, 
  for, 
  

   as 
  we 
  have 
  seen, 
  a 
  few 
  prophetic 
  types 
  of 
  it 
  appeared 
  in 
  the 
  North 
  

   American 
  Jurassic, 
  while 
  the 
  earliest 
  recorded 
  existence 
  in 
  the 
  

   Old 
  World 
  of 
  species, 
  which 
  seem 
  intimately 
  related 
  to 
  it, 
  is 
  in 
  the 
  

   later 
  Cretaceous 
  or 
  earlier 
  Tertiaiy. 
  While 
  some 
  eight 
  or 
  ten 
  groups 
  

   of 
  Unios 
  and 
  Anodontas, 
  now 
  living 
  in 
  the 
  Oriental 
  region, 
  bear 
  such 
  

   a 
  strong 
  resemblance 
  to 
  similar 
  assemblages 
  in 
  the 
  United 
  States 
  that, 
  

   at 
  first 
  sight, 
  they 
  seem 
  to 
  be 
  the 
  same, 
  I 
  believe 
  every 
  one 
  of 
  them 
  

   to 
  be 
  distinct, 
  and 
  it 
  seems 
  probable, 
  when 
  it 
  is 
  taken 
  into 
  con- 
  

   sideration 
  how 
  slowly 
  the 
  Naiades 
  change 
  and 
  the 
  fact 
  that 
  the 
  forms 
  

   of 
  the 
  Laramie 
  groups 
  have 
  scarcely 
  altered 
  specifically 
  in 
  our 
  own 
  

  

  