﻿avalker: 
  on 
  margaritana 
  margaritifkba. 
  139 
  

  

  Ohio 
  aud 
  Tennessee 
  systems 
  is 
  rather 
  the 
  result 
  of 
  an 
  ancient 
  

   migration 
  from 
  the 
  north-east 
  than 
  one 
  from 
  the 
  south-west. 
  

  

  The 
  occurrence 
  of 
  M. 
  margaritifera 
  in 
  the 
  lower 
  Saskatchewan 
  

   near 
  Lake 
  Winnipeg, 
  quoted 
  by 
  Dall 
  (1905), 
  is 
  certainly 
  remarkable, 
  

   and 
  from 
  our 
  present 
  knowledge 
  difficult 
  of 
  explanation. 
  According 
  

   to 
  Dall, 
  the 
  specimens 
  are 
  of 
  the 
  eastern 
  wliite-nacred 
  (typical) 
  race. 
  

   It 
  seems 
  quite 
  impossible 
  that 
  this 
  colony 
  could 
  be 
  a 
  survivor 
  from 
  

   any 
  pre-Glacial 
  fauna. 
  That 
  its 
  present 
  range 
  in 
  that 
  region 
  is 
  a 
  very 
  

   limited 
  one 
  is 
  apparently 
  shown 
  by 
  the 
  fact 
  that 
  the 
  very 
  considerable 
  

   collecting 
  in 
  that 
  region 
  done 
  by 
  the 
  Canadian 
  naturalists 
  in 
  recent 
  

   years, 
  and 
  reported 
  upon 
  from 
  time 
  to 
  time 
  by 
  Whiteaves, 
  has 
  so 
  far 
  

   failed 
  to 
  discover 
  it. 
  Nor 
  is 
  there 
  any 
  record 
  of 
  its 
  occurrence 
  in 
  the 
  

   Hudson 
  Bay 
  system 
  to 
  the 
  south 
  or 
  east. 
  If 
  the 
  colony 
  was 
  of 
  the 
  

   purple-nacred 
  western 
  race, 
  its 
  occurrence 
  in 
  the 
  Saskatchewan 
  might 
  

   be 
  accounted 
  for 
  in 
  the 
  same 
  way, 
  that 
  it 
  is 
  probable 
  it 
  obtained 
  

   a 
  foothold 
  in 
  the 
  head-waters 
  of 
  the 
  Missouri. 
  But 
  there 
  is 
  as 
  yet 
  no 
  

   record 
  of 
  its 
  presence 
  in 
  any 
  of 
  the 
  headwaters 
  of 
  the 
  Saskatchewan 
  

   or 
  its 
  tributaries. 
  In 
  view 
  of 
  the 
  fact, 
  however, 
  that 
  the 
  western 
  

   race 
  not 
  infrequently 
  has 
  the 
  nacre 
  quite 
  as 
  light-coloured 
  as 
  that 
  of 
  

   the 
  eastern 
  or 
  typical 
  form, 
  there 
  would 
  seem 
  to 
  be 
  no 
  intrinsic 
  

   improbability 
  in 
  the 
  western 
  derivation 
  of 
  the 
  Saskatchewan 
  colony. 
  

   It 
  is 
  possible, 
  of 
  course, 
  that 
  its 
  occurrence 
  iu 
  this 
  particular 
  locality 
  

   is 
  one 
  of 
  those 
  cases 
  of 
  sporadic 
  colonization 
  (possibly 
  by 
  bird 
  trans- 
  

   portation) 
  that 
  occasionally 
  occur 
  to 
  puzzle 
  the 
  zoo-geographer. 
  The 
  

   discovery 
  of 
  the 
  Planorhis 
  hicarmatus, 
  Say, 
  at 
  Antioch, 
  California, 
  

   many 
  years 
  ago 
  is 
  a 
  case 
  in 
  point 
  (see 
  "Walker, 
  1909, 
  p. 
  23). 
  But, 
  

   of 
  course, 
  that 
  is, 
  at 
  the 
  best, 
  a 
  mere 
  possibility. 
  It 
  is 
  very 
  desirable 
  

   that 
  its 
  occurrence 
  in 
  the 
  Saskatchewan 
  should 
  be 
  verified, 
  and 
  if 
  

   rediscovered, 
  that 
  the 
  extent 
  of 
  its 
  range 
  should 
  be 
  accurately 
  

   determined 
  with 
  the 
  purpose 
  of 
  finding 
  the 
  true 
  explanation 
  for 
  its 
  

   presence 
  there, 
  and 
  until 
  that 
  is 
  done 
  there 
  is 
  very 
  little 
  that 
  can 
  be 
  

   said 
  that 
  would 
  be 
  of 
  any 
  value. 
  

  

  In 
  conclusion, 
  it 
  is 
  submitted 
  that 
  fi'om 
  all 
  the 
  data 
  accessible 
  at 
  

   the 
  present 
  time 
  the 
  following 
  inferences 
  as 
  to 
  the 
  distribution 
  of 
  

   M. 
  margaritifera 
  may 
  fairly 
  be 
  deduced: 
  — 
  

  

  1. 
  That 
  it 
  is 
  not 
  probable 
  that 
  the 
  species 
  originated 
  iu 
  North 
  

   America. 
  

  

  2. 
  That 
  it 
  is 
  probable 
  that 
  it 
  did 
  originate 
  in 
  Asia. 
  

  

  3. 
  That 
  its 
  presence 
  on 
  the 
  western 
  coast 
  of 
  North 
  America 
  is 
  due 
  

   to 
  a 
  migration 
  in 
  Miocene 
  or 
  early 
  Pliocene 
  times 
  from 
  Asia 
  over 
  the 
  

   Behring 
  bridge, 
  or 
  perhaps 
  even 
  earlier. 
  

  

  4. 
  That 
  its 
  occurrence 
  on 
  the 
  east 
  coast 
  of 
  North 
  America 
  is 
  best 
  

   explained 
  by 
  a 
  similar 
  immigration 
  from 
  Europe 
  over 
  the 
  Greenland 
  

   bridge. 
  

  

  5. 
  That 
  there 
  is 
  no 
  evidence 
  to 
  show 
  that 
  it 
  was 
  an 
  inhabitant 
  of 
  

   central 
  British 
  America 
  in 
  pre-Glacial 
  times. 
  

  

  6. 
  That 
  there 
  is 
  reason 
  to 
  believe 
  that 
  the 
  causes 
  Avhich, 
  since 
  the 
  

   Glacial 
  Epoch, 
  have 
  pi'evented 
  it 
  from 
  invading 
  that 
  region 
  were 
  

   equally 
  efficacious 
  in 
  restricting 
  its 
  western 
  range 
  before 
  that 
  time, 
  

   and 
  that 
  it 
  consequently 
  was 
  not 
  exterminated 
  in 
  that 
  region 
  by 
  the 
  

   Glacial 
  ice-cap, 
  for 
  the 
  reason 
  that 
  it 
  was 
  not 
  there 
  to 
  be 
  exterminated. 
  

  

  