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  THE 
  DISTEIBUTION 
  OF 
  MARGAEITAXA 
  MARGAEITIFERA 
  

   (LINN.) 
  IN 
  NOETH 
  AMERICA. 
  

  

  Ey 
  Bryant 
  Walker. 
  

  

  Read 
  8th 
  April, 
  1910. 
  

  

  PLATE 
  II. 
  

  

  This 
  common 
  and 
  well-known 
  species 
  has 
  the 
  most 
  extensive 
  range 
  

   of 
  any 
  of 
  the 
  TJnionidie. 
  As 
  stated 
  by 
  Simpson 
  (1900, 
  p. 
  677), 
  

   it 
  inhabits 
  "all 
  Europe, 
  except 
  the 
  southernmost 
  portion; 
  northern 
  

   Asia; 
  Japan; 
  northern 
  North 
  America; 
  Iceland. 
  Its 
  southern 
  limit 
  

   seems 
  to 
  be 
  about 
  north 
  latitude 
  40°. 
  It 
  appears 
  to 
  be 
  entirely 
  

   circumboreal, 
  except 
  that, 
  so 
  far 
  as 
  is 
  known, 
  it 
  is 
  missing 
  in 
  the 
  

   central 
  part 
  of 
  the 
  JS'orth 
  American 
  continent". 
  

  

  Wetherby 
  (1881, 
  p. 
  7) 
  was 
  apparently 
  the 
  first 
  to 
  call 
  attention 
  

   to 
  the 
  peculiar 
  distribution 
  of 
  the 
  species 
  on 
  this 
  continent, 
  and 
  

   his 
  suggestion 
  that 
  its 
  absence 
  in 
  central 
  JS'orth 
  America 
  was 
  due 
  

   to 
  its 
  extinction 
  in 
  that 
  region 
  during 
  the 
  Glacial 
  Epoch, 
  and 
  that 
  

   for 
  ?ome 
  reason 
  it 
  had 
  not, 
  like 
  its 
  associated 
  species, 
  been 
  able 
  to 
  

   regain 
  its 
  former 
  range 
  upon 
  the 
  retreat 
  of 
  the 
  glacier, 
  has 
  been 
  

   approved 
  by 
  Simpson 
  (1893, 
  p. 
  594, 
  and 
  1896, 
  p. 
  339), 
  and 
  tenta- 
  

   tively, 
  perhaps, 
  by 
  Call 
  (1882, 
  p. 
  402) 
  and 
  Dall 
  (1905, 
  p. 
  132). 
  

  

  A 
  recent 
  study 
  of 
  the 
  known 
  distribution 
  of 
  M. 
  margaritifera 
  in 
  

   North 
  America, 
  and 
  the 
  possible 
  causes 
  for 
  its 
  anomalous 
  character, 
  

   has 
  raised 
  a 
  question 
  as 
  to 
  the 
  sufficiency 
  of 
  AVetherby's 
  supposition, 
  

   and 
  has 
  suggested 
  that 
  possibly 
  the 
  explanation 
  is 
  to 
  be 
  sought 
  for 
  

   along 
  an 
  entirely 
  different 
  line 
  of 
  argument. 
  

  

  But 
  before 
  pursuing 
  that 
  subject, 
  it 
  may 
  be 
  well 
  to 
  consider 
  the 
  

   exact 
  distribution 
  of 
  the 
  species 
  on 
  this 
  continent, 
  so 
  far 
  as 
  it 
  has 
  

   been 
  made 
  known. 
  The 
  accompanying 
  map 
  (Plate 
  II) 
  shows 
  with 
  

   substantial 
  accuracy 
  the 
  present 
  range 
  of 
  II. 
  margaritifera 
  in 
  North 
  

   America. 
  The 
  detailed 
  list 
  of 
  the 
  exact 
  localities, 
  from 
  which 
  it 
  

   has 
  been 
  compiled, 
  will 
  be 
  found 
  at 
  the 
  conclusion 
  of 
  the 
  paper. 
  

  

  On 
  the 
  Pacific 
  coast, 
  it 
  may 
  be 
  said, 
  in 
  general 
  terms, 
  to 
  range, 
  

   west 
  of 
  the 
  Kocky 
  and 
  Wasatch 
  Mountains, 
  from 
  southern 
  Alaska 
  

   south 
  to 
  Santa 
  Cruz 
  and 
  Merced 
  Counties, 
  Cal., 
  northern 
  Nevada 
  

   and 
  Utah, 
  and 
  possibly 
  into 
  Arizona. 
  The 
  most 
  northern 
  locality 
  

   recorded 
  is 
  that 
  of 
  Dall, 
  at 
  Naha 
  Bay, 
  Alaska, 
  lat. 
  50° 
  35' 
  N. 
  The 
  

   most 
  southern 
  authentic 
  records 
  are 
  the 
  Stanislaus 
  Iliver 
  and 
  

   Merced 
  Co., 
  Cal. 
  The 
  records 
  from 
  Idaho 
  and 
  Utah 
  are 
  both 
  very 
  

   considerably 
  north 
  of 
  these 
  Californiau 
  localities. 
  It 
  apparently 
  

   does 
  not 
  occur 
  anywhere 
  in 
  the 
  Colorado 
  basin, 
  unless 
  Call's 
  very 
  

   doubtful 
  citation 
  be 
  verified. 
  East 
  of 
  the 
  Ilocky 
  Mountains, 
  it 
  has 
  

   only 
  been 
  recorded 
  from 
  the 
  head-waters 
  of 
  the 
  Missouri 
  above 
  

   the 
  falls 
  (Cooper), 
  and 
  the 
  Gallatin 
  Eiver 
  (Call) 
  in 
  Montana. 
  

  

  This 
  western 
  race 
  of 
  margaritifera 
  is 
  peculiar 
  in 
  having 
  the 
  nacre 
  

   usually 
  of 
  a 
  dull 
  purple 
  colour, 
  and 
  has 
  twice 
  been 
  described 
  as 
  

   a 
  distinct 
  species, 
  first 
  by 
  Gould 
  in 
  1850 
  as 
  Alasmoion 
  falcata, 
  and 
  

  

  