﻿walker: 
  on 
  MAEGAEITAXA 
  MARGAniTIFEIiA. 
  129 
  

  

  there 
  spread 
  east 
  into 
  Europe 
  and 
  west 
  into 
  Asia. 
  But 
  there 
  are 
  

   several 
  objections 
  to 
  that 
  theory. 
  

  

  There 
  is 
  no 
  doubt 
  but 
  that 
  the 
  characteristic 
  TJnione 
  fauna 
  of 
  

   jS^orth 
  America 
  is 
  descended 
  from 
  the 
  Upper 
  Cretaceous 
  species, 
  which 
  

   then 
  lived 
  "in 
  the 
  region 
  now 
  included 
  in 
  the 
  states 
  of 
  Colorado, 
  

   Utah, 
  "Wyoming, 
  South 
  Dakota, 
  and 
  Montana, 
  and 
  in 
  the 
  Canadian 
  

   territories 
  of 
  Alberta, 
  Assinniboia, 
  and 
  Saskatchewan" 
  (see 
  "White, 
  

   1905, 
  p. 
  86). 
  If 
  M. 
  margaritifera 
  originated 
  in 
  central 
  JS'orth 
  America 
  

   during 
  the 
  extraordinary 
  development 
  which 
  took 
  place 
  in 
  the 
  

   Unionidae 
  of 
  that 
  time, 
  it 
  must 
  have 
  travelled 
  nearly 
  around 
  the 
  

   globe, 
  across' 
  Europe 
  and 
  Asia, 
  and 
  over 
  the 
  Behring 
  bridge 
  in 
  

   order 
  to 
  have 
  attained 
  its 
  present 
  foothold 
  on 
  the 
  Pacific 
  coast, 
  

   hecause 
  it 
  coicld 
  not 
  hare 
  reached 
  there 
  in 
  any 
  other 
  tvay. 
  

  

  During 
  the 
  Upper 
  Cretaceous 
  period, 
  this 
  portion 
  of 
  North 
  America 
  

   was 
  separated 
  from 
  the 
  present 
  Pacific 
  region 
  by 
  an 
  arm 
  of 
  the 
  sea 
  

   which 
  extended 
  from 
  the 
  Gulf 
  of 
  Mexico 
  to 
  the 
  Arctic 
  Ocean. 
  This 
  

   insurmountable 
  barrier 
  was 
  at 
  the 
  end 
  of 
  the 
  Cretaceous 
  and 
  in 
  the 
  

   beginning 
  of 
  the 
  Tertiary 
  replaced 
  by 
  another, 
  owing 
  to 
  the 
  elevation 
  

   of 
  the 
  Pocky 
  Mountains. 
  And 
  as 
  Ortmann 
  well 
  states 
  (1902, 
  p. 
  354), 
  

   "this 
  barrier 
  was 
  probablj' 
  emphasized 
  by 
  the 
  development 
  of 
  desert 
  

   conditions, 
  in 
  and 
  at 
  the 
  foot 
  of 
  this 
  mountain 
  range." 
  This 
  barrier 
  

   was 
  sufficient 
  to 
  prevent 
  the 
  crayfish, 
  which 
  reached 
  the 
  Pacific 
  coast 
  

   from 
  Asia 
  during 
  this 
  period, 
  from 
  passing 
  into 
  eastern 
  North 
  America 
  

   (Ortmann, 
  1902, 
  p. 
  356), 
  and 
  according 
  to 
  Pilsbry 
  (1904, 
  p. 
  xliii) 
  

   similarly 
  restricted 
  the 
  range 
  of 
  the 
  Asiatic 
  Helicidge, 
  which 
  came 
  in 
  

   at 
  the 
  same 
  time. 
  So 
  too 
  with 
  the 
  Unionidae, 
  this 
  barrier, 
  first 
  of 
  

   sea, 
  and 
  later 
  of 
  mountain, 
  has 
  unquestionably 
  been 
  an 
  absolute 
  one 
  

   to 
  the 
  extension 
  of 
  the 
  fauna, 
  either 
  to 
  the 
  east 
  or 
  to 
  the 
  Avest. 
  

  

  The 
  relations 
  of 
  the 
  present 
  Unione 
  faunas 
  east 
  and 
  west 
  of 
  the 
  

   Eocky 
  Mountains 
  at 
  the 
  present 
  time 
  are 
  very 
  significant 
  in 
  this 
  

   connexion. 
  "West 
  of 
  the 
  mountains 
  the 
  fauna, 
  meagre 
  in 
  species, 
  is 
  

   represented 
  by 
  only 
  three 
  genera 
  — 
  Margaritana, 
  Anodonta, 
  and 
  Gonidea. 
  

   The 
  latter 
  is 
  endemic 
  and 
  is 
  confined 
  to 
  that 
  area. 
  The 
  single 
  species 
  

   of 
  Margaritana 
  is 
  the 
  one 
  under 
  discussion. 
  The 
  Anodontas 
  are 
  all 
  of 
  

   a 
  common 
  type, 
  evidently 
  of 
  Asiatic 
  derivation, 
  and, 
  in 
  some 
  of 
  its 
  

   phases, 
  so 
  similar 
  to 
  the 
  common 
  European 
  species, 
  A. 
  cygnea, 
  that 
  an 
  

   eminent 
  conchologist 
  in 
  this 
  country 
  has 
  questioned 
  their 
  specific 
  

   distinction 
  from 
  that 
  species 
  (see 
  Stearns, 
  1882, 
  p. 
  17). 
  

  

  As 
  in 
  the 
  Helicidte, 
  so 
  in 
  the 
  Unionidae, 
  the 
  relations 
  of 
  the 
  Pacific 
  

   fauna 
  are 
  wholly 
  with 
  the 
  Old 
  "World, 
  and 
  are 
  no 
  doubt 
  the 
  results 
  

   of 
  early 
  immigrations 
  from 
  Asia 
  (see 
  Pilsbry, 
  1904, 
  p. 
  xliii). 
  The 
  

   Anodontas 
  have 
  never 
  succeeded 
  in 
  passing 
  the 
  mountains. 
  And 
  the 
  

   only 
  known 
  instance 
  of 
  the 
  Margaritana 
  doing 
  so 
  is 
  the 
  occurrence 
  in 
  

   the 
  head- 
  waters 
  of 
  the 
  Missouri 
  in 
  Montana. 
  (The 
  Saskatchewan 
  

   locality 
  may, 
  perhaps, 
  be 
  another 
  instance.) 
  That 
  this 
  is, 
  probablj', 
  

   of 
  comparatively 
  recent 
  date, 
  is 
  shown 
  by 
  the 
  fact 
  that 
  it 
  has 
  not 
  as 
  

   yet 
  extended 
  its 
  range 
  any 
  further 
  east. 
  This 
  colony 
  "may 
  have 
  

   been 
  captured 
  with 
  streams 
  by 
  orographic 
  changes 
  or 
  transported 
  

   in 
  the 
  giochidium 
  stage 
  attached 
  to 
  fishes" 
  (Dall, 
  1905). 
  The 
  

   Margaritana, 
  moreover, 
  preferably 
  inhabits 
  rapidly 
  running 
  water, 
  

  

  