﻿WALKER 
  : 
  ON 
  JIAHGAHITANA 
  MARGAIUTIFERA. 
  133 
  

  

  coiintry, 
  that 
  if 
  such 
  migration 
  and 
  separation 
  took 
  place 
  it 
  occurred 
  

   a 
  long 
  time 
  ago." 
  

  

  In 
  this 
  connexion, 
  it 
  is 
  to 
  be 
  noted, 
  as 
  Simpson 
  has 
  already 
  

   pointed 
  out, 
  that 
  the 
  southern 
  migration 
  of 
  the 
  Cretaceous 
  fauna 
  

   into 
  Mexico 
  and 
  Central 
  America 
  has 
  resulted 
  in 
  the 
  evolution 
  of 
  

   several 
  distinct 
  groups 
  in 
  that 
  region, 
  which 
  are 
  not 
  represented 
  

   in 
  the 
  present 
  Mississippi 
  fauna, 
  and 
  as 
  he 
  states, 
  "These 
  older 
  

   Central 
  American 
  faunal 
  groups 
  bore 
  about 
  the 
  same 
  relation 
  to 
  

   those 
  of 
  the 
  Mississippi 
  Valley 
  as 
  do 
  many 
  of 
  those 
  of 
  the 
  Oriental 
  

   region." 
  This 
  would 
  suggest 
  the 
  possibility 
  that 
  if 
  the 
  Asiatic 
  

   fauna 
  was 
  derived 
  from 
  the 
  Cretaceous 
  fauna 
  of 
  North 
  America, 
  

   it 
  was 
  a 
  migration 
  from 
  the 
  ancient 
  fauna 
  of 
  Mexico 
  and 
  Central 
  

   America 
  rather 
  than 
  from 
  that 
  portion 
  of 
  the 
  Cretaceous 
  fauna 
  

   that 
  remained 
  in 
  the 
  north, 
  to 
  become 
  the 
  progenitors 
  of 
  the 
  

   present 
  Mississippi 
  fauna. 
  In 
  view 
  of 
  the 
  fact 
  that 
  the 
  way 
  was 
  

   clear, 
  from 
  the 
  Upper 
  Cretaceous 
  to 
  the 
  later 
  Tertiary, 
  for 
  the 
  

   Asiatic 
  crayfish 
  and 
  Helices 
  to 
  pass 
  down 
  tlie 
  Pacific 
  coast 
  into 
  

   Mexico, 
  it 
  would 
  seem 
  possible 
  at 
  least 
  for 
  the 
  ancient 
  Unionidse 
  

   of 
  Mexico 
  to 
  find 
  their 
  way 
  into 
  Asia 
  by 
  the 
  same 
  route. 
  This 
  

   would 
  supply 
  the 
  necessary 
  link 
  between 
  the 
  two 
  faunas, 
  and 
  would 
  

   not 
  in 
  the 
  least 
  militate 
  against 
  the 
  argument 
  already 
  advanced 
  

   as 
  to 
  the 
  improbability 
  of 
  any 
  migration 
  from 
  central 
  jS'orth. 
  America 
  

   into 
  Asia. 
  In 
  this 
  connexion 
  the 
  following 
  extract 
  from 
  a 
  recent 
  

   letter 
  from 
  Mr. 
  Harold 
  Hannibal, 
  of 
  Stanford 
  University, 
  Cal., 
  is 
  

   of 
  interest: 
  "I 
  have 
  been 
  working 
  on 
  the 
  vertical 
  distribution 
  of 
  

   this 
  species 
  myself, 
  and 
  can 
  give 
  the 
  following, 
  whicb 
  vou 
  are 
  

   welcome 
  to 
  use 
  : 
  Unio 
  [Margaritana) 
  margaritifera 
  immigrated 
  from 
  

   Eurasia 
  in 
  the 
  earliest 
  Quaternary. 
  It 
  first 
  appears 
  on 
  the 
  west 
  

   coast 
  in 
  the 
  Upper 
  Bonneville, 
  which 
  should 
  be 
  correlated 
  with 
  the 
  

   later 
  glacial 
  epoch. 
  A 
  ,very 
  closely 
  related 
  species 
  occurs 
  in 
  the 
  

   Tejon 
  formation 
  (Eocene) 
  at 
  Tesla, 
  Cal. 
  There 
  are 
  practically 
  no 
  

   distinguishing 
  characters, 
  except 
  the 
  latter 
  is 
  a 
  survivor 
  of 
  the 
  

   occurrence 
  of 
  this 
  sub-genus, 
  Quadrula 
  and 
  a 
  number 
  of 
  other 
  genera 
  

   from 
  the 
  Cretaceous 
  and 
  Laramie, 
  while 
  the 
  former 
  is 
  an 
  immigrant. 
  

   That 
  this 
  species 
  migrated 
  to 
  Europe 
  during 
  the 
  Eocene, 
  survived 
  

   there, 
  and 
  migrated 
  back 
  in 
  the 
  Quaternary, 
  is 
  possible, 
  but 
  to 
  

   avoid 
  confusion 
  I 
  will 
  name 
  it 
  something 
  else." 
  

  

  The 
  difficulties 
  suggested 
  by 
  this 
  latter 
  theory 
  might 
  be 
  avoided 
  

   by 
  the 
  assumption 
  that 
  this 
  Eocene 
  Margaritana 
  was 
  a 
  remnant 
  of 
  

   the 
  earliest 
  invasion 
  of 
  this 
  continent 
  by 
  this 
  species 
  from 
  Asia. 
  

   The 
  association 
  of 
  Quadrula 
  with 
  it 
  is 
  of 
  great 
  interest, 
  and 
  may 
  

   determine 
  which 
  hypothesis 
  is 
  correct. 
  It 
  is 
  to 
  be 
  noted, 
  however, 
  

   that 
  the 
  theorj- 
  that 
  this 
  Eocene 
  Margaritana 
  was 
  of 
  American 
  

   origin, 
  involves 
  the 
  assumption 
  that 
  the 
  species 
  or 
  its 
  prototype 
  

   originated 
  on 
  this 
  continent, 
  and 
  had 
  practically 
  attained 
  its 
  specific 
  

   characters 
  before 
  it 
  passed 
  over 
  into 
  Asia. 
  There 
  is, 
  as 
  yet, 
  " 
  no 
  

   evidence 
  except 
  this 
  of 
  that 
  fact, 
  and 
  while 
  its 
  occurrence 
  in 
  the 
  

   Eocene 
  with 
  Quadrula 
  would 
  seem 
  to 
  argue 
  in 
  favour 
  of 
  the 
  Mexican- 
  

   Asian 
  migration 
  above 
  suggested, 
  on 
  the 
  other 
  hand 
  the 
  fact 
  that 
  

   neither 
  it 
  nor 
  any 
  congeneric 
  form 
  is 
  found 
  in 
  the 
  present 
  fauna 
  

  

  V 
  

  

  VOL. 
  IX.— 
  .irxE, 
  1910. 
  10 
  

  

  