﻿138 
  PROCEEDINGS 
  OF 
  THE 
  MA 
  LACOLOGICAL 
  SOCIETY. 
  

  

  p. 
  393). 
  It 
  does 
  not 
  appear 
  to 
  range 
  as 
  far 
  east 
  as 
  western 
  Penn- 
  

   sylvania 
  (Ortmann, 
  1909). 
  In 
  Indiana 
  it 
  is 
  cited 
  by 
  Call 
  (1900, 
  

   p. 
  526) 
  and 
  Daniels 
  (1903, 
  p. 
  650) 
  from 
  the 
  Ohio 
  and 
  Wabash, 
  and 
  

   from 
  there 
  ranges 
  west 
  through 
  Illinois 
  (Baker, 
  1906, 
  p. 
  76). 
  into 
  the 
  

   Mississippi 
  drainage 
  of 
  eastern 
  Iowa 
  (Keyes, 
  1889, 
  p. 
  19). 
  It 
  has 
  

   been 
  listed 
  from 
  the 
  Elkhorn 
  and 
  Elue 
  Elvers 
  in 
  jS'ebraska'.by 
  

   Aughey 
  (1877, 
  p. 
  703), 
  but 
  these 
  records, 
  like 
  many 
  others 
  of 
  that 
  

   author, 
  require 
  verification. 
  In 
  the 
  Mississippi 
  it 
  is 
  recorded 
  by 
  

   Baker 
  from 
  Adams 
  County, 
  Illinois, 
  which 
  is 
  the 
  extreme 
  south- 
  

   westerly 
  record. 
  Its 
  range 
  northward 
  in 
  that 
  river 
  is 
  apparently 
  

   a 
  limited 
  one, 
  as 
  it 
  is 
  not 
  recorded 
  from 
  Wisconsin 
  by 
  Chadwick 
  

   (1906), 
  nor 
  from 
  Minnesota 
  by 
  Grant 
  (1885 
  and 
  1887). 
  It 
  is 
  found 
  

   also 
  in 
  the 
  Tennessee 
  drainage 
  system 
  at 
  Knoxville, 
  Tenn., 
  in 
  the 
  

   Holston 
  and 
  Little 
  Rivers, 
  and 
  in 
  the 
  Tennessee 
  Biver 
  at 
  Florence, 
  

   Alabama 
  (Walker). 
  It 
  has 
  not 
  yet, 
  however, 
  been 
  recorded 
  from 
  the 
  

   head-waters 
  of 
  the 
  Tennessee 
  system. 
  Its 
  distribution 
  is 
  apparently 
  

   very 
  local 
  and 
  discontinuous, 
  as 
  shown 
  by 
  the 
  chart 
  (Plate 
  II). 
  In 
  

   the 
  absence 
  of 
  any 
  record 
  of 
  its 
  occurrence 
  in 
  the 
  south 
  and 
  west 
  

   beyond 
  the 
  points 
  mentioned, 
  the 
  inference 
  would 
  be 
  that 
  its 
  original 
  

   point 
  of 
  dispersal 
  was 
  in 
  the 
  east, 
  and 
  that 
  it 
  had 
  migrated 
  westward 
  

   by 
  two 
  routes, 
  one 
  down 
  the 
  Ohio 
  and 
  thence 
  into 
  the 
  Mississippi 
  

   Yalley, 
  and 
  the 
  other 
  down 
  the 
  Tennessee 
  from 
  its 
  tributaries 
  or 
  

   head-waters. 
  That 
  it 
  reached 
  its 
  present 
  range 
  by 
  a 
  migration 
  from 
  

   the 
  south-west 
  is, 
  in 
  view 
  of 
  the 
  known 
  facts 
  of 
  its 
  present 
  distribution, 
  

   quite 
  improbable. 
  

  

  On 
  the 
  other 
  hand, 
  that 
  it 
  was 
  originally 
  an 
  immigrant 
  from 
  the 
  

   head-waters 
  of 
  the 
  Atlantic 
  drainage 
  into 
  those 
  of 
  the 
  Ohio 
  and 
  

   Tennessee 
  is 
  most 
  probable. 
  AVhile 
  it 
  has 
  not 
  as 
  yet 
  been 
  recorded 
  

   from 
  the 
  head-waters 
  of 
  those 
  river 
  systems, 
  that 
  is 
  no 
  reason 
  why 
  it 
  

   may 
  not 
  yet 
  be 
  discovered 
  in 
  both, 
  for 
  as 
  already 
  remarked, 
  the 
  

   species 
  is 
  remarkably 
  local 
  in 
  its 
  distribution, 
  and, 
  even 
  if 
  no 
  longer 
  

   resident, 
  it 
  may 
  not 
  have 
  formerly 
  lived 
  there. 
  The 
  head-waters 
  of 
  

   the 
  eastern 
  and 
  western 
  drainage 
  systems 
  of 
  the 
  Appalachian 
  slopes 
  

   are 
  not 
  so 
  widely 
  separated 
  that 
  there 
  has 
  not 
  been 
  in 
  past 
  ages 
  

   stream 
  transference 
  from 
  one 
  system 
  to 
  the 
  other, 
  with 
  the 
  consequent 
  

   introduction 
  of 
  the 
  fauna 
  from 
  one 
  system 
  into 
  the 
  other. 
  This 
  is 
  

   shown 
  by 
  the 
  occurrence 
  of 
  Anculosa, 
  a 
  characteristic 
  western 
  genus, 
  

   in 
  the 
  Atlantic 
  drainage 
  (see 
  Pilsbry, 
  1894, 
  p. 
  26). 
  The 
  presence 
  of 
  

   Quadrula 
  and 
  Pleiirobema, 
  two 
  of 
  the 
  most 
  prevalent 
  genera 
  of 
  the 
  

   Tennessee 
  system, 
  in 
  the 
  Atlantic 
  drainage 
  of 
  Virginia 
  and 
  North 
  

   Carolina 
  points 
  unmistakably 
  to 
  the 
  same 
  fact. 
  And 
  if 
  the?e 
  

   distinctly 
  westeru 
  forms, 
  by 
  means 
  of 
  ancient 
  orographic 
  changes, 
  

   were 
  enabled 
  to 
  cross 
  the 
  divide 
  and 
  obtain 
  a 
  foothold 
  in 
  the 
  Atlantic 
  

   rivers, 
  there 
  is 
  no 
  reason 
  why 
  in 
  the 
  same 
  way 
  the 
  molluscan 
  

   fauna 
  of 
  the 
  early 
  eastern 
  streams 
  should 
  not 
  at 
  some 
  time 
  have 
  

   succeeded 
  in 
  obtaining 
  access 
  to 
  the 
  head-waters 
  of 
  the 
  western 
  

   drainage 
  system. 
  That 
  the 
  migration 
  was 
  a 
  very 
  ancient 
  one 
  is 
  

   shown 
  by 
  the 
  specific 
  differentiation 
  that 
  has 
  taken 
  place 
  in 
  the 
  

   species 
  thus 
  transferred 
  from 
  one 
  area 
  into 
  the 
  other. 
  It 
  may 
  be 
  

   fairly 
  concluded, 
  therefore, 
  tliat 
  the 
  presence 
  of 
  Margaritana 
  in 
  the 
  

  

  