﻿128 
  PROCEEDINGS 
  OF 
  THE 
  MALACOLOGICAL 
  SOCIETY. 
  

  

  M. 
  crassa 
  is 
  restricted 
  to 
  South 
  Europe, 
  wliere 
  it 
  apparently 
  replaces 
  

   margaritifera 
  and 
  extends 
  ' 
  ' 
  possibly 
  into 
  Asia 
  Minor, 
  and 
  south- 
  

   west 
  Siberia" 
  (Simpson, 
  1900). 
  M. 
  Laosensis 
  is 
  known 
  only 
  from 
  

   Cambodia 
  and 
  Burmah. 
  21. 
  Eembeli 
  is 
  confined 
  to 
  the 
  gulf 
  drainage 
  

   of 
  southern 
  Louisiana 
  and 
  Alabama. 
  M. 
  monodo9ita 
  is 
  found 
  only 
  in 
  

   the 
  central 
  United 
  States, 
  where 
  it 
  inhabits 
  the 
  Ohio, 
  Tennessee, 
  and 
  

   Cumberland 
  river 
  systems 
  and 
  ranges 
  west 
  to 
  Illinois, 
  eastern 
  Iowa, 
  

   and 
  possibly 
  Ts'ebraska 
  (Simpson, 
  1900). 
  The 
  tj'pe 
  and 
  only 
  known 
  

   specimen 
  of 
  If. 
  decuvihens 
  came 
  from 
  Alabama, 
  but 
  the 
  exact 
  locality 
  

   is 
  unknown. 
  It 
  must 
  be 
  very 
  rare, 
  as 
  the 
  exhaustive 
  collections 
  

   made 
  in 
  that 
  state 
  during 
  the 
  last 
  five 
  years 
  by 
  Mr. 
  H. 
  H. 
  Smith 
  have 
  

   failed 
  to 
  rediscover 
  it. 
  The 
  recorded 
  ranges 
  of 
  the 
  several 
  American 
  

   species 
  are 
  indicated 
  on 
  the 
  map 
  (Plate 
  II). 
  

  

  M. 
  Hembeli 
  is 
  the 
  most 
  aberrant 
  member 
  of 
  the 
  group, 
  and 
  differs 
  

   from 
  all 
  the 
  other 
  species 
  by 
  the 
  plicate 
  dorsal 
  area. 
  Too 
  little 
  is 
  

   known 
  of 
  31. 
  decumhens 
  to 
  base 
  an 
  opinion 
  on. 
  Dr. 
  Lea 
  compares 
  it 
  

   with 
  one 
  of 
  the 
  many 
  forms 
  of 
  Unio 
  com2}lanatus, 
  Dill., 
  and 
  if 
  it 
  came 
  

   from 
  the 
  Atlantic 
  faunal 
  area 
  of 
  Alabama 
  the 
  comparision 
  is, 
  perhaps, 
  

   suggestive. 
  If, 
  however, 
  it 
  came 
  from 
  the 
  Alabama 
  drainage 
  system, 
  

   it 
  is 
  possible 
  that 
  it 
  may 
  prove 
  to 
  be 
  an 
  offshoot 
  from 
  the 
  monodonta 
  

   stock 
  of 
  the 
  Tennessee 
  drainage, 
  modified 
  specifically 
  in 
  the 
  same 
  way 
  

   that 
  so 
  many 
  of 
  the 
  species 
  of 
  that 
  system 
  have 
  been 
  since 
  it 
  was 
  

   separated 
  from 
  its 
  ancient 
  connexion 
  with 
  the 
  Tennessee. 
  It 
  is 
  

   possible, 
  however, 
  that 
  both 
  these 
  species 
  are 
  of 
  an 
  entirely 
  different 
  

   line 
  of 
  descent, 
  and 
  a 
  more 
  extended 
  knowledge 
  of 
  their 
  anatomy 
  may 
  

   necessitate 
  their 
  removal 
  from 
  the 
  genus. 
  

  

  The 
  remaining 
  four 
  species 
  are 
  essentially 
  homogeneous. 
  Margariti- 
  

   fera 
  and 
  crassa 
  are 
  closely 
  related, 
  and 
  Laosensis 
  and 
  monodorda 
  stand 
  

   in 
  about 
  the 
  same 
  relation 
  to 
  margaritifera. 
  The 
  range 
  of 
  the 
  two 
  

   latter 
  species 
  is 
  of 
  interest 
  in 
  connexion 
  with 
  the 
  range 
  of 
  the 
  

   margaritifera 
  and 
  not 
  without 
  importance. 
  

  

  Margaritifera 
  itself 
  is 
  a 
  very 
  ancient 
  species, 
  which, 
  through 
  an 
  

   enormous 
  extent 
  of 
  time, 
  during 
  which 
  it 
  has 
  wandered 
  nearly, 
  if 
  not 
  

   quite, 
  around 
  the 
  globe, 
  has 
  preserved 
  its 
  peculiar 
  characters 
  and 
  

   specific 
  identity 
  to 
  a 
  remarkable 
  degree. 
  The 
  essential 
  similarity 
  of 
  

   the 
  species 
  as 
  it 
  exists 
  at 
  the 
  present 
  time 
  on 
  the 
  diS'erent 
  continents 
  

   is 
  very 
  remarkable, 
  and 
  indicates 
  that 
  its 
  persistent 
  specific 
  characters 
  

   were 
  well 
  established 
  before 
  its 
  long 
  migration 
  was 
  begun. 
  As 
  North 
  

   America 
  has 
  been 
  permanently 
  separated 
  from 
  Asia 
  and 
  Europe 
  since 
  

   the 
  close 
  of 
  the 
  Tertiary 
  period, 
  and 
  the 
  progress 
  of 
  the 
  species 
  in 
  its 
  

   long 
  journey 
  must 
  have 
  necessarily 
  been 
  slow, 
  there 
  would 
  seem 
  to 
  

   be 
  no 
  doubt 
  but 
  that 
  the 
  evolution 
  of 
  the 
  species 
  must 
  have 
  long 
  

   antedated 
  that 
  period, 
  and 
  quite 
  possibly 
  may 
  go 
  back 
  even 
  to 
  

   Cretaceous 
  times. 
  

  

  "Where 
  the 
  species 
  did 
  originate 
  is 
  by 
  no 
  means 
  clear. 
  It 
  must 
  

   have 
  been 
  either 
  in 
  Europe, 
  Asia, 
  or 
  North 
  America. 
  That 
  it 
  is 
  an 
  

   immigrant 
  into 
  Europe 
  is 
  generally 
  conceded. 
  Dr. 
  ScharfP, 
  in 
  his 
  

   recent 
  work 
  on 
  European 
  Animals 
  (1907, 
  p. 
  34), 
  expresses 
  the 
  

   opinion 
  that 
  it 
  reached 
  Europe 
  via 
  Greenland 
  and 
  Iceland. 
  If 
  so, 
  

   the 
  inference 
  would 
  be 
  that 
  it 
  originated 
  in 
  North 
  America, 
  and 
  from 
  

  

  