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  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol. 
  los 
  

  

  secondarily 
  concentric 
  operculum. 
  Its 
  complete 
  biological 
  relation- 
  

   ship 
  with 
  the 
  other 
  Tanganyika 
  genera 
  and 
  with 
  extra-Tanganyika 
  

   forms 
  must 
  be 
  clarified 
  by 
  future 
  studies. 
  

  

  Genus 
  Bathanalia 
  Moore, 
  1898 
  

  

  The 
  genotype, 
  Bathanalia 
  howesi 
  Moore, 
  1898, 
  is 
  stated 
  by 
  Moore 
  

   (1898a) 
  to 
  be 
  "almost 
  identical 
  anatomically 
  with 
  Tiphobia 
  horei." 
  

   This 
  genus 
  differs 
  from 
  Tiphobia 
  by 
  shell 
  characters 
  such 
  as 
  height 
  of 
  

   spire 
  and 
  the 
  perforate 
  columella. 
  In 
  addition, 
  Bathanalia 
  possesses 
  

   an 
  unmodified 
  (paucispiral) 
  operculum. 
  

  

  Family 
  Thiaridae 
  

  

  The 
  pattern 
  of 
  reproduction 
  is 
  the 
  same 
  for 
  all 
  known 
  members 
  of 
  

   the 
  family 
  Thiaridae. 
  A 
  high 
  degree 
  of 
  specialization 
  in 
  this 
  one 
  

   character 
  is 
  indicated 
  by 
  this 
  observed 
  pattern 
  of 
  ovoviviparous 
  

   parthenogenicity. 
  In 
  other 
  words, 
  there 
  is 
  no 
  male 
  indiv^idual 
  known 
  

   in 
  this 
  entire 
  family 
  of 
  snails. 
  To 
  my 
  knowledge, 
  the 
  parthenogenesis 
  

   of 
  the 
  Thiaridae 
  has 
  not 
  been 
  examined 
  cytologically 
  to 
  determine 
  

   whether 
  it 
  is 
  haploid 
  or 
  diploid. 
  

  

  There 
  is 
  required 
  only 
  one 
  individual 
  (any 
  individual) 
  to 
  start 
  

   a 
  new 
  population 
  or 
  colony 
  if 
  it 
  reaches 
  a 
  new 
  fresh-water 
  locality 
  by 
  

   stream 
  capture 
  or 
  by 
  adventitious 
  transportation. 
  The 
  resultant 
  

   ability 
  of 
  these 
  snails 
  theoretically 
  to 
  spread 
  more 
  rapidly 
  may 
  partly 
  

   explain 
  their 
  wider 
  distribution 
  in 
  suitable 
  habitats 
  on 
  islands 
  in 
  both 
  

   the 
  Pacific 
  and 
  the 
  West 
  Indies 
  and 
  their 
  wider 
  expansion 
  southward 
  

   across 
  South 
  America 
  than 
  that 
  exhibited 
  at 
  present 
  by 
  the 
  family 
  

   Pleuroceridae. 
  Certain 
  corollaries 
  are 
  assumed 
  in 
  this 
  theoretical 
  

   explanation 
  of 
  the 
  differences 
  and 
  peculiarities 
  of 
  distribution 
  of 
  the 
  

   two 
  fresh-water 
  families 
  of 
  Pleuroceridae 
  and 
  Thiaridae. 
  Structurally, 
  

   the 
  animals 
  of 
  these 
  families 
  are 
  relativel}^ 
  primitive, 
  exceedingly 
  so 
  

   as 
  regards 
  their 
  fundamental 
  reproductive 
  characters. 
  Without 
  any 
  

   proof 
  to 
  the 
  contrary, 
  we 
  may 
  assume 
  them 
  to 
  be 
  of 
  approximately 
  

   equal 
  antiquity, 
  geologically 
  speaking. 
  Reasoning 
  from 
  the 
  special- 
  

   ized 
  pattern 
  of 
  reproduction 
  of 
  the 
  Thiaridae, 
  we 
  may 
  logically 
  argue 
  

   that 
  they 
  are 
  the 
  younger 
  group 
  evolved 
  from 
  the 
  older 
  ancestral 
  type 
  

   represented 
  now 
  in 
  fresh 
  water 
  by 
  the 
  Pleuroceridae. 
  If 
  this 
  be 
  true, 
  

   the 
  younger 
  has 
  outstripped 
  the 
  more 
  primitive 
  older 
  type 
  in 
  geo- 
  

   graphic 
  expansion 
  over 
  geologic 
  time 
  from 
  the 
  era 
  of 
  their 
  differentia- 
  

   tion 
  up 
  to 
  the 
  present. 
  If 
  the 
  Thiaridae 
  were 
  ever 
  in 
  North 
  America 
  

   we 
  must 
  assume 
  that 
  they 
  are 
  no 
  longer 
  represented 
  there 
  because 
  the 
  

   conditions 
  of 
  existence 
  are 
  not 
  now 
  or 
  were 
  not, 
  during 
  some 
  previous 
  

   geologic 
  era, 
  favorable 
  to 
  their 
  continued 
  existence 
  in 
  those 
  northern 
  

   areas. 
  Their 
  expansion 
  across 
  the 
  North 
  American 
  continent 
  may 
  be 
  

  

  