﻿OLD 
  AND 
  NEW 
  WORLD 
  MELANIANS 
  — 
  MORRISON 
  375 
  

  

  unrepresented 
  in 
  any 
  fossil 
  record, 
  known 
  or 
  yet 
  undiscovered, 
  be- 
  

   cause 
  successfully 
  living 
  fresh-water 
  moUusk 
  species 
  seldom 
  undergo 
  

   fossilization. 
  They 
  usually 
  enter 
  the 
  fossil 
  record 
  only 
  when 
  a 
  

   habitat 
  change 
  brings 
  about 
  the 
  extinction 
  of 
  populations 
  or 
  species. 
  

   On 
  the 
  other 
  hand, 
  the 
  present 
  geographic 
  distribution 
  of 
  the 
  Thiaridae 
  

   argues 
  for 
  their 
  greater 
  ability 
  to 
  spread 
  across 
  oceanic 
  areas 
  on 
  island 
  

   stepping 
  stones 
  without 
  the 
  necessity 
  for 
  any 
  continuous 
  "land 
  

   bridges." 
  

  

  There 
  is 
  one 
  important 
  taxonomic 
  problem 
  present 
  in 
  the 
  Thiaridae 
  

   that 
  has 
  not 
  often 
  been 
  acknowledged 
  in 
  the 
  study 
  of 
  these 
  or 
  any 
  

   other 
  parthenogenetic 
  animal 
  species. 
  We 
  know 
  by 
  observation 
  the 
  

   great 
  variability 
  of 
  individuals 
  within 
  the 
  species 
  of 
  the 
  dioecious 
  

   type 
  such 
  as 
  the 
  Pleuroceridae. 
  This 
  variability 
  is 
  the 
  greatest 
  

   single 
  factor 
  contributing 
  to 
  confusion 
  in 
  past 
  and 
  present 
  studies 
  of 
  

   these 
  fresh-water 
  shells. 
  The 
  variability 
  of 
  individuals 
  of 
  the 
  related 
  

   Thiaridae 
  may 
  be 
  either 
  increased 
  or 
  decreased 
  because 
  of 
  their 
  

   reduction 
  to 
  unilateral 
  ancestry. 
  What 
  we 
  actually 
  have 
  in 
  the 
  

   entire 
  family 
  of 
  the 
  Thiaridae 
  is 
  the 
  possibility 
  of 
  clone 
  formation 
  in 
  

   every 
  existing 
  population 
  of 
  these 
  animals. 
  These 
  is 
  no 
  biparental 
  

   or 
  cross-inheritance 
  control 
  or 
  check 
  on 
  individual 
  variation 
  in 
  their 
  

   present 
  parthenogenetic 
  state. 
  Wise 
  indeed 
  is 
  the 
  scientist 
  who 
  can 
  

   tell 
  whether 
  a 
  clone 
  is 
  a 
  species 
  or 
  not, 
  and 
  be 
  right 
  every 
  time, 
  in 
  the 
  

   case 
  of 
  the 
  Thiaridae. 
  

  

  Family 
  Thiaridae 
  (in 
  the 
  Americas) 
  

  

  The 
  earliest 
  generic 
  name 
  for 
  any 
  of 
  the 
  indigenous 
  American 
  

   species 
  of 
  this 
  family 
  is 
  Aylacostoma 
  Spix 
  (1827, 
  p. 
  15, 
  pi. 
  8). 
  This 
  

   manuscript 
  name 
  of 
  Spix 
  was 
  first 
  published 
  in 
  the 
  synonymy 
  of 
  both 
  

   Melania 
  tuberculata 
  Wagner 
  (= 
  Aylacostoma 
  tuberculatum 
  Spix) 
  and 
  

   Melania 
  scalaris 
  Wagner 
  ( 
  = 
  Aylacostoma 
  glabrum 
  Spix) 
  in 
  connection 
  

   with 
  the 
  species 
  description, 
  and 
  also 
  on 
  plate 
  8 
  (Spix, 
  1827) 
  without 
  

   the 
  indication 
  of 
  synonymy. 
  This 
  is 
  in 
  the 
  first 
  edition 
  (Munich) 
  of 
  

   plates, 
  not 
  the 
  second 
  edition 
  (Leipsic). 
  As 
  a 
  manuscript 
  name 
  of 
  

   Spix, 
  published 
  in 
  direct 
  connection 
  with 
  two 
  validly 
  described 
  and 
  

   figured 
  species, 
  Aylacostoma 
  is 
  valid 
  and 
  available. 
  The 
  genotype, 
  

   by 
  subsequent 
  designation 
  by 
  Morrison 
  (1952, 
  p. 
  8), 
  is 
  Melania 
  

   scalaris 
  Wagner 
  (= 
  Aylacostoma 
  glabrum 
  Spix). 
  This 
  species, 
  as 
  

   comparison 
  of 
  the 
  figures 
  will 
  demonstrate, 
  is 
  that 
  form 
  called 
  behnii 
  

   by 
  Reeve 
  (1859-1861, 
  pi. 
  2, 
  fig. 
  8) 
  in 
  his 
  1860 
  monograph 
  on 
  Hemisinus. 
  

  

  Swainson 
  (1840, 
  pp. 
  200, 
  341) 
  was 
  next 
  to 
  furnish 
  a 
  name 
  for 
  Ameri- 
  

   can 
  Thiaridae. 
  Hemisinus 
  is 
  his 
  generic 
  name 
  given 
  to 
  the 
  species 
  

   lineolata 
  Gray 
  from 
  Jamaica. 
  The 
  later 
  emendations, 
  Aulacostoma 
  

   Agassiz 
  1846 
  and 
  Semisinus 
  Crosse 
  and 
  Fischer 
  1885, 
  are 
  both 
  

  

  