Although  the  establishment  of  this  genus  is  due  to
Van  Beneden,  the  probability  of  its  separation  from
Mytilus  was  first  suggested  by  an  equally  distinguished
zoologist  of  our  own  country.  Dr.  Gray.  The  Dreissena^
may  be  called  "freshwater  Mussels''  with  greater  pro-
priety than  the  Uniones,  or  Anodontie ;  and  the  habit  and
faculty  which  the  Dreissena  possess,  in  common  with  the
true  Mussels,  of  mooring  or  attaching  themselves  by  a
strong  byssus  to  extraneous  substances,  put  us  very
much  in  mind  of  their  marine  analogues.  They  are  also
equally  gregarious  and  capable  of  tiving  for  a  long  time
out  of  water.
*  Named  after  M.  Dreiaaens,  a  druggiat  at  ICaieylh.
?


DBEI88BNA.  47

Only  one  species  is  known  in  Europe ;  and  it  was  first
noticed  (in  1754)  by  the  Russian  traveller  and  natura-
list^ Pallas^  in  the  Biver  Wolga,  as  well  as  in  the  Black
Sea.  The  epithets  of  '' fluviatilis ''  and  ''marinus''
whidi  he  applied  to  these  two  forms,  coupled  with  the
dnmmstance  that  this  was  antecedent  to  the  era  and
usage  of  binomial  appeUationSj  have  given  rise  to  con-
siderable controversy  as  to  whether  these  forms  belong
to  different  species  or  to  varieties  of  the  same  species,
one  of  which  has  a  fi:eshwater,  and  the  other  a  marine,
habitat.  The  last  supposition  would  be  quite  consistent
with  the  tact  observed  by  Nilsson,  that  several  shells,
which  are  usually  inhabitants  only  of  firesh  water,  live
in  the  Baltic  Sea  together  with  other  shells  which  are
peculiarly  marine. '


Dbeisssna  polyhob'fha  *,  Pallas.
Mytilta  pofymorpkus^  &&,  PalUu,  It.  Run.  i.  p.  478.      Dreisaena  pofy-
morpka,  F.  &  H.  ii  p.  165,  pi.  xlii.  f.  4, 5,  and  (animal)  pL  Q.  f.  4.
