The  little  mollusks  which  are  comprised  in  this  genns,
although  very  doeely  related  to  the  true  and  marine
Risso4B,  appear  to  have  as  much  right  to  be  generically
separated  from  the  latter  as  Neritina  has  to  form  a
distinct  genus  from  Nerita.    Nearly  all  the  HydrobuB
are  inhabitants  of  fi^sh  and  pure  water;  but  one  of
them  {H.  ventrosa)  frequents  estuaries,  as  well  as  pools
and  ditches  close  to  the  sea^shore  which  are  liable  to  be
occasionally  overflowed  by  the  tide,  and  the  water  of
which  is  more  or  less  brackish.     Those  species  which
inhabit    fresh  water  have  been  arranged  by  Moquin-
Tandon  in  a  sul^enus  otBythinia,  named  by  him  Bytku
neUa.    WithjByMtnta,  as  well  as  with  Sissoaj  this  genus
has  undoubted  relations.     It  differs,  however,  from  the
former  in  the  eyes  not  being  sessile,  but  placed  on  short
tubercles,  and  from  the  latter  in  wanting  the  caudal
filament  which  is  appended  to  the  foot.    The  shell  of
Hydrobia  has  besides  an  umbilical  cleft  which  does  not
exist  in  Rissoa;  while  its  operculum  is  decidedly  not
Bythinian,  but  Rissoan,  or  rather  LUtorinan.    It  may
therefore  be  considered  as  in  many  respects  forming  an
intermediate  link  between  those  two  genera,  but  having
a  greater  affinity  to  Bissoa.    The  genus  Hydrobia  was

*  liTing  in  water.
/


64  PALUDINIDA.
