46 
and  become  dextral,  which  is  the  rule  ;  in 
the  opposite  case,  it  will  be  sinistrai.  When 
the  shell  is  altogether  elongated,  conical,  fu¬ 
siform  or  globose,  the  opening  in  the  dextral 
is  opposed  to  the  right  hand  of  the  obser- 
server,  when  the  apex  is  directed  upwards 
and  the  mouth  forwards.  In  the  sinistrai,  it 
takes  an  opposite  position,  to  the  left  hand 
of  the  observer,  and,  in  this  case,  not  only 
the  direction  of  the  shell,  but  also  the  po¬ 
sition  of  the  organs  of  the  inhabitant  is  re¬ 
versed. 
The  gasteropods  of  fresh  water  are 
either  operculate  or  not  operculate.  The  first 
include  the  largest  forms  in  the  genus  Am- 
pullaria ,  characterised  by  four  antennae  and 
e  respiratory  syphon.  Their  shell  is  usually 
globose,  as  in  the  common  snails.  They 
are  generally  known  by  the  name  of  “ aruá ” 
in  Brazil  and  principally  in  the  north, 
where  they  are  eaten.  The  family  Melaniidae 
contains  other  species,  abundant  in  rivers, 
the  shell  being  thick  and  calcareous,  of  elon¬ 
gated  conical  form,  showing  longitudinal 
sculptures  and  often  a  corroded  apex.  They 
frequently  contain  trematodes,  but  none  of 
them  is  known  to  be  of  greater  importance. 
Many  Melaniidae  are  viviparous. 
The  most  important  species  belong  to 
the  family  Limnaeidae ,  formed  by  non-oper- 
culate  fresh-water  snails  provided  with 
lungs.  In  their  form  they  are  like  many  ter¬ 
restrial  species,  often  found  near  the  water, 
but  they  are  easily  distinguished  by  their 
way  of  living;  they  are  always  found  in  the 
water  (or  close  by,  when  they  exceptionally 
leave  it).  Also  they  never  have  more  than 
two  antennae  and  the  eyes  are  at  their  base. 
With  exception  of  Ancylus  they  have  a  large 
respiratory  cavity,  easily  seen  through  the 
transparent  shell. 
For  distinguishing  our  genera,  the  fol¬ 
lowing  key  may  be  used: 
1.  Shell  small,  flattened  like  a 
shield .  Ancylus 
Shell  with  distinct  whorls.  .  2 
2.  Shell  discoid,  twisted  like  a 
watch-spring .  Planorbis 
Shell  ovoid  or  sub-conical.  .  3 
3.  Shell  sinistrai.  Antennae  fi¬ 
liform .  Physa 
Shell  dextral.  Antennae  en¬ 
larged  at  the  base . Limnaeus 
The  three  last  genera  have  sub-genera  ; 
if  these  are  given  the  value  of  genera  the 
first  pass  to  the  rank  of  super-genera. 
Of  Ancylus  I  observed  a  species  cor¬ 
responding  probably  to  A.  moricandi  and  a 
few  others. 
Of  Limnaeus  I  observed  a  species  which 
may  be  viator  of  D’ORBIQNY  and  is  pro¬ 
bably  the  intermediary  host  of  F.  hepática; 
there  are  a  few  other  species  rarely  found. 
Of  Physa  I  found  two  species,  one  of 
them  common. 
Of  Planorbis  four  species,  found  in  Rio, 
and  seven  more,  observed  in  the  North,  are 
discussed  in  this  paper. 
While  the  genus  Planorbis  is  easily  re¬ 
cognized  by  the  typical  shell-form,  the  de¬ 
termination  of  the  sub-genera  and  species 
is  often  very  difficult.  The  former  were  most¬ 
ly  established  long  ago,  with  an  insufficient 
knowledge  of  many  species.  These  are  dis¬ 
tributed  all  over  the  world  and  already  in 
1850  numbered  nearly  120.  Considering  that 
the  species  are  often  widely  spread  and 
rather  variable,  we  might  expect  a  large  sy¬ 
nonymy.  The  difficulty  in  obtaining  all  the 
literature,  as  well  as  ¡the  insufficiency  of  de¬ 
scriptions,  often  made  from  empty  shells,  form 
other  obstacles. 
If  the  characters,  taken  from  the  shell,  are 
not  sufficient,  as  a  rule,  the  other  ones,  also, 
are  not  very  satisfactory.  The  radula  or 
scraper,  which  covers  the  tongue,  has  a  very 
variable  structure,  used  for  the  classification 
of  families  and  genera  but  seemingly  less 
useful  for  distinguishing  sub-genera  and 
species.  •  S' 
Our  species  may  be  divided  in  two 
groups,  of  which  the  first  contains  species 
with  plenty  of  black  pigment  and  abundant 
hémoglobine,  dissolved  in  the  blood  and 
colouring  it  red.  Form  and  size  of  the  shell 
are  variable  but  the  whorls  are  always  rather 
wide  in  relation  to  the  diameter  of  the  shell. 
With  the  animal  inclosed  the  shell  appears 
