48 
this  new  proceeding  represents  a  real  pro¬ 
gress,  as  it  clearly  shows  as  much  in  one 
drawing,  as  three  oidinary  drawings,  and 
makes  a  description  unnecessary. 
Whether  the  shell  in  the  genus  Planor¬ 
bis  be  dextral  or  sinistrai  or  variable,  ac¬ 
cording  to  species,  is  a  question  on  which 
the  authors  do  not  agree.  This  is  due  to  the 
fact  that  the  apex  is  not  well  defined.  If  we 
put  the  mouth  in  sinistrai  position,  the  upper 
face  may  be  depressed  and  even  distinctly 
umbilicated,  which,  to  several  authors,  is  a  suf¬ 
ficient  reason  for  orienting  the  shell  in  the 
opposite  way.  At  the  mouth  the  under  side 
is  longer,  which  to  MOQUIN-TANDON 
is  a  sufficient  reason  for  considering  it  as  the 
upper  one.  Admitting  these  objections,  I 
however,  attribute  such  characters  to  secon¬ 
dary  changes  and  adaptations,  while,  at  least 
in  the  species  I  examined,  the  animal  is 
sinistrai  (as  in  the  nearest  genus  Physa );  in 
this  case  the  shell  also  must  be  considered 
sinistrai.  Drawing  the  shells  in  this  posi¬ 
tion,  we  gain  the  advantage  of  exposing  the 
mouth  opening. 
The  animals  of  most  european  species 
of  Planorbis  are  well  described  (for  instance 
in  the  book  of  MOQUIN-TANDON),  but 
those  of  other  countries  are  little  known. 
In  our  drawings  of  various  species  of 
Planorbis  the  form  and  the  position  of  the 
animals,  inside  and  outside  of  the  shell,  is 
well  shown.  In  walking,  the  animal  rests  on 
the  sole  of  the  foot,  over  the  front-parts  of 
which  appears  the  head  with  two  lateral 
lobes.  On  the  under  side  the  mouth  is  seen 
and  its  working  may  be  appreciated  when 
the  animal  feeds  on  the  side  of  a  glass 
jar.  On  the  upper  side  we  find  the  an¬ 
tennae  which  may  be  somewhat  retracted, 
but  not  invaginated,  as  the  eye-bearing  an¬ 
tennae  of  the  terrestrial  snails.  The  eyes  are 
situated  inwards  of  theii  base,  while  outside 
and  to  the  left  the  head  and  the  male 
genital  duct  may  be  seen.  The  part  behind 
the  head  and  the  foot  and  supporting  both 
of  them,  may  be  called  the  neck,  as  it  moves 
and  turns  with  great  ease.  Behind  the  neck 
appears  a  kind  of  diaphragm,  shutting  the 
shell  like  a  curtain,  when  the  animal  partly 
leaves  it.  It  is  the  fold  of  the  pallium  and 
its  posterior  side  runs  backwards,  forming 
the  covering  of  the  visceral  cavity  and  the 
lining  of  the  shell.  The  first  part  of  the  vis¬ 
ceral  sack  is  mostly  occupied  by  the  respira¬ 
tory  cavity  which  occupies  the  length  of  at 
least  one  whorl.  It  is,  as  a  rule  full  of  air 
and  communicates  with  the  outside  by  means 
of  an  opening  provided  with  a  sphincter,  the 
foramen  respiratorinm.  The  extension  of  the 
cavity  shows  through  the  shell  by  increased 
transparency,  which  in  smaller  forms  is  al¬ 
most  complete.  ¿} 
The  intestine  and  the  genital  ducts  run¬ 
ning  along  the  wall  are  indicated  by  a  more 
intense  pigmentation.  There  are  also  spots, 
or  a  continuous  layer  of  pigment  situated  in 
the  pallium.  Behind  and  near  the  posterior 
end  of  the  breathing  cavity  the  beating  of 
the  heart,  formed  by  two  cavities,  may  be 
faintly  seen  through  the  shell.  Near  it  is  the 
kidney,  showing  a  glandular  aspect,  and  be¬ 
hind  it  the  large  liver  with  its  yellowish  or 
greenish-brown  colour.  Above  and  inside 
its  apical  portion  lies  the  sexual  gland  which 
furnishes  the  products  of  both  sexes.  It  is 
known  by  its  granular  or  vesicular  appearance 
and  well  distinguished  from  the  liver  by  its 
lighter  colouring. 
The  intestine  is  divided  in  pharyngeal 
bulb  (containing  the  radula),  oesophagus,  sto¬ 
mach  and  posterior  gut.  It  extends  back¬ 
wards  to  the  liver  where  it  forms  a  loop  re¬ 
turning  forwards.  The  anus  is  found  near  to 
the  opening  of  the  breathing  cavity. 
I  shall  now  discuss  several  brasilian 
species  of  Planorbis  which  I  observed  alive, 
four  of  them  being  found  near  Rio  de  Ja¬ 
neiro.  The  drawings  given  are  so  accurate  that 
they  make  the  description  almost  unneces¬ 
sary.  I  shall  also  mention  other  brasilian  and 
south-american  species,  reproducing  the  des¬ 
criptions  and  drawings  I  found  in  the  lit¬ 
térature. 
The  three  first  species,  given  in  natural 
size, are  distinguished  principally  by  the  shell; 
the  animals  of  all  of  them  are  blackish  ;  the  pal¬ 
lium  of  the  respiratory  cavity  shows  velvety 
