50 
shows  a  greater  height,  although  this  is  infe¬ 
rior  to  the  width.  Considering  the  shell  sinis¬ 
trai,  the  upper  half  of  the  whorl  is  always 
narrower  than  the  under  one.  Generally,  the 
upper  face  is  more  concave  and  the  last 
whorl  a  little  deflected  upwards  (PI.  XV  b), 
but  sometimes  the  contrary  may  be  seen 
(Pi.  XV  e).  The  opening  does  not  show 
an  angle  on  top,  but  in  exceptional  cases 
there  may  be  a  very  narrow  curve. 
The  colour  of  the  shell  varies  from 
transparent  corneous  to  light  ocraceous  in 
life  ;  after  death  it  may  become  more  or  less 
opaque,  whitish  or  straw-coloured.  The  part 
occupied  by  the  animal  always  appears 
dark. 
The  animal  itself  is  dark  blackish  ;  when 
little  pigmented  and  full  of  blood,  it  appears 
reddish-brown;  other  specimens  are  dark  olive 
in  colour.  The  part  over  the  respiratory  ca¬ 
vity  does  not  show  as  dark  and  velvety 
black  as  it  may  do  in  the  two  following 
species.  The  specimens  from  the  north  did 
well  in  captivity,  but  would  not  breed  in  the 
cooler  season.  They  are  easily  infected  with 
Schistosomum  mansoni  and  spontaneous  in¬ 
fection  is  pretty  frequent. 
Planorbis  olivaceiis  was  observed  by 
SPIX  in  Uheos  and  Almada,  where  we  could 
not  obtam  it  now;  also  by  PIRAJÁ  in  the 
city  of  Bahia  where  it  is  quite  abundant  and 
the  only  large  species.  Other  specimens 
were  obtained  in  Âracajú  where  they  were 
exceedingly  common.  It  often  occurs  alone  and 
sometimes  wi.h  other  species.  As  for  the  sub¬ 
genus  it  agrees  with  Menetus  ADAMS,  though 
in  size  it  is  larger  than  the  average.  Pla¬ 
norbis  bahiensis  DUNKER,  a  possible  syno¬ 
nym,  is  considered  by  VON  MARTENS  as 
intermediary  between  Menetus  and  Helisoma 
SWA1NSON,  1840. 
2.  Planorbis  confusus  n.  n. 
(Plate  15  fig.  2,  a,  b,  c,  d.) 
This  species  was  referred  to  by  D’ORBIG- 
NY  as  ferrugineus  SPIX,  but  differs  by  the 
shape  of  the  mouth,  as  described  by  D’OR- 
BIGNY  and  figured  in  REEVE-SOWERBY, 
where  confusus  appears  a  second  time  with 
the  erroneous  name  tenagophilus  D’ORBIGNY. 
(For  this  and  other  reasons  the  name  of 
SPIX  cannot  be  used.)  I  found  it  in  the  same 
place  as  D’ORBIGNY.  Fortunately  it  does  not 
seem  to  exist  together  with  the  first  spe¬ 
cies,  which  would  make  the  determination  of 
I  1 
not  very  typical  specimens  rather  difficult. 
The  largest  specimens,  as  shown  in  the  fi¬ 
gure,  never  come  up  to  the  size  of  the  pre¬ 
ceding  species  and  the  last  whorl  is  gene- 
nally  bent  a  little  upwards  and  narrower, 
but  relatively  higher,  showing,  in  sinistrai  po¬ 
sition,  a  rounded  keel  near  the  suture.  The 
number  of  complete  whorls  does  not  exceed 
five,  while  in  the  preceding  species  there 
may  be  six.  The  animal  differs  little  in  both 
species,  though  that  of  confusus  is  a  little 
darker. 
The  shell  occurs  in  ditches  and  in  pools 
of  standing  water,  with  or  without  aquatic 
plants,  sometimes  in  very  large  numbers,  of 
which  only  a  few  have  reached  full  size.  It 
is  rarely  found  alone  ;  in  Rio  it  is  often  ac¬ 
companied  bv  the  next  species.  The  shell  is 
transparent  horny-yellow  but  looking  black 
when  filled  by  the  living  animal.  It  is  often 
attacked  by  small  algae,  producing  excava¬ 
tions,  where  the  shell  appears  calcareous  and 
white,  becoming  very  weak  and  brittle.  After 
death,  the  whole  shell  may  become  opaque 
and  calcareous. 
In  his  “Journey  in  South  America”  D’OR¬ 
BIGNY  mentions  this  species  and  gives  the 
following  description: 
“Planorbis  ferrugineus  SPIX. 
Planorbis  ferrugineus  et  P.  olivaceus 
SPIX,  pi  XVIII,  fig  2. 
1.  Planorbis  olivaceus  WAGNER,  id. 
D’ORBIGNY  Syn.  Mag.  de  zool.  (1835)  pag. 
26  no  1. 
P.  corpore  nigrescente.  Testâ  discoideo- 
depressâ,  subdiaphanâ,  ferrugineâ,  superne 
plano-depressâ  stristâ  subtus  concavâ  sublae- 
vigatâ,  anfractibus  sex,  subangulato  apertura 
semilunari. 
Diam.  30  millim.  ait.  10  centim. 
Cette  belle  espèce,  remarquable  par  sa 
partie  supérieure  peu  déprimeé,  par  sa  partie 
inférieure  concave  mais  étroite  comparative- 
