On  Brasilian  fresh-water  shells  of  the  genus 
Planorbis 
by 
DR.  ADOLPHO  LUTZ 
(With  plates  15-18). 
The  fresh-water  mollusks  are  interme¬ 
diary  hosts  of  the  Trematoda,  to  which  be¬ 
long  many  parasites  of  importance  for  man 
and  domestic  animals.  On  account  of  this 
fact  their  study  is  not  merely  of  malaco-zo- 
ological  interest,  but  also  belongs  to  medi¬ 
cal  zoology,  as  well  as  the  study  of  blood¬ 
sucking  insects. 
One  of  the  most  important  genera  of 
these  mollusks  is  Planorbis,  because  the  in¬ 
termediary  hosts  of  Schistosomum  mansoni 
belong  to  it,  as  was  shown  in  Egypt  by  a 
medical  commission,  directed  by  LEI  PER, 
and  afterwards  confirmed  by  studies,  made 
by  me  on  brasilian  species.  In  the  pre¬ 
sent  paper  I  shall  give  the  description  and 
determination  of  the  species,  used  in  these 
studies;  they  a-re  accompanied  by  good  illus¬ 
trations  and  pieceeded  by  some  general  no¬ 
tions  for  +he  use  of  readers  less  familiar 
with  the  matter.  I  add  a  catalogue  of  other  Bra¬ 
silian  and  South-American  species,  as  found 
in  littérature,  reproducing  the  descriptions 
and  drawings,  so  as  to  help  the  determina¬ 
tion  of  any  species  observed. 
The  fresh-water  mollusks  may  be  divided 
in  Gasteropods  and  Bivalves.  The  gastero- 
pods  walk  on  a  single  foot,  provided  with  a 
sole  ;  the  head  shows  two  or  four  antennae 
and  the  body  is  elongated.  The  simplest  type 
is  seen  in  the  slugs  which  have  no  shell 
and  show  bilateral  symmetry.  Supposing  a 
slug  forming  a  shell,  open  at  the  cephalic 
end  and  increasing  in  width  while  the  ani¬ 
mal  grows,  the  shell  will  take  the  form  of  a 
more  or  less  elongated  paper  cornet.  If  one 
side  grows  more  than  the  other,  it  will  take 
a  form  like  a  ram’s  horn  ;  the  circumvolu¬ 
tions  may  be  twisted  round  each  other  in 
various  ways,  flattening  themselves  more  or 
less  by  contact  and  so  forming  the  enormous 
variety  of  shells,  used  chiefly  for  the  clas¬ 
sification  of  these  animals.  If  the  right  side 
grows  less,  the  shell  will  turn  on  this  side 
