47 
very  dark;  however,  when  the  mollusk,  in 
consequence  of  a  kind  of  albinism,  shows 
little  pigment,  the  result  is  a  bright  orange  or 
reddish  colour,  due  to  the  red  blood.  Our 
species  belong  apparently  to  the  subgenera 
Menetus  and  Taphius,  the  latter  forming  a 
transition  to  the  second  group. 
The  second  group  consists  of  small  flat 
species  of  light  hue,  due  to  the  absence  of 
red  colour  in  the  blood  and  of  black  pigment 
in  the  skin.  The  whorls  are  numerous,  narrow 
and  more  or  less  flattened.  The  head,  at 
least  in  three  species,  shows  a  yellow  spot  ; 
the  shell  assumes  a  horizontal  position.  This 
group  seems  to  belong  to  the  sub-genus 
Spirulina. 
Of  the  first  group  I  observed  about 
seven  ana  of  the  second  three  species,  figu¬ 
red  at  the  end  of  this  paper. 
The  shells  of  the  mollusks  generally 
show  three  layers,  the  exterior  being  formed 
by  the  epidermis  ;  the  middle  one  has  a  cal¬ 
careous  appearance,  while  the  inner  consists 
of  mother-of-pearl.  In  the  Limnaeidae  the 
layers  are  not  distinct;  the  fresh  shells, 
formed  principally  by  a  corneous  substance, 
the  conchiolin,  are  very  thin  and  transparent. 
Only  in  pathological  conditions,  when  the 
epidermis  is  destroyed  and  the  media  attacked 
by  small  algae  or  other  aquatic  organisms, 
or  in  dead  and  old  shells,  the  calcareous  as¬ 
pect  appears,  accompanied  by  a  brittle  con¬ 
sistence.  The  inner  layer  only  shows  at  the 
opening  of  the  shell  where  the  last  whorl 
ends  in  contact  with  the  preceeding  one, 
forming  a  milky  spot. 
The  colour  of  the  fresh  shell  is  variable 
in  the  same  species,  also  the  thickness,  which 
seems  to  depend  on  external  conditions. 
Some  shells  are  almost  hyaline  of  amber  or 
honey  colour,  or  rusty,  or  blackish.  During 
life  the  colour  of  the  animal  shines  through 
the  transparent  shell  which,  after  death, 
shows  a  more  variable  colour,  in  consequen¬ 
ce  of  decomposition.  The  form,  resulting 
from  the  direction  of  the  whorls,  is  also 
rather  variable;  if  the  specimens  are  nume¬ 
rous,  some  aberrations  always  exist,  which, 
if  found  alone,  might  make  the  determina¬ 
tion  very  difficult. 
The  colour  of  the  animal  may  be  light, 
of  almost  transparent  or  of  opaque,  more 
or  less  dirty  white,  or  ocraceous.  Many 
species  have  abundant  black  pigment  which, 
however,  may  be  much  reduced  in  some 
individuals.  In  the  larger  and  darker  species 
the  blood  is  distinctly  reddish,  on  account 
of  the  haemoglobin,  dissolved  in  the 
plasma.  Such  species,  in  specimens  with 
much  blood  and  little  pigment,  may  appear 
reddish-brown,  instead  of  black.  In  two  spe¬ 
cies  we  even  found  individuals  almost  with¬ 
out  black  pigment,  showing  animals  of 
bright  orange  colour  which  seemed  to  be¬ 
long  to  another  species.  Such  specimens  are 
very  useful  for  anatomical  studies. 
Dimensions  ought  to  he  taken  from  the 
largest  specimens,  which  are  comparatively 
rare,  as  only  a  small  proportion  attains  the 
age  limit.  (In  many  mollusks  the  propagati¬ 
on  takes  place  before  full  size  is  attained). 
In  our  group  adult  individuals  do  not  show 
the  thickening  of  the  lip  or  free  edge  of  the 
opening,  seen  in  other  shells,  and  so  we 
run  the  risk  of  taking  for  small  forms  young 
specimens,  belonging  to  large  species.  The 
dilation  at  the  mouth  of  the  shell  may  be 
observed,  as  well  in  young,  as  in  old  speci¬ 
mens  of  certain  species. 
Characters  are  taken  from  the  width  or 
largest  diameter  of  the  shell,  from  the  number 
of  whorls  and  the  way  in  which  they  cover 
each  other,  from  the  form  of  the  perpendi¬ 
cular  section  and  of  the  mouth  or  opening 
of  the  last  circumvolution.  The  height  at  the 
mouth  does  not  necessarily  correspond  to 
the  largest  diameter  of  the  opening  which 
may  be  oblique.  Even  the  mouth  is  not  per¬ 
pendicular  to  the  whorl,  but  more  or  less 
inclined  and  the  last  whorl  may  be  deflec¬ 
ted,  upwards  or  downwards. 
For  understanding  all  these  relations, 
nothing  is  more  useful  than  a  perpendicular 
section,  passing  through  the  center  of  the 
shell,  opening  all  the  whorls  and  showing 
the  mouth,  like  those  we  give  in  the  dra¬ 
wings  of  almost  all  the  species.  I  think  that 
