GENUS  GYMNODINIUM.  443

envelope,  the  animalcules  assume  after  death  various  irregular  shapes,  and  speedily
undergo  complete  disintegration.  As  indicated  by  Stein,  Gymnodimum  may  be
regarded  as  the  ground  or  stock  form  of  the  various  more  complex  representatives
of  the  Peridiniidae.
Oymnodl2iiimi  ftuK^nm,  Ehr.  sp.    Pl.  XXV.  Figs.  17  and  18.
Body  oval,  slightly  compressed,  rounded  posteriorly,  acuminately  pointed
anteriorly,  nearly  twice  as  long  as  broad  ;  surface  smooth,  colour  light  brown  ;
no  eye-like  pigment-spot    Length  1-430"  to  1-280".     Hab. — Fresh  water.
In  the  figures  of  this  species,  as  given  by  Ehrenberg  in  connection  with  the
title  of  Paidinium  fuscum^  and  here  reproduced,  the  flagellate  appendage  was
apparently  overlooked,  while  the  vertical  as  well  as  the  transverse  groove  is
represented  as  bordered  with  cilia.  The  presumed  example  of  longitudinal  fission
delineated  by  the  same  authority  (see  PI.  XXV.  Fig.  18),  is  regarded  by  Stein
as  an  advanced  phase  of  the  process  of  fiision  or  genetic  union  between  two
previously  independent  zooids,  which  he  has  frequently  observed  to  take  place  under
similar  auspices  in  connection  with  the  species  next  described.
Oymnodininm  pulvlflcaliui,  Ehr.  sp.    Pl.  XXV.  Fios.  19  and  20.
Body  subspheroidal,  slightly  trilobate,  surface  smooth ;  flagellum  con-
spicuous, two  and  a  half  times  as  long  as  the  body ;  colour  light  brown  or
greenish-yellow.    Length  1-2300"  to  1-1150".
Hab. — ^Amongst  Conferva  with  Chlamydomotuis  fulvisculus^  often  in
great  numbers.
As  represented  in  Ehrenbetg's  figures  of  this  species,  coloured  matter  is  fireely
incepted.  Perty  reports  the  presence  of  an  e^e-like  pigment-spot  in  the  anterior
body-half,  but  such  a  structure  is  not  indicated  m  Ehrenberg's  original  drawings  and
descriptions.  The  process  of  conjugation  or  fusion  has  been  observed  by  Stein,  who
identifies  with  this  phenomenon  the  representations  of  presumed  longitudinal  fission
given  by  Ehrenberg.
Oymnodininm  oorpnacnlnsii  Perty  sp.
