A January P/9oom*}     Contamination  of  Drinking  Water.  29 
similar  in  general  appearance,  consisting  of  a  gelatinous  mass  in 
which  are  embedded  bluish-green  cells  of  either  spherical  or  oblong 
shape.  T«here  is  no  connection  between  the  cells,  and  the  colonies 
are  capable  of  being  broken  up  into  smaller  groups,  each  cell  of 
which  divides  rapidly,  and  thus  new  masses  are  formed.  In  ClatJiro- 
cystis  the  colony  sends  out  buds  or  projections  which  soon  fall  off, 
and  these  grow  into  new  colonies  like  the  original  one.  Among 
the  Hormogoneae  there  are  quite  a  number  of  genera  which  have 
been  reported  to  be  the  cause  of  various  odors  and  tastes  in  drink- 
ing water.  The  members  of  the  nostoc  tribe  are  particularly  obnox- 
ious, and,  since  all  can  produce  thick-walled  spores  in  addition  to 
their  vegetative  method  of  propagation,  it  is  often  a  most  difficult 
matter,  after  they  once  make  their  appearance,  to  get  rid  of  them. 
The  following  key  will  give  some  idea  of  the  various  forms  of 
Schizophyceae  which  have  been  found  to  contaminate  the  water  in 
which  they  occur,  and  will  bring  out  the  morphological  points  which 
are  depended  upon  for  their  determination. 
-  SCHIZOPHYCE/E. 
(A)  Plants  consisting  of  a  single  cell,  occasionally  united  into  colo- 
nies by  being  embedded  in  a  gelatinous  matrix. — I.  Cocco- 
goneae. 
(Z?)  Plants  always  of  more  than  one  cell,  forming  simple  or  branched 
filaments,  which  may  or  may  not  be  enclosed  in  an  outer  gela- 
tinous layer  or  sheath. — II.  Hormogoneae. 
(/)  Coccogoneae. 
(/)  Cells  free  or  only  slightly  held  together,  not  forming  a  de- 
finite colony. —  Chroococcus. 
(2)  Cells  held  together  in  a  gelatinous  matrix  and  forming 
colonies  of  regular  outline. 
[a)  Colonies  at  first  solid,  several  rows  of  cells  thick, 
becoming  saccate  and  perforated. — Clathrocystis. 
{b)  Colonies  hollow,  cells  only  on  outer  surface. — 
Ccelosphmdum. 
(//)  Hormogoneae. 
(/)  Cells  generally  differentiated  into  three  kinds:  (1)  vegeta- 
tive cells;  (2)  spores,  and  (3)  heterocysts.  The  latter 
usually  being  of  different  color,  clearer  contents,  and 
