64 The American Naturalist. [January, 
ORDER I.—CysTIPHOR&. 
Plants one-celled or associated in loose groups in a gelatinous matrix. 
Family 1.—Chroococcacee.—Our genera may be disposed as follows: 
A. Cells globose, dividing irregularly in three planes, Chroococcus, 
Glceocapsa, Aphanocapsa, Microcystis, Polycystis, Anacystis, Gom- — 
phospheeria, Cœlophærium, Clathrocystis. 
B. Cells globose, dividing ragulariy.i in two or three planes. Merismo- 
pedia, *Sarcina. 
C. Cells cylindrical, dividingin one plane only. Synechococcus, Glæo- 
theca, Aphanotheca. 
ORDER II.—NEMATOGENEÆ. 
. Plants several- to many-celled by division mainly in one plane, form- 
ing simple or branched filaments ; cell-walls often thickish, and sepa- — 
rating an outer continuous layer as a sheath, which encloses the row — 
of cells. 
Five families may be distinguished as follows: 
A. Cells in each filament alike ; no heterocysts. Filaments cylindric 
motile, Oscillariacee. 
B. Cells differentiated ; heterocysts present. 
a. Division of cells in one plane only. 
1. Filaments moniliform, unbranched, Nostocaceé. 
2. Filaments cylindrical, sometimes spuriously branched, | 
Scyton 
3. Filaments tapering, sometimes spuriously branched, 
Rivulariacee. 
b. Division of cells ultimately in three planes. Filaments with 
true branches, Sirosiphoniacee. 
The relationship of these families to each other may be indicated by 
the following diagram : 
Sirosiphoniacee 
Rivulariacee 
Scytonemacece 
Nostocacece 
Oscillariacee 
l 
Chroococcaceee 
