Blackman and Tans ley. 
2 14 
27. Didymoprium. Kiitzing, 1843. 
Cells oval, each with a slight equatorial furrow ; connected 
by their flat ends to form spirally wound threads; in 
end-view elliptical, with two opposed promontaries, 
Chromatophore of eight plates diverging from four 
pyrenoids in each semi-cell. 
28. Gymnozyga. Ehrenberg, 1840. 
Cells barrel-shaped, with a slight equatorial furrow, on 
each side of which is a ridge; in end-view round, 
with two opposed projections of the wall; membrane 
with transverse ridges, and between them, longitu¬ 
dinal striae. Chromatophore of six plates diverging 
frcm a pyrenoid in each semi-cell. 
29. Hyalothcca , Ehrenberg, 1840. 
Cells oblong in side-view, intimately connected by their 
flat ends to form spirally-wound cylindrical threads ; 
equatorial constriction very shallow, with gently 
sloping sides ; in end-view circular. Chromatophore 
of six to ten plates diverging from a pyrenoid in each 
semi-cell. 
Series II. Zygnemoideae. 
Plant-body filamentous, attached or free, consisting of equivalent 
cylindrical cells (only the attachment-cell differs). Chromatophores 
symmetrical with reference to the long axis of the cell, of three well- 
marked types, characterising the three families. 
Multiplication by fragmentation of the filament. 
Reproduction sometimes by the formation of aplanospores (one 
in each cell), usually by zygotes, which are formed either iso- 
gamously in the conjugation canal, or anisogamously in one of the 
conjugating cells. Parthenogenesis of the gametes sometimes occurs. 
Earn. I. Spirogyr ace^e. / 
Chromatophores one to nine in each cell, each a parietal spirally- 
wound band with a single row of pyrenoids. 
Genera. 
1. Spirogiyra. Link, 1820. 
Chromatophores usually making a considerable angle 
with the long axis of the cell, rarely parallel with it. 
Gametes derived directly from ordinary vegetative 
cells. Conjugation anisogamous. 
2. Sirogonium. Kiitzing, 1843. 
Chromatophores almost parallel with the long axis of 
the cell. Gamete-mother-cells formed from vegetative 
cells by the cutting off a small cell in the case of the 
functionally female, and of a large, sometimes also of a 
small, cell in the case of the functionally male. Conju¬ 
gation anisogamous. 
