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Pasythea (Lamour) Nutting. 
"Hydrothecæ' biserial, strictly opposite, årranged in groups of 
pairs, a- group to an internode, the upper pair being smaller and 
differing in shape from the lower, margin bilabiate, with a too- 
flapped operculum.” 
Sertularella (Gray) Nutting. 
"Hydrothecæ biserial, strictly alternate, usually with three 
Or four marginal teeth and a well-marked operculum with three or 
four flaps." Rarely the teeth are obliterated, in which case. the 
Operculum is stretched across the hydrothecal - aperture like. a 
drumhead.  Branches never regularly anastomosing to form a 
reticulate flabellate structure.” 
Dictyocladum Allm. 
"Colony flabellate in form. Branches anastomosing and forming 
a rudely reticulate structure or network.  Hydrotheca on more 
than two sides of the stem. Aperture without conspicuous. teeth. 
Operculum variable.” 
Diphasia (Agass.) Nutting. 
"Hydrothecæ biserial, opposite or alternate, aperture broad, 
Operculum evident, of a single adcauline flap.” 
Abietinaria (Kirchenp.). Nutting. 
Hydrothecæ non strictly opposite, more or less bottle-shaped (the 
proximal portion turgid, distal portion narrowed), operculum of a 
single adcauline flap, margin usually without teeth. 
Selaginopsis (Allm.) Nutting. 
"Hydrothecæ arranged in more than two longitudinal series, 
at least -on distal parts of branches, or in two or more series each 
of which has the distal ends of the hydrothecæ turned alternately 
to the right and left. Operculum of a single abcauline flap. Inter- 
nodes long or absent.” 
As chief characters for the two- genera Sertularia and Thu- 
Jaria, to which the author only refers species with two-serial 
" hydrothecæ, he uses both the different arrangement of the hydro- 
thecæ and the length of the internodes. The hydrothecæ may be 
