382 THE AMERICAN NATURALIST. [VOL. XXXV. 
i, O. commisuralis McCr. (Fic. 17). 
Trophosome: Colony long, slender, profusely branching, branches spread- 
ing in graceful curves on each side of the main stem, which may attain 
a height of six to eight inches. 
Gonosome: Gonangia elongate, slen- 
der, obconical, opening by terminal, 
circular orifice arising from the base 
of the cone on a short conical neck. 
Medusz when first liberated have six- 
teen marginal tentacles, four radial 
canals, beneath which later the gonads 
develop. 
2, O. adichotoma Linn. 
Trophosome: Colony rather small, 
stem slender, irregularly branched, an- 
nulated just distal to origin of branches, 
the latter annulated at irregular inter- 
Fic. 17.— Obelia commisuralis McCr. vals. Hydrothecze large, deeply cam- 
(After Agassiz.) ; . 
panulate, borne on annulated pedicels. 
Gonosome : Gonangia axillary, slender and smooth, somewhat obconical, 
and similar to those of former species. Medusz with sixteen tentacles, 
manubrium somewhat trumpet-shaped. 
3, O. flabellata Hincks. 
Trophosome: Stem filiform, alternately branching, giving the stem a 
somewhat zigzag character. Both stem and branches variously annulated. 
Hydrothece alternate, short, widely open and with entire margins, borne 
on tapering annulated pedicels. 
Gonosome: Gonangia axillary, obovate, with tubular orifice. Medusa? 
4, O. geniculata Linn. (FIG. 18). 
Trophosome: Colony inconspicuous, rarely attaining a height of more 
than an inch. Stem somewhat zigzag in form as in former species, but 
apparently jointed at each bend. Hydrothece obconical, rather short, 
with plain orifice, borne on short annulated pedicels. 
Gonosome: Gonangia axillary, urceolate, borne on short pedicels. 
Medusz discoid, with twenty-four tentacles when liberated, greatly increas- 
ing in number with age. 
Habitat : Common along Massachusetts and north Atlantic coast, on 
Fucus and Laminaria. 
