Vou. 2] Torrey.—Hydroids of the San Diego Region. 15 
18. Obelia dichotoma (Linn.). 
Sertularia dichotoma, Linnaeus, 1767, p. 1312. 
Obelia dichotoma, Hincks, 1868, p. 156, pl. 28, fig. 1. 
Obelia dichotoma, Calkins, 1899, p. 356, pl. 3, fig. 16. 
Obelia dichotoma, Torrey, 1902, p. 57. 
Trophosome. ‘‘Stem filiform, slender, nearly straight, irregularly 
branched, ringed above the origin of the branches, of a deep horn color: 
branches suberect, often very long, and more or less ramified, ringed at 
intervals, a single calycle in the axils; hydrothecae alternate, broadly com- 
panulate and deep, polyhedral above, each side corresponding with a very 
slight sinuation of the margin, borne on ringed pedicels, which vary in 
length from 4 or 5 to as many as 16 rings.’’ 
Gonosome. ‘‘Gonotheeae axillary, slender, smooth, widening from 
the base upwards, and terminating above in a raised, somewhat conical 
aperture.’’ (Hincks). Medusae liberated with 16 tentacles (Hincks), 20 
to 24 (Southern California specimens). 
Distribution. San Diego; San Pedro, Cal. Puget Sound 
(Calkins). Alaska (Nutting). Eastern United States (Nut- 
ting). Helgoland (Schulze). N. Asia (Thompson). 
It is possible that the California species is the stock which 
produces an undescribed medusa of the genus Obelia that is very 
abundant in the neighborhood of San Diego. If this prove to be 
the case, the hydroid, though identical with O. dichotoma as 
regards the trophosome, will become a new species. 
19. Obelia geniculata (Linn.). 
Sertularia geniculata, Linnaeus, 1767, p. 1312. 
Obelia geniculata, Allman, 1864, p. 372. 
Obelia geniculata, Torrey, 1902, p. 58. 
Trophosome. ‘‘Stem zigzag, sometimes sparingly branched, jointed 
at each of the flexures, and thickened immediately below them, so as to 
form a series of projections or rests, from which the pedicels arise, hydro- 
thecae somewhat obconical, rather short, the length slightly exceeding the 
width, with a plain margin, borne on short, annulated stalks (rings (4-6), 
which are suberect and taper slightly upwards.’’ ° 
Gonosome. ‘‘Gonothecae axillary, urnshaped, attached by a short 
ringed stalk (3-4 rings).’’ (Hincks.) Medusae at time of liberation with 
24 tentacles. 
Distribution. Coronado, Cal., at surface; Catalina I., 42 fath- 
oms; San Francisco, between tides. Eastern United States 
(Agassiz, Nutting). Europe (Hineks). White sea (Meresch- 
kowsky). New Zealand (Hartlaub). 
Gonosome present, July 1, 1903. 
