356 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
shaped; the chitinous periderm is exceedingly delicate and easily 
wrinkled or folded. Hydrothecae placed alternately and at some 
distance apart. Margin sinuous; stems short and annulatedthrough¬ 
out. A single hvdrotheca in the axil of each branch. 
O 4/ 
Gonophores unknown. 
Diaphragm a simple well-defined shelf below the hydranth. 
Basal chamber a part of the stem (PI. 6, fig. 15c). 
Dimensions. Length of colony, 30 mm.; length of branches, 
9 mm. ; distance' between branches, 1.5 mm.; length of hydrotheca, 
.5 mm.; width of margin, .4 mm. ; number of tentacles, 22—24. 
This exceedingly graceful hydroid recalls Marktanner-Turneret- 
scher’s 0 . chinensis, which according to this author resembles 
Hincks’s Gonothyraea hyalina. I cannot carry the resemblance so 
far, and it differs also from 0 . chinensis in its much smaller dimen¬ 
sions ( O . chinensis -r~ internodes .2 mm. thick and often 4.5 mm. 
long) and in the length of the stem bearing the hydrothecae. 
Habitat. Dredged in Port Townsend Harbor on Aglaophenia 
struthionides. 
17. Obelia dichotoma Linn. PI. 3, figs. 16, 16 a, 16 b, 
16 c, 16 b. PI. 6, fig. 16 e. 
A few good specimens of this species were found at Bremerton. 
Hincks’s description is as follows: “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 and with a single 
calycle in the axils. Hydro thecae alternate, broadly campanulate 
and deep; polyhedral above, each side corresponding with a very 
slight sinuation of the margin borne on ringed pedicels. Gono- 
thecae axillary, slender, smooth, widening from base upwards and 
terminating above in a raised somewhat conical aperture.” 
Diaphragm. A shelf consisting of two portions, the upper form¬ 
ing the main part of the diaphragm and extending inwards to con¬ 
fine the coenosarc; the other or lower portion bends downwards to 
form a tube confining the coenosarc, and connects with a second 
diaphragm at the first annulation. The coenosarc at this portion is 
smooth and even, but below the first annulation it becomes ragged 
and irregular. 
Dimensions. Length of hydrotheca, .35 mm.; diameter at 
margin of hydrotheca, .2 mm.; number of tentacles on hydranth, 
22-24; length of gonotheca, .8 mm.; diameter at extremity, .3 mm. 
