76 AMEKICAN HYDKOIDS. 



Obelia genindata 3 at>i:ksoi,m, Northern and Arctic Invert., pt. 4, Hydroiden, 1909, p. 62. 



Obelia geniculata Mayer, Medusae of the World, vol. 2, 1910, p. 250. 



Obelia geniculata Fraser, West Coast Hydroids, 1911, p. 39. 



Obelia geniadata Stechow, Hydroidpolypen der japanischen Ostkiiste, part 2, 1913, p. 70. 



Trophosome} — Colonies growing from a creeping rootstock on a laminarian and attaining 

 a height of 14 mm. Stems usually unbranched, regularly geniculate and divided into regular 

 internodes, each of wliich bears a strong projecting shoulder or expansion on its distal end 

 where it is much broader than at its proximal end, the internodes sometimes being rudely tri- 

 angular in outline. The outer side of each internode has a remarkable thickening of the peri- 

 sarc which reaches its maximum just below the insertion of the pedicel, and which is very well 

 illustrated by Hincks in figure la, plate 25 of liis "British Hydroid Zoophytes." The branches, 

 when present, are like the stem. Pedicels alternate, very short, usually shorter than the hydro- 

 thecEe, with usually 3 to 5 annulations and the diameter decreasing rapidly from proximal to 

 distal end. Hydrothecse rather broadly campanulate, often triangular in outline and with a 

 strongly marked, perfectly even margin. The diaphragm is well marked. 



Gonosome. — The gonangia are usually borne in the axils of the pedicels. They are oblong- 

 ovate in shape, the distal end being broader than the proximal and sometunes showing a sort 

 of shoulder, beyond which it rapidly narrows to a short conical neck terminating in a small round 

 aperture. Within the gonangia are numerous developing medusse thickly crowded around a cen- 

 tral blastostyle. Medusae, at hberation, with a disk-shaped bell, 16-24 marginal tentacles, a short, 

 square-hpped manubrium, 4 radial canals under which the ovaries are hung, and 8 hthocysts on 

 the lower sides of the bases of 8 of the tentacles. Adult medusse are described by Mayer ^ as differ- 

 ing from the freshly hberated ones mainly in the greater size (6 mm. in diameter), about 100 

 stiff tentacles and the position of the ovaries which he nearer the margin than the gastric cavity. 



Distrihution . — This is one of the most widely distributed of all the hydroids, and one of the 

 best known. It was originally reported from British coasts, and has been found in practically 

 all suitable locahties on European shores from the Mediterranean to the polar region and on the 

 Atlantic coast of America south to the West Indies (Versluys) . On the west coast of America 

 it has been reported from San Diego, Cahfornia (Torrey). It has been reported by various 

 writers from the Antarctic, Terra del Fuego, Straits of Magellan, coast of Chile. In the Western 

 Pacific it is reported from Japan, PhiUppine Islands, Austraha, New Zealand, Moluccas, Aru 

 Islands, Malay Archipelago, etc. 



It is therefore world-wide in distribution, and being a shaUow-water form, has probably been 

 collected more frequently than any other species. 



It is probable that the habit of growing on floating seaweed, timbers, etc., together with 

 its profuse production of medusse has had much to do with the success of tliis species in attain- 

 ing a cosmopohtan distribution. 



Bathymetric distribution, 1 to 35 fathoms. 



OBELIA HYALINA Clarke. 



Plate 18, figs. 6-7. 



Obelia hyalina Clarke, Bull. Mus. Comp. Zool., vol. 5, No. 10, 1879, p. 241. 



Obelia hyalina Versluys, Hydrairea Calyptoblastes recueillis dans les Mer des Antilles, 1899, p. 39. 



Obelia hyalina Pictet and Bbdot, Hydraires provenant des campagnes de VHirondelle, 1900, p. 8. 



Obelia hyalina Thornley, Ceylon Pearl Oyster Report, The Hydroida, 1904, p. 113. 



Obelia hyalina Congdon, Hydroids of the Bermudas, 1907, p. 468. 



Obelia hyalina Billard, Travailleur et Talisman, Hydroides, 1907, p. 170. 



Obelia congdoni Hargitt, Hydroids of Woods Hole, 1909, p. 375. 



? Obelia hyalina Stechow, Hydroiden der Miinchener Zoologischen Staatssammlung, 1912, p. 354. 



TropTiosome? — Colony delicate, consisting of a number of stems springing from a creeping 

 rootstock, and attaining a height of 1 cm. Stem simple, rather feebly but regularly geniculate, 



' Description of a specimen collected at Woods Hole, Massachusetts, by the author in 1904. 



2 Medus* of the World, vol. 2, the Hydromedus*, 1910, p. 250. 



* Description of specimen collected by C. M. Fraser at Beaufort, North Carolina. 



