^E. diaphana.^ This species is often abundant on the fronds of /Laminaria/ washed ashore, and also on /Rhodymenia/ and /Fucus/. It has a creeping base, zigzag in form, but keeping a straight course, and in its branching often forming a network over the surface of the flat fronds. At each angle of the creeping stems rises a pedicel about an inch high, which inclines in the direction of the stem and terminates in a zoöid-cup similar in form to that of /Obelia/. The medusa which this hydroid liberates is called /Thaumatias diaphana/. The swimming-bell is very shallow and thin, turning inside out at almost every pulsation. The tentacles are numerous and rigid like stiff hairs. This little medusa is very active and is abundant. The species is found from Long Island Sound northward. GENUS ^Oceania^ ^O. languida.^ This medusa is one inch in diameter and one half of an inch high, and is so delicate and transparent that it is hardly visible except in its outlines. In its early stages it is nearly spherical and has no tentacles; later the disk flattens and has from thirty-two to thirty-six [pg126] tentacles and numerous eye-spots. When disturbed it flattens its disk and folds together, leaving its tentacles sprawled in every direction. It is very languid in its movements, and often remains in one position for hours. These medusæ are found only in the hottest hours of the day, but are very plentiful then, shoals of them often stretching for miles, and so thick as to touch one another. Their habitat is the New England coast. GENUS ^Clytia^ ^C. poterium.^ This hydroid is found creeping over seaweeds in tide-pools from Long Island Sound northward. The main stem is prostrate, or root-like, running over the body to which it is attached. The stems rise as do the tubularians. The sterile zoöids are on single stems about one quarter of an inch high. The stems are faintly ringed for their entire length, and at the top have a distinct ring, on which rests an open, bell-shaped cup, which is smooth around the rim. The reproductive zoöids are on very short pedicels, and the cups are long and cylindrical, with a wavy outline. (Plate XLII, A.) ^C. bicophora.^ This species is found in the same places as the preceding, and is of about the same size. The long stems are more or less ringed and sometimes branched. The edges of the cups are notched. The medusa-buds are urn-shaped and ringed, and are on very short pedicels. (Plate XLII.) THE SERTULARIANS