Professor Allman has recently described under the name of Stephanoscyphus mirabilis, a very remarkable Hydrozoon, which he believes to form the type of a new order. This singular organism is invariably associated with a species of Sponge, in the substance of which it is embedded. It consists of a congeries of chitinous tubes, which permeate the sponge-substance, and which open on its surface by large openings resembling oscula. At their bases the tubes are connected by horizontal branches, and they expand widely as they approach the surface; where their contents become developed into a remarkable body, which has the power of extending itself beyond the mouth of the tube, and again of withdrawing within it. This body is furnished with a crown of tentacles, and is essentially medusiform in its structure. There is a circular canal at the base of the tentacular crown, surrounding the central opening, with four radiating canals proceeding backwards from this; but no lithocysts, ocelli, nor velum have been detected. For this curious organism, Professor Allman proposes the formation of a new order under the name of Thecomedusae.