Among the very few known instances of fossil hydroids, the genus Hydractinia must be included. Under the name of Cellepora echinata, M. Michelin has described a fossil from the Subapennine group of Asti, and from the Superior Fallunian of Bordeaux and Dax (Michelin, Icon. Zooph., p. 74, pl. xv., fig. 4). M. Fischer has drawn attention to the fact, that the Cellepora echinata of Michelin is really a Hydractinia encrusting a Murex or a Nassa, while he has himself added another fossil Hydractinia from the Upper Greensand of Mans (Fischer, Bull, de la Soc. Géol. de France, 2me sér. t. xxiv., p. 689). This he found in the collection of M. Alc. d’Orbigny, where it encrusted numerous specimens of Natica tuberculata, d’Orbig., from that formation. M. Fischer has assigned to this species the name of Hydractina cretacea, while to Michelin's species he has given that of Hydractina Michelini.