Obs.—As distinguished from Serpulites, the term Serpula is here made to include those tubes which were attached, or in all probability attached by some portion of their surface to other objects, and in which there is an absence of the thickened margins, and the posterior bifurcating shelly tubes of the former. The name can only be used in an approximate sense, for it is quite within the bounds of possibility that any of the species here referred to Serpula may be shown to have other affinities. Already two out of the limited number of British Carboniferous species have been so indicated. The fine rigid siliceous rods formerly called Serpula parallela, M'Coy, are now known to be the anchoring fibres of a sponge allied to Hijalonema, whilst it is more than probable that Serpula hexicarinata, M'Coy, is the corallum of a species of Heterophyllia.