Professor M'Coy, in his “British Palæzoic Fossils” (1854), speaks of transverse diaphragms being present at the base or proximal termination of the calycles (hydrothecæ) of certain graptolites, dividing the calycles from the common canal or perisarc. No further allusion appears to have been made to the presence of any diaphragms or septa until in 1868 the writer mentioned (Journ. Quek. Micros. Club, vol. i.) having observed “an impressed line between the hydrothecæ and the periderm” (perisarc), which was compared with that “at the base of the hydrothecæ in the sertulariadæ.” More recently Professor Allman (Monogr. Tubularian Hydroids, 1872), not admitting the presence of any septum or constriction, has compared the calycles of the Ehabdophora to the nematophores of the Plumularidaa.