Reported are findings of calcitic pseudomorphs of monaxonic sclerites (heloclones and ophirhabds) occuring as highly ordered vertical tracts and subhorizontal strands in the midwall of dalcareous skeletal tubes of a common Silurian favositid species, Favosites hisingeri, from Gotland. The discovery ends conclusively the current controversy about the nature of favositids and related tabulomorphs in favour of the neglected early suggestion of Kirkpatrick ( 19 1 1) that these fossils can be basal calcareous secretions of siliceous sponges similar to those in living Merlia normani. The type of sclerites found in F. hisingeri and other favositids indicate that favositids are closely related to fossil and extant sponges classified within the order Lithistida (class Demospongia) as the so-called sublithistids.

Key words: favositids, tabulomorphs, sclerites, affinity, sponges, Silurian.