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e ralized bodies with rounded ends and a through-going axial cavity, occur in the base of glauconitic sandstones of the Lower Ordovician Leetse Formation in the vicinity of St. Petersburg (northwestern Russia). These objects were identified by the 19th century palaeontologist Karl Eduard Eichwald as fossil sponges. In 1840, Eichwald ascribed these bodies to the genus Siphonia Goldfuss, 1826 and gave them a brief description under the name S. cylindrica. Later, Eichwald significantly enlarged his description of S. cylindrica and accompanied the descriptions with what seemed to him to be evidence of their sponge origin. The attribution of S. cylindrica Eichwald, 1840 both to the genus Siphonia and to Porifera has been questioned repeatedly, however, the authors of the two subsequent revisions (Lamansky 1905, Asatkin 1949), despite the lack of obvious evidence of the sponge nature of the fossil, preferred to use the name given by Eichwald, but in the open nomenclature. In the course of systematic study of the Lower Ordovician deposits in the vicinity of St. Petersburg, the author noticed that the stratigraphic level bearing S. cylindrica is the same as the level with sandy phosphorite concretions described in the Tosna and Sablinka river valleys (Ershova et al. 2006). Dark brown cryptocrystalline cement of the sandy phosphorite concretions looks completely identical to the mineral substance composing the bodies of S. cylindrica. Moreover, on the surface of several specimens of S. cylindrica thin transverse encircling grooves, unusual for sponges were detected. These features were not reported in the descriptions of the 19th and 20th centuries. Therefore, the author undertook a new study of S. cylindrica from the collection of Eichwald and his personal collection; the results are given below. Review of Cretaceous sponges, which were also called ‘Siphonia cylindrica’ in the first half of the 19th century, is beyond the scope of this article. Information about these sponges may be found in the publications by Sollas (1877) and Zittel (1878). The authorship and history of the study of the genus Siphonia are not discussed.

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Lower Ordovician Siphonia cylindrica Eichwald, 1840 from north-western Russia: a pseudo-sponge and a natural &amp;#39;recorder&amp;#39; of geological history