Thues H, Nascimbene J. 2008. Contributions toward a new taxonomy of Central European freshwater species of the lichen genus Thelidium (Verrucariales, Ascomycota). The Lichenologist 40: 499-521.
Thallus white, whitish grey to grayish green, on wood occasionally with red, K+ purple patches, epilithic to semi-endolithic, 45–145 μm thick, proso- to paraplectenchmatous. Cortex not differentiated, but uppermost 10–15 μm of the thallus occasionally yellowish brown. Basal layer absent. Photobiont a coccoid green alga, cells ± clustered, 4–7 μm diam.
Perithecia immersed to semi-immersed in the thallus, epilithic or basal part to one third immersed in the substratum, naked. Exciple 200–320 μm diam., black-brown. Periphyses up to 30 μm. Involucrellum apical to almost reaching the thallus base, thin (10–35 μm), adjacent to the exciple or laterally extending in the thallus, 200–360 μm diam. (n=7, N=7). Asci 58–113 × 13·4–50 μm (n=8, N=4). Ascospores eventually with three septa (1–2 septa in young ascospores), (22–) 30 ± 4·9 (–36) × (9·5–) 11·7 ± 1·8 (–14) μm (n=28, N=7).
Habitat and distribution. This species occurs in damp to amphibious sites on base rich substrata (limestone, lime-containing sandstone, on silt-impregnated wood, also on mica schist). It is widely distributed but rare, occurring from low mountain ranges to (sub-) alpine altitudes.
Similar species. Thelidium zwackhii does not have an involucrellum and its perithecia are usually largely exposed to almost sessile on the thallus surface.
Taxonomic notes. The isotypes of Thelidium fontigenum differ from those of T. cataractarum only in the development of the thallus (very thin in T. cataractarum, thicker in T. fontigenum), the reddish colour and the I+ blue reaction of the hymenium (I+ red in T. cataractarum). However, the reddish colour in the type specimens of T. fontigenum is not restricted to the lichen thalli but is also present within the wood and maybe caused by co-occurring saprobic fungi or bacteria. Thalloid features in Thelidium species from calcareous substrata are in general notoriously variable, often even within a single collection, and the iodine reaction of the hymenium alone appears insufficient for species delimitation in this genus. Thelidiumriparium (Hepp) Zschacke, from submerged willow bark, differs from the type material of T. fontigenum only by the I + red reaction instead of a blue reaction as in the type specimens of T. fontigenum (according to the description given in Zschacke 1920). As in the case of T. cataractarum (from which it differs only by its substratum) this character alone is not sufficient for species delimitation. Thelidium rodnense was separated from T. zwackhii by the presence of an involucrellum. All anatomical details as given in the protologue are within the range of T.fontigenum. It is most probable that Zschacke did not consider T. fontigenum (nor its synonym T. cataractarum) when he described T.rodnense. He corrected this error in a subsequent paper (Zschacke 1920), where he placed T. rodnense in synonymy under T.cataractarum without further comment.