Orthotrichiaceae. Genus Zygodon

1.Gemmae with longitudinal cell walls 2

— Gemmae without longitudinal cell walls 3

2.Leaves coarsely serrate distally 1. Z. dentatus

A rare species in Russia, Zygodon dentatus is inadequately known and has often been considered as a variety of the widespread Z. viridissimus. In Russia it is known from a single locality in the northern Caucasus (Karachay-Cherkessia) region (Fedosov et al., 2017a). Zygodon dentatus occurs mainly in Europe (frequent in southern Scandinavia and the Alps) and is also found eastward to Ukraine and Georgia with an isolated locality in North America (Arizona). It is a corticolous moss that grows in montane beech-hornbeam forests at moderate elevations.

— Leaves with entire margins [Z. viridissimus]

Zygodon viridissimus was reported in Russia in a broad sense, and all so named Russian collections are misdeterminations for either Z. rupestris or Z. sibiricus. However, because Z. viridissimus occurs in Scandinavia and Poland, it should be expected in nearby regions of Russia.

3.Gemmae filiform, composed of 7–8 cells with hyaline wals [Z. conoideus]

This species with oceanic distribution occurs in Finland near the Russian border, and therefore is expected in the Karelian Republic. Historical records of this species in Asia are misdeterminations of Z. sibiricus.

— Gemmae composed of 3–6 cells with brownish wals 4

4.Peristome present, single, with 16, short exostome teeth 3. Z. sibiricus

Zygodon sibiricus was described from the Bureya River, a tributary of the Amur River. It differs from Z. rupestris in the peristome presence and can be distinguished from this species with confidence only by collectiosn with mature sporophytes, which occur in the Russian Far East, Yakutia and south of East Siberia. We refer to this species also plants without sporophytes collected further to the west, in West Siberia, the Urals and NE European Russia, because of similar habitats in mesic boreal forest primarily on aspen and occasionally on poplar and willow trunks.

— Peristome absent 2. Z. rupestris

Zygodon rupestris has been reported from various regions of Russia, however it was never collected with sporophytes, which differ this species from closely related Z. sibiricus. Here we refer to Z. rupestris collections from the Caucasus where the species grows in somewhat dry forests on trunks of Juniperus, Quercus, Fraxinus, Carpinus, Pyrus, Populus and cultivated Metasequoia.