Pottiaceae subfamily Barbuloideae Hilp.

Twelve genera are recognised in subfamily Barbuloideae for N.Z. Characteristic substrates are soil or rock, often calcareous; no members are epiphytic.

Perichaetia may be either lateral or terminal, and peristomes are usually well developed, as in Barbula, but may be vestigial or absent, as in Anoectangium, Gymnostomum, and in some species of Geheebia and Tridontium.

In earlier treatments, five or six species of Didymodon were accepted for N.Z. (Zander 1993; Fife 1995; Gibb et al. 2018​​​​​​​). All but one, Didymodon calycinus Dix. (see incerta sedis), are now placed in other genera, namely in Geheebia, Gertrudiella, Trichostomopsis or Tridontium​​​​​​​ (Zander 2013; Jiménez et al. 2022​​​​​​​; Beever et al. 2023​​​​​​​; Allan Herbarium 2023​​​​​​​​​​​​​​​​​​​​​; Zhang et al. 2023​​​​​​​).

Incertae sedis

The N.Z. endemic Didymodon calycinus (Beever 2024) was described by Dixon (1915) from material collected in the vicinity of Mt Bruce, Wairarapa (W. Gray 176, 18 Sep. 1913 "in a boggy place in grass paddock, among rushes", BM 000845861) with a further specimen seen by Dixon, probably from the same locality (W. Gray s.n., 8 Nov. 1914, BM 000845863) and with duplicates in Sainsbury’s herbarium (WELT M001770, WELT M005813a and b). Sainsbury himself made subsequent collections (G.O.K. Sainsbury 18349, WELT M021900, M010124; G.O.K. Sainsbury 18350, WELT M010125) at a second site 150 km to the NW. Both sites were accurately pin-pointed from extra information on the herbarium packets, but despite considerable search effort the species has not been refound. These two remain the only known collection sites; all other putative specimens of Didymodon calycinus that have been located for the present study appear to be misidentified. Two collected by Sven Berggren, from Napier (S. Berggren 193, AK 123949, BM 000845862) and from Wellington (S. Berggren 388, AK 123948, BM 000845864, CHR 486020,) are referable to Geheebia tophacea and Trichostomopsis australasiae, respectively. A voucher specimen (WELT M008396) for a published report of D. calycinus by Bartlett (1985) is also misidentified.

The correct generic and family placement of Didymodon calycinus Dixon is problematic. It bears a resemblance to the very variable Ceratodon purpureus (Ditrichaceae) in many features (see Burley & Pritchard 1990), namely in the lanceolate leaves with recurved leaf margins, stout costa, smooth and irregularly subquadrate upper laminal cells (with KOH colour reaction yellow), short-rectangular basal laminal cells, and sheathing perichaetial bracts. The stem and leaf cross-sections are likewise very similar in the two taxa (cf. Beever 2024, fig. B–E, O with Seppelt 2004 fig. 57 9–14). There is a similarity in sporophyte characters (as seen in WELT M010124 for D. calycinus): a well-developed annulus, a peristome basal membrane with prominent abaxial trabeculae, rami incurved when dry, straight when moist, papillose and nodose. In its typical form, C. purpureus differs from D. calycinus in having acute leaf apices, with a short-excurrent costa and the leaf margin irregularly and bluntly toothed near the apex (vs a rounded leaf apex, failing costa and entire leaf margins in D. calycinus), theca inclined or suberect, curved, and sulcate at least when old and dry (vs theca erect and cylindric in D. calycinus). Rarely C. purpureus, at least in N.Z., may have "obtuse-rounded" leaf apices and "nearly entire" upper leaf margins (A.J. Fife, pers. comm., July 2023). Elsewhere, in Arctic and Antarctic regions, a form of Ceratodon with failing costae and entire leaf margins (Seppelt & Green 1998; Seppelt 2004; Zander et al. 2007), and thus reminiscent of Didymodon calycinus, has been described as Ceratodon heterophyllus Kindb. There are also resemblances between D. calycinus and species of Geheebia, including obtuse to rounded leaf apices with failing costa, recurved leaf margins, a stem sclerodermis, and well-developed stem central strand. If fresh material of Didymodon calycinus is found, DNA sequencing may help establish its true relationships.

Illustrations: Beever 2024. Dixon 1915, pl. 9, fig. 3; Zander 1993, pl. 50, 1–3.