38. HYPODONTIUM                 Plate 51.

Hypodontium C. Müll., Hedwigia 38: 96, 1899. Type: Hypodontium dregei (Hornsch.) C. Müll. (lectotype by Magill 1981).

     The genus is endemic to southern Africa, where it is found on soil, rock or tree trunks.

     Magill's (1981) treatment of the genus is detailed and up to date. Reese et al. (1986), in an instructive review of the genera of Calymperaceae, where Hypodontium has heretofore been put, suggested this genus might be pottiaceous. Edwards (1980a) accepted Hypodontium as Calymperaceae, and did not discuss it extensively. Edwards (1979, p. 328) pointed out the fragmentary remains of 32 narrow exostome teeth that remain attached to the inside of the operculum of Hypodontium and fall with it; scattered, narrowly elliptical, transversely oriented plates attached to the external faces of joints of the peristome were seen in this study. An examination of a series of collections of H. dregei (Pl. 51, f. 1–12) and H. pomiformis (Pl. 51, f. 13–16) from PRE indicates that Hypodontium lacks many of the central characters of genera of Calymperaceae, e.g. the campanulate calyptra of Calymperes, a stem central strand, the intramarginal bands of elongated cells, and the enlarged but short-rectangular to quadrate basal cells of Calymperes, Mitthyridium and Syrrhopodon. The elongate, hyaline basal cells of Hypodontium are similar to those of many of the genera of Pottiaceae. Characters of Hypodontium most characteristic of the Calymperaceae are the peristome teeth (Pl. 51, f. 12) being short and incurved, 16, deep orange, triangular, smooth or weakly verrucose (compare those of Syrrhopodon, or even Octoblepharum cf. Magill 1981); upper basal cells of at least the outer perichaetial leaves sharply differentiated and inflated-quadrate like the cancellinae of Calymperes; cauline leaves lanceolate, bordered by much-elongate cells with thick walls, apex broadly acute to rounded and margins plane to incurved and often spinose, base with high shoulders; the upper laminal cells usually bulging ventrally and weakly convex dorsally; and upper laminal and dorsal costal papillae very thick (Pl. 51, f. 7, 16); none of these characters is, however, unique to the Calymperaceae. The peristome teeth, in being red, flat and triangular, are quite like those of Oreoweisia (Dicranaceae), a genus, which, however, has a rather dissimilar gametophyte morphology.

            Unusual in both the Pottiaceae and Calymperaceae are the inner perichaetial leaves (Pl. 51, f. 9) sheathing below but narrowly subulate or awned apically (as in Bryobartramia of the Encalyptaceae, or species of Diphyscium, Diphysciaceae, and approached in the awned perichaetial leaves of some species of Pseudocrossidium). Calymperes has a persistent calyptra, opening by slits, which is unique to Calymperaceae, while the cucullate calyptra of Syrrhopodon often has a distinct collar on the lower margin that opens late. The calyptra of Hypodontium (Pl. 51, f. 11) is comparatively small, cucullate, and perched on the rostrum of the operculum, and is more typical of Pottiaceae, although also found in Mitthyridium, Calymperaceae (Nowak 1980).

     Except for the peristome, Hypodontium has the same  capsule morphology of fleshy appearance; large, thick-walled exothecial cells; annulus largely undifferentiated; and spores relatively large, as has Tridontium (here removed to the Grimmiaceae near Scouleria) and Tetracoscinodon (Merceyoideae), the last with similarly sheathing, distally narrowly subulate (but not quite awned) perichaetial leaves. The peristome teeth of these three genera are also somewhat alike, being broadly triangular-lanceolate, while Hypodontium and Tetracoscinodon both have smooth teeth, but the gametophytes are disparate in appearance. If there were some distinctive autapomorphy or compelling combination of unusual characters, Hypodontium might be placed in a family of its own.