Basionym: Navicula intercedens A. Schmidt in Schmidt et al., Atlas der Diatomaceenkunde, pl. 160, fig. 3. 1890. Synonyms: Caloneis intercedens (Schmidt) Fricke (1902: 20), Progonoia musca (Gregory) Schrader (1971: 922), partim.

LM (Figs 5, 7–10): Valves lanceolate-elliptical, strongly constricted in the middle, length 31–49 μm, greatest width 18–19 μm, width of central constriction 7.5–9.0 μm. Striae 9 in 10 μm, uniseriate, alveolate, divided by a longitudinal rib (Fig. 5). Axial areas in each half-valve broad, lanceolate, hyaline, central area small, the striae reaching or almost reaching the raphe. Raphe and central endings straight.

SEM (Figs 11–17): Mantle very shallow, not clearly separated from valve face. External openings of the alveoli comprised four main longitudinal series of variously sized pores with smaller pores between, especially in the widest parts of the valve (Figs 11 and 12). The two middle rows of areolae are closed by a membrane almost flush with the valve surface (Fig. 12, arrowhead). Those on the axial edge of the alveoli are slightly elongated apically, those along the mantle short, curved slits that were convergent along the constriction, radiate elsewhere (Figs 11 and 12). Both these rows appear to be pits with deeper membranes (Figs 12 and 13, arrows) and occur at the ends of the alveoli away from the dividing wall. A series of perforations were seen through marginal slits (Fig. 13, arrow), but these do not align with the perforations on the internal membrane, so that the membrane appears to be a deeper external one and not the inner membrane. In terms of a stria with the two underlying alveoli, the two large pores in the middle lie on opposite sides of the wall dividing the two alveoli, as can be seen from the degraded valve in Fig. 17. There are reduced striae at the central constriction, with fewer, smaller pores, and the central striae are slightly closer together; internally the two chambers are somewhat offset in the central striae (Figs 14–17). Externally, the axial area is slightly textured but not perforated. Central raphe endings are straight, slightly expanded externally; internally they end in the characteristic beak-like double helictoglossa (Fig. 16). Terminal raphe endings deflected to the secondary side, passing onto the mantle. Internally, the two chambers of the alveoli are covered by a membrane (Figs 14–16), with tiny pores along the edges of the inner portion of one and the outer portion of the other (Fig. 15). The inner portion of the inner chamber appears thinner than the rest of the alveoli, and there is a ridge roughly parallel to the valve margin; this was not evident in LM, in contrast to the rib formed by the wall between the two chambers. The valve wall in the axial area appears to be only the basal siliceous layer; no longitudinal canals were present. Girdle bands striate (Figs 10, 15 and 17), striae about 30 in 10 μm.

Type:— Lectotype (designated here): Schmidt et al. (1874 –1959), pl. 160, fig. 3. Epitype (designated here): Navarro 1982, fig. 101; slide #155A in his collection (as Progonoia musca).

Materials examined: — GUAM. Outhouse Beach, Apra Harbor, GU 52O-4, slides 1695–1697. A voucher specimen is located 15.3 mm E and 10.2 mm S of reference mark on slide 1696! deposited at California Academy of Sciences (CAS) for the Progonoia diatreta n. sp. holotype (see below); Fig. 9.