Zygoceros rhombus Ehrenberg (1839: 156

Sims, Pat A., Williams, David M. & Ashworth, Matt, 2018, Examination of type specimens for the genera Odontella and Zygoceros (Bacillariophyceae) with evidence for the new family Odontellaceae and a description of three new genera, Phytotaxa 382 (1), pp. 1-56 : 20

Zygoceros rhombus Ehrenberg (1839: 156 ; Ehrenberg 1839 [ 1841]: 160, pl. IV, fig. XII; Ehrenberg 1840b: 80, pl. IV,

fig. XII) ( Figs 65–71 )

Synonyms:— Biddulphia rhombus (Ehrenberg) W. Smith (1856: 49 , pl. 45, fig. 320; pl. 61, fig. 320) Odontella rhomboides R. Jahn & Kusber (2004: 584)

Type:— Cuxhaven [ Germany], 540043-4 BHUPM, lectotype ( Jahn & Kusber 2004: fig. 45); 540211-5 BHUPM ( Jahn & Kusber 2004: fig. 44) and 360805-c BHUPM (“Cuxhaven I”, Sar et al. 2007: fig. 3), isolectotypes.

In girdle view frustules square to rectangular, apical axis 50–108μm, transapical axis 40–60μm, with central set of girdle bands. Valves with narrow mantles, indented, extending to narrow mantle flange, with polar elevations that extend outwards to lie in line with girdle ( Fig. 69 ).

Valve outline lanceolate or triangular ( Figs 65, 66 ), length 40–180μm, breadth 30–50μm, with rim of mantle flange extending slightly beyond valve outline ( Fig. 67 ). Valve surface almost flat with a barely raised elevation at each pole ( Fig. 69 ). Summit of elevation with hyaline rimmed ocellus positioned mostly on dorsal side ( Fig. 68 ). Areolae loculate with continuous cribrum linked by passage pores ( Figs 67, 68 ) with 8 areolae in 10μm in transapical rows that meet at central apical line (annulus), with 8 striae in 10μm.

A regular series of buttressed spines with branched tips and branched spines situated above locular walls ( Fig. 67 ). 2–6 sub-marginal rimoportulae open through short spine ( Fig. 67 ) buttressed at base on valve exterior (if 2, each positioned at base of elevation– Fig. 68 ); on interior through short barely raised slits ( Fig. 70 ). Girdle bands open, valvocopula with well-developed pars interior ( Fig. 71 ). Each band with narrow hyaline margins bordering vertical rows of poroid areolae, c. 15 in 10μm ( Fig. 71 ).

Observations:— The description above is based on material from Tampa, Florida.

Figures

FIGURES 65–71. Zygoceros rhombus Ehrenberg (Tampa, Florida). Fig. 65. Valve exterior, short elevations with ocelli at apices and submarginal spines, which are the openings from rimoportulae; scale bar = 20 μm. Fig. 66. Valve interior with radial rows of foramina and submarginal openings of rimoportulae; scale bar = 20 μm. Fig. 67. Exterior with ridged opening from fractured rimoportula, continuous cribrum linked by passage pores and buttressed branched spines covering valve; scale bar = 2 μm. Fig. 68. Rimmed ocellus at summit of elevation, narrow hyaline valve margin with raised border; scale bar = 5 μm. Fig. 69. Exterior with barely raised elevations, submarginal rimoportulae opening through spines; scale bar = 20 μm. Fig. 70. Interior with foramina and two openings from rimoportulae; scale bar = 5 μm. Fig. 71. Triangular valve with attached girdle bands, three short elevations with ocellus at summit; scale bar = 20 μm.