Fragilaria deformis (W.Sm.) Van de Vijver & Ector, comb. nov., type population

(Fig. 1)

Basionym: Synedra deformis W.Sm., A Synopsis of the British Diatomaceae: 98 (1856). — Lectotype (here designated): BR-4613 (Meise Botanic Garden, Belgium). — To be excluded as synonym: Synedra vaucheriae var. deformis Grunow in Van Heurck 1881 (pl. 40, fig. 18), Synedra vaucheriae var. deformis (W.Sm.) Van Heurck (1896, p. 310, pl. 30, fig. 833) (see below for discussion).

MATERIAL EXAMINED. — United Kingdom. Sussex, Lewes, P. Mill Pond; coll. date 8.III.1853 (Van Heurck diatom collection, Meise Botanic Garden, Belgium). The same material and a slide are present in the Natural History Museum, London, United Kingdom (D.M.Williams, pers. comm.).

MORPHOLOGY

LM (Fig. 1 A-R)

Frustules in girdle view rectangular, solitary never linked to form chains. Occasionally, two frustules linked together (Fig. 1A). Valves linear with usually parallel margins, giving the valves a rectangular outline, occasionally asymmetrical with one straight and one convex margin. Smaller valves becoming elliptic-lanceolate (Fig. 1 O-R). Apices clearly protracted, rostrate to occasionally weakly capitate, placed on clearly developed shoulders. Valve dimensions (n = 25): valve length 14-29 µm, valve width 4.5-5.0 µm. Axial area narrow but distinct, linear, gradually but weakly widening towards the central area. Central area asymmetrical due to shortening or absence of striae on one side. Ghost striae often present. Striae parallel near central area becoming weakly radiate towards the apices, 14-16 in 10 µm, often irregularly spaced. Areolae not discernible in LM.

SEM (Fig. 1 S-Y)

Girdle composed of several open, perforated copulae. Mantle edge covered by irregularly shaped, large mantle plaques (Fig. 1S). Mantle striae restricted to the upper part of the mantle leaving the lower part up to the edge hyaline. Striae uniseriate, composed of very small, rounded areolae (Fig.1 T-W). Virgae between the striae weakly raised, usually flat. Areola series often interrupted due to absence of one to several striae (Fig. 1T, arrows). One rimoportula present at one apex, oblique or in line with the last stria at the apex (Fig. 1 U-W, arrows). Apical pore field present at both apices, relatively large, composed of up to six rows of relatively large, rounded to squarish pores (Fig. 1W). Spines never present. Internally, rimoportula distinctly present at one apex (Fig. 1 X-Y), superimposed on one of the final striae at the apex, centrally placed (Fig. 1X) or asymmetrically on one side (Fig. 1Y).