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FIGURE 4 Gross morphology of the frustule, types of girdle bands and segments, and overlapping of bands.

Connecting band(s)—pleura(e)—element(s) in the middle of the girdle when intercalary bands are present or any element when no intercalary bands are present.

Septum—a sheet or ridge in the valvar plane projecting from a girdle band into the interior of the frustule, often with several openings.

Hyaline band—element of girdle with no perforations (see Hemidiscaceae).

Fine Structure of the Siliceous Cell Wall (Figs. 5 and 7)

Basal siliceous layer—the layer that forms the basic structure of the various components of the frustule.

Annulus (von Stosch, 1977)—a ring of costal thickness, often surrounding one or more processes and with a structure different from that of the rest of the valve (see Porosira and Actinocyclus).

Areola—regularly repeated perforation through the valve wall, often marked by more or less elaborate multiangular walls or ribs (definition slightly deviating from Ross et al., 1979, p. 527).

Velum—a thin perforated layer of silica across an areola.

Cribrum—a velum perforated by regularly arranged pores.

Foramen—the passage through the constriction at the surface opposite the velum.

Poroid areola or poroid—an areola not markedly constricted at one surface of the valve.

Loculate areola or loculus—an areola markedly constricted at one surface of the valve and occluded by a velum at the other.

Alveolus—an elongated chamber running from the central part of the valve to margin, open to the inside and covered by a perforate layer on the outside.

Stria—one or more rows of areolae or pores, or an alveolus. Uniseriate, one row; biseriate, two rows; multiseriate, many rows.

Interstria—the nonperforate siliceous strip between two striae.

Processes (Figs. 6-8)

Process—projection with homogeneously silicified walls.

Labiate process—rimoportula—a tube or an opening through the valve wall with an internal flattened tube or longitudinal slit surrounded by two lips.

Spine—a closed or solid structure projecting out from the surface of the frustule.

LOCULATE AREOLAE /'.''O» ' CD = LOCULI . - -. , - - , -

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