Lindavia praetermissa

Cells are cylindrical with shallow mantles. The valve face is nearly flat, with a marginal ring of short striae and a central area of many scattered, uniform-sized and evenly-spaced areolae. The areolae may be arranged in a weakly radiate pattern in some specimens. The short marginal striae are slightly unequal in length and composed of very fine areolae. Every fourth or fifth costa is thickened. The costae appear in LM number about 5 in 10 µm.

Håkansson (2002), Houk et al. (2010), Krammer and Lange-Bertalot (1991) and Tanaka (2007) published SEM images and descriptions of P. praetermissa from regions outside of North American. External views show a domed central area, with external openings of areolae and central fultoportulae surrounded by a slight depression. Surrounding the central area are 1 to 5 rimoportula. The marginal area consists of multiseriate striae that continue onto the mantle. Areolae in the striae are much finer than those in the central area. A ring of marginal fultoportulae occurs at the junction of the valve face and mantle.

Internally, striae are expressed as short oblong alveoli separated by costae. The costae are sometimes divided and every fourth or fifth costa is thickened and bears a marginal fultoportula. The fultoportula which consist of a short tube with two lateral satellite pores. One to 5 rimoportulae are located between the central area and the marginal alveoli. The central area is concentrically undulate, with many areolae and central fultoportulae scattered or arranged in radial rows. Each central areola is covered with a domed cribrum. Central fultoportulae consist of a short tube surrounded by three satellite pores.

Note that stria density is variously reported in the literature as 15-18 in 10 µm (Houk et al. 2010), 13-15 in 10 µm (Håkansson 2002), and 13-19 in 10 µm (Krammer & Lange-Bertalot 1991). The stria density of Montana specimens ranges from 19 to 20 in 10 µm. This variability may be due, in part, to different measurement techniques employed by various investigators.