Cocconeis grovei

Valves are broadly elliptic, with rounded apices. The valve length to width ratio is 1.3-1.7. The raphe valve has a hyaline marginal ring that separates a marginal area from an inner area. Striae are radiate. Striae in the central marginal area have many areolae, as compared to fewer marginal areolae at the apices. Striae in the marginal area are more widely spaced than striae in the inner area. Areolae are elliptic and somewhat disorganized near the valve center. Areolae number 14-16 in 10 µm for both the marginal and inner striae (measured at the hyaline band at middle of the raphe valve). The axial area is very narrow. The central area is small and rounded. The raphe is filiform and straight, with slightly inflated proximal ends.

The rapheless valve lacks a hyaline ring. The axial area is narrow. A central area is lacking. Striae are radiate. Areolae appear as short dashes and are arranged in zig-zag longitudinal rows. Areolae number 7-11 in 10 µm measured near the central area.

Our observations are based on specimens collected from a Pleistocene deposit near Terrebonne, Oregon. VanLandingham (1990) remarked that C. grovei became extinct during the Pleistocene epoch. We are uncertain that this species is extinct because the type locality of the Deschutes River suggests that this species may be extant.

We also observed specimens from other regions in the Northwest that share many of the features of C. grovei, but are much smaller in size. Further work is needed to determine if these represent separate, new species.