Valve linear, narrowing toward rounded ends. Axial area narrow, one-fourth to one-fifth breadth of the valve. Central area variable; small, somewhat elliptical, and usually asymmetrical; or widened unilaterally or bilaterally into a transverse fascia. Raphe complex; terminal fissures small but distinct. Striae slightly parallel throughout most of the valve, almost parallel at the center and parallel to slightly convergent toward the ends; crossed by a band which varies in distinctness. This taxon is distinguished from the nominate variety by its finer striae and central area of variable shape(Patrick and Reimer 1966).