Craticula cuspidata

Valves are rhombic-lanceolate and widest at the center of the valve before tapering to narrow apices. Striae are parallel and equidistant throughout the entire valve, with thin transapical costae intersecting perpendicular to the longitudinal striae forming an orthostichous pattern. Striae are composed of small, elliptic areolae, approximately 12-14 in 10 µm in the transverse direction and 24-26 in 10 µm longitudinally. The axial area is narrow with a slightly widened central area with slightly concave margins. The raphe is filiform. The proximal raphe ends are expanded and either straight, or slightly hooked in the same direction. The distal raphe ends form hooks, deflected in the same direction.

This species is polymorphic and forms modified valves termed craticulae and heribaudii valves, as in other species within the genus Craticula. The polymorphic valves are formed as the result of increases in salinity. Craticular valves possess a central rib with plates that extend transapically across the valve, but the ribs lack a consistent definite shape or pattern. Craticula valves lack a raphe system. Heribaudii valves have radial, punctate striae that appear to retain an equal distance between striae. Unlike the craticula valves, heribaudii valves possess a simple raphe system similar to vegetative valves.

Patrick and Reimer (1966, plate 43, fig 10) illustrate a capitate form within the species Craticula cuspidata. However, Lange-Bertalot (2001) note that this smaller morph is incorrectly included within C. cuspidata, and should be recognized as the distinct species C. ambigua (Ehrenberg) Mann 1990. This web flora also recognizes C. ambigua. Note, however, that many previous reports of C. cuspidata in North America include C. ambigua.