All Taxa

Hemidiscaceae: Actinocyclus

Cells disc-like or in the form of a short barrel, with radiating striae arranged in sectors (fascicles). Valve face sometimes flat, but often concentrically undulate, so that in LM the whole of the valve face cannot all be in focus at the same time. Mantle usually clearly differentiated from the valve face and possessing more densely spaced striae, sometimes bearing a shallow ridge at the junction with the valve face. Areolae large, closely packed, and chambered (SEM). A ring of widely and equally spaced rimoportulae (labiate processes) is present near the periphery of the valve face or on the mantle, appearing in LM as a series of dark spots or flared structures. Also present near the periphery of the valve face or on the mantle is a 'pseudonodulus': a small area in which the valve is differently structured and thinner. This can be extremely difficult or impossible to detect in freshwater species like A. normanii, but is often obvious in large-celled marine species, making them easy to separate in LM from otherwise rather similar species of Coscinodiscus.