Usually two latticed shells, outermost spherical to oval-shaped, often thick-walled with irregular pores; medullary shell spherical (Stylatractus spp. group). Two polar spines of equal or different size, with or without secondary spines. Two-spined and usually two-shelled actinommids comprise a large, sometimes abundant, highly variable and very poorly studied group. Outer shell diameter without spines: 100-150 µm.
Ref.: Nigrini and Moore (1979).