Although identifiable in the field, P. carolinianus and P. laevis are treated together because they show no differences apart from their sexes. Both grow as opaque, dark green rosettes up to about 3-4 cm across, with a smooth upper surface and non-frilly thallus edges. The dioicous P. laevis has colonies composed of small male plants with almost their entire upper surface covered with male pits, and much larger female plants with scattered reproductive organs (sometimes only visible as bumps on the upper surface) or horns. The monoicous P. carolinianus has a few male pits and a few female organs mixed together on a single, medium-sized thallus. P. carolinianus has been overlooked in Britain in the past, and plants should not be assumed to be P. laevis. Mature spores on the tip of the horns are orange in both species.