Like all Porella species, this one has a complex structure with two rows of complicate-bilobed leaves and an additional row of obvious underleaves with decurrent bases. Both lobules and underleaves are large and have recurved margins, which offer good clues to its identity.
Its habitat of dry calcareous rock or base-rich bark helps to rule out the other species of Porella. In such places it sometimes grows with P. arboris-vitae but it lacks the hot peppery taste of that species, or the gloss and smoothness of its shoots. You are unlikely to find it in the habitat of P. cordaeana which rarely strays far from water and even less likely to confuse it with P. pinnata, which always grows where it is regularly submerged by running water, such as on rocks in rivers. P. obtusata is another species of dry places, but is a calcifuge and prefers acidic rocks.
P. platyphylla sometimes creeps across rock and then it can resemble the uncommon Marchesinia mackaii. This lovely liverwort is usually nearly black rather than green, but any doubt can be settled by carefully prising a shoot off and examining its underleaves, which in M. mackaii are large and almost round, without any hint of recurvature.Read the Field Guide account