Phymatolithon is a genus of nongeniculate

Geniculate (alga)

A geniculate habit, with reference to the red algae, is one in which the alga branches, tree-like, forming "fronds" that attach to the substrate with a holdfast. Non-calcified "genucila" serve as "knees" or hinges between the calcified intergenicula. The geniculate or non-geniculate form of algae...

 coralline red alga

Coralline algae

Coralline algae are red algae in the order Corallinales. They are characterized by a thallus that is hard because of calcareous deposits contained within the cell walls...

, known from the UK and Australia. It is encrusting, flat, and unbranched; it has tetrasporangia and bisporangia borne in multiporate conceptacle

Conceptacle

Conceptacles are specialised cavities of marine and freshwater algae that contain the reproductive organs. They are situated in the receptacle and open by a small ostiole. Conceptacles are present in Corallinaceae, and Hildenbrandiales, as well as the brown Fucales. In the Fucales there is no...

s. Some of its cells bear small holes in the middle; this distinctive thallus texture is termed a "Leptophytum-type" thallus surface, and has been posited as a taxonomically informative character. It periodically sloughs off its epithallus

Epithallus

The epithallium or epithallus is the outer layer of a crustose coralline alga, which in some species is periodically shed to prevent organisms from attaching to and overgrowing the alga.- Structure :...

, reducing its overgrowth by algae by as much as 50% compared to bare rock.