Dorsiventral coralline algal thalli are generally composed of basal filaments ("hypothallus" in older literature), which run +/- parallel to the substrate, and ventral filaments (former "perithallus") which are arranged +/- at right angles to the substrate.
|core = core filaments (former
peri = peripheral filaments (former "perithallus")
primi = primigenous filaments (former "hypothallus")
posti = postigenous filaments (former "perithallus")
Algal thalli can be dimerous or monomerous.
Monomerous thalli (A, B) consist of basal multilayered core filaments ("hypothallium" of older literature) which can be coaxial (A) (i. e., they are arranged in tiers) or non-coaxial (B) (= plumose). Some derivates of core filaments curve outward to form the peripheral filaments ("perithallium" of older literature).
Dimerous thalli (C) consist of unilayered basal primigenous filaments ("hypothallium" of older literature), from which the postigenous filaments ("perithallium" of older literature) arise dorsally at right angles.