Mesophyllum alternans (Foslie) comb. nov. (Corallinales, Rhodophyta), a Mediterraneo-Atlantic species, and new considerations on the Lithothamnion philippii Foslie complex

Mesophyllum alternans (Foslie) comb, nov. is a crustose, foliaceous coralline alga that makes up a large part of the calcareous bioconstructions of coralligene bottoms in the Mediterranean. A detailed study of recent samples, compared to specimens of the Foslie collection, clarifies the nature of this entity. Described by Foslie from the Atlantic as Lithiothamnion philippii f. alternans, this form is, after examination of the type specimen, transferred to the genus Mesophyllum and elevated to the rank of species. Morphological and ecological characters of this mediterraneo-atlantic species are given. After examination of their type specimens, Lithothamnion philippii Foslie and Lithothamnion crispatum Hauck, remain as distinct species.