Subgen. Pliostroma Fosl. mser. 
(sub Melobesia). 
Thallus inter conceptacula stratis cellularum 5—12 compositus. 
In species typically developed of the Corallinacé-genus Melo- 
besia the thallus is monostromatic with mostly small cortical cells 
which form no continuous layer, but only occur solitary and each 
only covering the foremost part of the subjacent cell.*) In parts 
of the thallus in which conceptacles are developed, a slight growth 
of thickening takes place. Here the layers of cells is usually two 
or three, seldom four (e. g. sometimes in M. Lejolisii). In old 
forms of Humelobesia and Heteroderma, with or without hetero- 
cysts, respectively, the conceptacles are often densely crowded and 
developed over the greater part of the thallus, and when several 
thalli become confluent and form a crust somewhat extended, the 
main part is composed of the said number of layers of cells. 
Then in most cases only the marginal parts are monostromatic. 
On the other hand, there are species Which, even at a younger 
stage of development, but peculiarly when fully developed, only 
show a narrow monostromatic marginal zone, while in other parts 
of the thallus there is a stronger growth of thickness. These parts 
are composed of five to twelve layers of cells by a thickness of 
up to 150 p, even if the conceptacles are not crowded. When 
several thalli are confluent or encompassing, for instance, a terete 
1) In one species, M. subtilissima, I have found no cortical cells, and in 
another, M. minutula, they seem partly to be wanting, partly to occur in 
very small number. Of these species, however, I have only had a scarce 
material. 
