No. 11] PLIOSTROMA A NEW SUBGENUS OF MELOBESIA. 5 
section the cells are subquadrate, slightly horizontally or vertically 
elongated, 7 (6)—18 by 7—16 p. "The conceptacles of sporangia 
are scattered or somewhat crowded, convex or convex-conical, 
140—250 (300) p in diameter, when seen from above. The con- 
ceptacles of cystocarps are of about the same size. The sporangia 
are tetrasporic, 45 -60 p long and 30—40 p broad. 
The species proves to be very nearly connected with M. Lejo- 
list, from which it is distinguished essentially only by its greater 
thickness and by a little narrower cells which are more frequently 
somewhat elongated in the direction of the radius. It occurs on 
glass, china, and small shells. At Banyuls sur Mer some years 
ago a Melobesia-like form was found on glass in the aquarium of 
the laboratory Arago. Professor Sauvageau kindly sent me åa 
piece of such aquarium glass with numerous and particularly 
young crusts of this calcareous alga on it. I first supposed that 
it belonged to M. Lejolisti, but have later on come to the con- 
clusion that it has to be referred to M. zonalis. 
An authentic specimen of M. myriocarpa which I have seen, 
corresponds in all essentials with typical M. zonalis. According 
to Crouan, however, it is somewhat varying. I, therefore, as I 
observed I. c., take it in the sense that it corresponds to M. far:- 
nosa f. Solmsiana and M. minutula f. lacunosa with a very irre- 
gular, often subdichotomous thallus. The typical form I consider 
as å more regular, sometimes almost circular one. 
M. zonalis occurs at Brest (Crouan!), Plymouth (Batters!) 
and Banyuls sur Mer (Sauvageau!). 
Melobesia (Pliostroma) mauritiana Fosl. mser. 
Lithophyllum (Carpolithon) mauritianum Fosl. Alg. Not. III (1907) p. 32. 
In this species the peripherical part of the thallus is mostly 
irregular, crenulate or provided with short and rather narrow, 
irregular offshoots. This part is monostromatic. The cells are 
here, as seen from the surface, elongated in the direction of the 
radius, 11—18 (22) p long and 6—9 (11) p broad. The cor- 
tical cells are nearly linear, or oblong. The central parts of 
the thallus attain to a thickness of about 80, and sometimes 
