Fry. — Some Types of Endolithic Limestone Lichens. 551 
crowded with small yellow immersed apothecia. To the naked eye the 
thallus appears white, but under the microscope the surface is seen to be 
speckled with black or dark-brown spots and patches. It is granular and 
more irregular than in the three types previously described. After scratch- 
ing away the superficial layers of limestone, comparatively large, bright 
green nests of gonidia are exposed. The apothecium originates within 
the rock, as in Lecidea immersa and V. calciseda—Xhz limestone above 
gradually breaking down, caving in, and falling on the top of the young 
apothecium. This accounts for many of the hollows appearing too big 
for the young apothecia. 
After decalcification the upper surface of the thallus appears coarsely 
woolly, the comparatively large white clumps of tissue being rather far 
apart and apparently having little or no connexion with each other. From 
the underside as well as from the upper, the apothecia can be seen ; in the 
former case they appear as hemispherical, pale yellowish, protruding lumps. 
The thallus is very thin, the average thickness being i6o/ut, and there are 
only a very few extremely short rhizoids. The small black spots on the 
surface have no special significance. Occasionally a brownish patch caps 
one of the white clumps of tissue, but this feature is not common. 
In vertical section several striking differences from the former types are 
seen. The thallus, as indicated above, is made up of a number of almost 
isolated masses of tissue (Plate XXI, Figs. 8 and 9). At the top of each 
mass, which is situated in a pocket in the limestone, there is a more or less 
solid layer, about 23 /x thick, of quadrate hyphal cells. This acts as a plug 
protecting the tissue below, which is composed of hyphae and gonidia — the 
latter being scattered about without any definite grouping of the cells 
(Plate XXI, Fig. 9). The algal cells are unusually large for the size of 
the thallus. From below arise a few hyphae which, for want of a better 
term, may be called rhizoids, and in these there are no inflated cells. In 
a typical case (Plate XXI, Fig. 9) from the outside of the c cap’ to the base 
of the pocket measures 100 /x, and including the rhizoids 125 jx may be reached. 
The whole mass is usually narrower towards the outside of the limestone, 
being only 46 /x in diameter, while at the base it widens or branches and may 
have a breadth of 92 /x or more. Very few hyphae link up these gonidial 
groups, so that nowhere does one find either a continuous cortical, gonidial, 
or rhizoidal zone. 
The apothecia usually penetrate the limestone to a depth below the 
base of the gonidial pockets. They seem to have no more connexion with 
their thallus than have the parts of the thallus one with another. 
From the above description it is seen that P. ' rupestre , var. calvum , 
forma incrustans , almost merits a position in the B ( homoiomerous ) section. 
Against this position there are, however, the caps of hyphal cells above the 
gonidia which correspond to the cortical tissue in the other endolithic lichen 
