108 TRANSACTIONS AND PROCEEDINGS OF THE [Szss. uxrx, 
Placodium-structure. The marginal podetia show a dorsi- 
ventral arrangement, the short assimilators springing from 
the upper side only. But even here, near the margin the 
dorsiventral and free podetia can be distinguished perfectly 
from the protothallus, which is firmly attached to the rocky 
substratum, 
The protothallus consists of fine strands of fungal hyphe, 
which, white in colour, radiate out in an irregular manner 
from the base of the podetia. At this latter point the proto- 
thallus is often very thick. 
The gonidia are fairly evenly distributed in the podetia, 
where these are exposed to light, but the gonidia are massed 
together at those points where a new branch or an assimilator 
is about to sprout. 
The general structure of the apothecium is that typical 
of Placodium species. It is up to 4 mm. in diameter, with 
vrange epithecium and distinct thalline margin, which, how- 
ever, gradually sinks below the level of the epithecium. The 
light hyaline spores are polar-bilocular; parathecium and 
amphithecium are colourless, and green gonidia are found 
under the hypothecium. 
This plant is very nearly related to Placodiwm coralloides, 
Tuck. (Synopsis of the North American Lichens, 1. p. 169), 
and P. cladodes, Tuck. (lc.). It differs from the latter by 
having colourless spores in each ascus instead of one brown 
one. It is also stouter and bigger than both species of 
Tuckerman. I have only seen specimens of P. coralloides. 
The big apothecia of P. fruticuloswm also retain their amphi- 
thecium throughout life. 
Placodiuwm fruticulosum is an interesting plant which belongs 
to the subgenus Thamnoma of Placodiwm, created by 
Tuckerman for his species coralloides and cladodes. The 
thallus is throughout distinctly diploblastic, the protothallus 
being easily separated from the podetia, even when the latter 
are prostrate, near the margin of the plants. 
Several species of Placodiwm have a tendency to become 
fruticulose. Thus in H. Lojka Lich. Regni Hung. exsic. i. 
(1882), n. 26, Lecanora elegans Lk v. compacta (Arn.) Nyl. 
( = Placodiwm) shows fruticulose podetia in the centre of the 
thallus. 
Xanthoria lychnea (Ach.) Th. Fr., N. and S. America, N. 
