3 OTTO VERNON DARBISHIRE, (Schwed. Südpolar-Exp. 
Placodium ambitiosum nov. spec. 
(Plate 2, fig. 13.) 
Thallus flavescens, margine Placodium miniatum simulans, crustaceus, sed medio 
plus minus fruticulosus et eximie granulosus; podetia apice intense flavescentia et 
parte inferiori pallidiora, ad 3 mm alta; K sanguineo-rubescens; hypothallus nullus 
distinctus; apothecia apicalia, lecanorina, primo rotundata dein multo difformia, 
1..—2.o mm lata; amphithecium marginem thallinum producens bene distinetum et 
thallo concolorem; epithecium aurantiacum, K sanguineo-rubescens semper planum 
aut parce convexum; parathecium et hypothecium decolor; sporae octonae, poly- 
seriatim dispositae, hyalinae, polariter bicellulares, 14—16x4—6 u magnae; spermo- 
gonia nulla visa; habitat ad saxa, Halkland Islands, Port Louis. 
Notes: I think that this species is a new one though it is not far distant from 
the species which HUE calls Polycauliona Charcotii. This latter species has however 
a minutely granular margin to its thallus whereas our plant is more or less con- 
tinuously crustaceous at that point not unlike Placodium miniatum. Towards the 
centre the species becomes quite fruticulose though only to the height of 3 mm. 
HUE has separated the fruticulose species of Placodium from those clearly crusta- 
ceous. I think that in the case of this genus such a proceeding is not necessary 
and not really advisable. It is often very difficult in this particular genus to sepa- 
rate the crustaceous forms from the fruticulose forms. His Polycauliona Charcotii 
is a good example of this. He gives it the following description: “Thallus pallide 
vel obscure spadiceus, opacus primum granulatus granulis parvis, contiguis aut disper- 
sis, mox fruticulosus, erectus, caespitem nunec satis densum, nune passim interruptum 
et hypothallo nigro ceircumdatum (hoc etiam in lacunis visibile) formans.” (HUuE, 
CHARCOT No. 9.) This description clearly shows that this particular species is 
crustaceous at the margin and only becomes fruticulose nearer the centre. This is 
a feature common to many species of the yellow genera Placodium and Xanthoria. 
The margin is more or less crustaceous or feebly foliaceous or properly foliaceous 
and towards the centre where increases of assimilating surface cannot otherwise be 
gained the thallus becomes fruticulose. For these reasons I think that the genus 
Polycauliona should be included in that of Placodium. If it is retained or rather 
the concept for which the generic name stands then we must employ the older name 
introduced by TUCKERMAN namely Zhamnoma |and not Thamnonoma, as spelt by 
HuE (loc. eit.) and Waınıo (Waın. Belg. p. 23)] in his Genera Lichenum p. 107 in 
1872. 
Per 
