8 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. 

 1.0 — 2.0 mm lata; amphithecium marginem thallinum producens bene distinctum 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 11 magnae; spermo- 

 gonia nulla visa; habitat ad saxa, Falkland Islands, Port Louis. 



Notes: I think that this species is a new one though it is not far distant from 

 the species which Hl/E 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 ver} - 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 nunc satis densum, nunc passim interruptum 

 et hypothallo nigro circumdatum (hoc etiam in lacunis visibile) formans." (HUE, 

 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 Thamnoma [and not Thamuonoma, as spelt by 

 Hue (loc. cit.) and WAINIO (Wain. Belg. p. 23)] in his Genera Lichenum p. 107 in 

 1872. 



