384 ECOLOGY 



An examination 1 of the west coast of Ireland yielded much the same 

 results, but with a still higher " white belt " formed mainly of Lecanora 

 parella and L. atra which covered the rocks lying above high-water mark, 

 "giving them the appearance of having been whitewashed." A more 

 general association for the same position as regards the tide is given by 

 Wheldon and Wilson 2 on the coasts of Arran as : 



Physcia aquila. Placodium tegularis. 



Xanthoria parietina. Ramalina cnspidata. 



Lecanora parella. Physcia stellar is. 



Lecanora atra. Physcia tenella. 



Lecanora campcstris. Verrttcaria maitra. 

 Placodium fcrriigineum var. festiinnn. 



A somewhat similar series of "formations" was determined by Sandstede 3 

 on the coast of Riigen. On erratic granite boulders washed by the tide he 

 found : 



Verrucaria maiira. Lecanora prosechoides. 



Lichina confinis. Placodium lobulatum. 



While in a higher position on similar boulders : 



Lecanora exigna. Lecanora parella. 



Lecanora dispersa. Lecidea colludens. 



Lecanora galactina. Lecidea lavala. 



'Lecanora sulphurea. Lecidea nigroclai'ata f. lenticularis. 



Lecanora saxicola. Xanthoria parietina and f. aureola. 



Lecanora caesiocinerea. Physcia subobscitra. 



Lecanora gibbosa. Physcia caesia. 



Lecanora atra. 



And more rarely a few species of Lecidea. 



b. LICHENS OF SAND-DUNES. These lichens might be included with those 

 of the terricolous communities, but they really represent a maritime com- 

 munity of xerophytic type, subject to the influence of salt spray but not 

 within reach of the tide. They are sun-lichens and react to the strong light 

 in the deeper colour of the thallus. In such a sun-baked area at Findhorn 

 a luxuriant association of lichens was observed growing among short grass 

 and plant debris. It consisted chiefly of: 



Parmelia physodes. Cladonia cervicornis. 



Evernia prunastri. Cladonia endiviaefolia. 



Cetraria aculeata. Peltigera spp. 



On very arid situations the species of Cladonia are those that have a well- 

 developed rather thick primary thallus, probably because such a thallus is 

 able to retain moisture for a prolonged period 4 . On shifting sand, as in the 

 desert, there are no lichens; it is only on surfaces more or less fixed by marram 



1 Knowles 1915. 2 Wheldon and Wilson 1913. 3 Sandstede 1904. 4 Aigret 1901. 



