98 Scientific Proceedings, Royal Dublin Society. 



very far down the cliff-faces. It is usually associated with PavmeUa perhifa 

 and P. sa.ratilis. P. phi/sodes is not fertile on the Howth coasts. 



T/ie Physcias. — Three species, Physcia aquila, Ph. parietina, and Ph. stellaris 

 subsp. tenella, are abundant. 



Physcia aquila likes shady sheltered situations, and above all a north-east 

 aspect. It is more frequently found on the shore-rocks than on the cliffs, and 

 often covers wide expanded areas with its brown thallus. In favourable 

 situations the growth is vigorous, and it may often enough be seen pushing 

 its way down the shore, swamping the Ramalinas and clearing a space for its 

 thallus on the rock. When growing amongst the Ramalinas the thallus of 

 Physcia aquila forms small circular patches round the attachment areas of the 

 fronds. These patches increase in size, and often overgrow the Ramalinas 

 and exterminate tliem. Where the Ramalina growths are close and sward- 

 like the Physcia thallus may often be seen growing on the surface of the 

 fronds working up towards the light and air much in the way that Ivy climbs 

 up the bark of trees, the Physcia thallus being so closely attached to the 

 Eamalina fronds that it seems to be epiphytic on them. If Physcia aquila 

 likes shelter and shade and a north-east aspect, Physcia parietina may be 

 looked for amongst the Ramalinas on sunny rocks in rather exposed situations 

 and especially on those facing south or south-west. In several places along 

 the Howth shores Physcia parietina was noticed growing on the Ramalina 

 fronds in a way similar to that already described for Physcia aquila, and also 

 on dead Thrift stems and over the Thrift leaves in the same way. Physcia 

 tenella is most frequently associated with Physcia parietina. Other species 

 also occur in the Ramalina belt, but not in any abundance. 



The Crustaceous Species. — The connexion between the thallus of the 

 crustaceous lichens and the rocks on which they grow is so much more 

 intimate than is the case with the foliaceous species, that they are enabled to 

 thrive under more adverse conditions and to colonize steeper surfaces. The 

 crustaceous subvegetation is therefore most often found on the steeper rocks 

 where conditions of shelter and moisture are insufficient to support the 

 foliose forms, and it is almost the only subvegetation met with in the lower 

 reaches of the Eamalina growths. 



A complete list of the crustaceous lichens found amongst the Ramalinas 

 would include almost all tliose that have been found on the rocky Howth 

 coasts, with the exception of the truly marine species. 



The impossibility of reeogniziug the majority of these minute lichens in 

 the field has made it a very difficult matter to obtain more than a scanty 

 knowledge of their distribution. The following broad general facts may be 

 stated about the crustaceous subvegetation as a whole : — 



