104 Scientific Proceedings, Royal Dublin Society. 



In the open the thallus is well developed, though it is seldom fertile, but 

 where it covers the stones amongst the flowering plants which are close 

 enough to give shelter without too much shade, it is usually well supplied 

 with apothecia. On these rock-slopes, which face towards the west and 

 south-west, Physcia parietina gets all the afternoon sun, and at the same time 

 faces the prevalent winds, which bring a good supply of moisture in wet 

 weather. These alternate bakings and soakings, combined with a fairly 

 sheltered habitat, seem to be the conditions under which Ph. parietina thrives 

 best. Along the eastern coasts Ph. parietina forms sheets similar to those 

 described above, but it is less often fertile. 



Placodhim murorum is much less common on the Howth shores than 

 either P. lobulatum or Physcia parietina. It is difficult to make out what 

 are the conditions under which it thrives best. On sheltered sliores it usually 

 forms a narrow band below Physcia parietina, and in more exposed places, 

 as on the tops of tlie cliils at Drumleck, it is mixed up witli the Physcia 

 growths. It seems to be absent from or very scarce on the east coast, where 

 it is usually replaced by Placodium tegularis. 



Placodium tegularis is very frequent on the eastern and northern coasts, 

 and occurs but seldom on the south and south-west shores. It grows very 

 freely on the cliffs at Gaskin's Leap. The rocky slopes of these cliffs 

 are very similar to those at Drumleck Point. The aspect also is mainly 

 south-west, but the situation is more shaded by surrounding high rocks. 

 The rocks are shales, and tlie surface is rough and much broken up, and 

 seabirds frequently congregate on it. On the sunnier projections Placodium 

 tegularis is of a deep orange colour and covered with apothecia ; but in shady 

 crevices and on the easterly sides of the projections it often covers quite 

 large patches of the surface with greenish barren growths. 



Placodium decipiens forms good colonies on the Needles and on similar 

 high quartzite rocks. Like P. tegularis, it is only fertile on sunny slopes 

 where the aspect is westerly. In shade, as, for instance, on the north-east 

 faces of the Needles, it forms a thick close greenish-yellow covering of 

 barren growths. 



The species of the second list are frequent all round the coast. In 

 shelter on the flat shores of the Broad Strand they are inconspicuous and 

 grow chiefly amongst the orange lichens, seldom forming patches of any 

 extent. On the cliffs and on steep rocks in more exposed situations, however, 

 they often form pure colonies, sometimes of considerable area, above the 

 orange lichens. The way in which Lecanora prosechoides grows on steep 

 quartzite rocks where there is a moderate amount of wave-action, has been 

 already described on page 103, tlie growths of Opegrapha calcarea f. hetero- 



