Knowles — The Maritime and Marine Lichens of Howth. 137 



Weddell, who has examined some of these specimens, says they correspond 

 almost exactly with those in the exsiccata of Th. Fries ; but he noticed 

 a difference in the form and size of the spores of the French specimens, 

 the majority of which were broadly elliptical or almost globose, 

 measuring 6-8)U x 6-7 /x. These correspond exactly with the measure- 

 ments of the spores of V. aquatilis, which is common where fresh 

 water flows on the Howth shores ; V. mucosa, on the contrary, is never 

 met with in these situations, but is confined to rocks between tide-marks. 

 The 'spores of V. mucosa from Howth measure 7-lOiU x 4/^, or occasion- 

 ally 7-10;U X bfl. 



fV. microspora Nyl. — On rocks inside neap-tide marks associated with V. 

 mucosa ; common. Tlie spores of the Howth specimens vary somewhat 

 in size and shape. The type, in which the spores are elliptical and 

 measure 9-10 fx x 4-5 ;u, is most abundant below the Pelvetia zone ; 

 towards low water the spores are frequently longer, often measuring 

 14/i X 5ju. 



*var. mucosula Wedd. — In the upper part of neap-range; frequent. 

 The thallus of this variety is thinner, the peritheoia are smaller 

 and more numerous, and the spores are shorter and broader than 

 in the type. Saudstede (24) records it from similar levels on the 

 German islands of the North Sea. 

 tV. striatula Wahlenb. — On steep shady quartzites in the upper part of neap- 

 range, on the more exposed parts of the coast, associated with Lichina 

 pygmaea, V. microspora, and V. mucosa. 



The main characteristics of V- striatula lie in — 



(1) the thallus, which consists of small scattered or deudritically 

 arranged ridges ; 



(2) the flat perithecia with large shallow apical depressions, which 

 in the Howth plant are usually surrounded by a shiny margin ; 



(3) the small obtuse-ended spores, which measure 6-9 /i x 4/x. 



A study of this lichen as it grows on the Howth coasts shows that 

 while there are specimens that answer to the above description there are 

 others which differ only in having a continuous thallus, with small 

 ridges and dots scattered irregularly on the surface, but sometimes 

 radiating towards the circumference. This state of the lichen, which I 

 call forma continual to distinguish it from the species, is much more 

 abundant, and is usually found on a greater variety of rock and in 

 more sheltered situations. 



' f . euntimia. Thallus continuus, niger vel nigro-virescens, thallina dorsa parva incondite 

 dispersa. 



u2 



