184 
Geo. K. Sutherland. 
This conjoint occurrence has so far prevented the exact deter¬ 
mination of the damage caused to the thallus by Qrcadia pelvetiatia, 
although a slight local browning of infected regions (Fig. 1, 7) 
would point to its being a distinct parasite like the other species 
on Ascophyllum. 
The fruiting bodies commence to form towards the end of 
August or in September, but so far no mature perithecia have been 
observed before October. These are deeply immersed, spherical, 
possessing, when completely formed, ostioles 150/a to almost 200/a 
long which pierce but rarely project beyond the decomposed layer 
overlying the tissues (Fig. 1, 2 b). These may be straight or curved, 
Fig. 1. Orcadia pelvetiana. 7, thallus showing infected areas ; 2, section 
of thallus showing immersed perithecia (a) with long ostioles merely reaching 
the surface coated with disorganised tissue (b) ; 3, view of perithecium with 
woven type of peridium ; 4, asci (a), ascospore (b). 
ending frequently in slightly dilated tips. The peridium, composed 
of loosely woven hyphae (Fig. 1, 3) remains soft and white even 
where it reaches the surface. This has been observed so far only 
in the two genera Trailia and Orcadia ; all the other marine 
Pyrenomycetes show a distinct blackening of exposed or unexposed 
ostioles even when the basal portions remain sub-membranous and 
slightly coloured, or almost hyaline as in Stigmatea. In the lower 
part of the perithecium the wall is lined by a layer, 15 to 20/x thick 
of fine mycelium, forming a substantial cushion from which the 
asci spring. While the periphyses are numerous as in Orcadia 
Ascophylli the paraphyses are few and soon disappear. 
