210 Proceedings of {he Royal Irish Academy. 



6. Pariiif/tK olivacea (L.). 



The olive-green of the upper-surface is due to the absence of a definite 

 cortical layer. The gonidial layer is an irrregular one, but contains large 

 numbers of gonidia reaching to the upper-surface; here and there they also 

 extend almost to the lower. The lower-surface is blackish and thickly 

 covered with rhizines towards the centre: it is paler, shining, and without 

 rhizines towards tin* edge. The corrugations of the lower surface frequently 

 appear in section as angular out-growths, the whole being covered by a thin 

 but definite and very darkly coloured cortex. The rhizines are stout and well 

 developed, with a central core of medullary tissue. 



The species represented in fig. 15 is P. exasperata, regarded formerly as a 

 papillate variety of P. olivacea, now accorded specific rank, but agreeing 



ntially in structure with the typical specimens of the parent species. 



7. Parnulia caperata (L.). 



The pale-gi eeniab upper-surface with its broad lobes is easily distinguished. 

 The under-surface is black, becoming ,i shining li.uht brown towards the edge. 

 Rhizines do noi reach the edge, but are large and numerous in the centre 

 (fig. 16). 



Fig. 17 Bhows .in extreme case of fusion of the thalline lobes. At («) the 

 of a lobe is overshadowed by a neighbouring lobe and has on its morpho- 

 logically upper surfaces black cortex continuous with its lower cortex. At (b) 

 a 1 • surrounded by a black cortical layer. The broad masses 



of black tissue an- lower cortices of connecting lobes, and not connecting 

 ma — such as occui between ('m and (c). 



8. Parnulia pi rlata {L.). 



The upper-s - light grey or bluish white, and, both cortices being 



very Btrougly developed, affords a striking contrast to the black under-surface 

 (fig. 18). The latter becomes pale blown towards the edge. The rhizines are 

 comparatively few, not reaching the edge, bul well developed and black. In 

 central portions of the thalius which are not adherent to the substratum, 

 papillae are present in large numbers ami present an irregular surface, 

 becoming smooth in the young rhizine state (fig. 19). Further, they arise 

 from a broad-spree inc.: base, whereas the rhizines are frequently without a 

 definite basal expansion or thalline cup. The hypothalline cup or disc is, on 

 the other hand, well developed, as in fig. L'U. 



These considerations suggest that the papillae are abortive rhizines which, 



