84 M. FOSLIE. [1905 



assume a more flattened form than usual. Cp. Norw. Lithoth. 

 pi. 1, fig. 3. The same is also the case with L. fornicatum and 

 other species, when the alga is so situated on a hard substratum 

 that the light can penetrate below it. 



The development of the alga, however, does not always pro- 

 ceed in the manner described above, but frequently more irregularly. 

 Thus there may be formed complexes of indefinable shape, partly 

 owing to the shape of the encompassed objects, partly to the 

 degree and the direction of the attacks and the havoc of the ani- 

 mals and the boring algse. Sometimes are also developed numerous 

 semicircular, now and then lobed, or irregular prolifications which 

 are growing more or less freely, often with concentric striae parti- 

 cularly on the under side, or furrowed almost as in Litlwphyllum 

 expansum and other species, Cp. Norw. Lithoth. pi. 22, fig. 3 — 4. 



The form of Ph. investiens here described represents f. torosa. 

 As far as I have seen, it never forms real branches, it is possible 

 that it sometimes develops small wartlike excrescences. But in 

 the cases I have examined, even such ones have been formed by 

 growing over and covering up extraneous objects, or new layers 

 of tissue have been heaped up round the surface opening of pas- 

 sages made by worms, or caused by other animals. Larger ex- 

 crescences or short branchlike formations may be solid in the 

 uppermost part and convey the impression of real branching, apart 

 from similar ones encompassing branches of a host plant as men- 

 tioned above. But being more closely examined they also prove 

 to have risen from one of the causes mentioned. Nor does it 

 seem that the leaf-like prolifications mentioned above are to be 

 considered as normal formations. 



The form ocellata forms thinner crusts than f. torosa, 0,5 — 1 

 mm. thick. They frequently cling more closely to the host plant 

 than in the said form, never distended between its branches, nor 

 covering several branches collectively. If several crusts are esta- 

 blished on the same substratum, they finally coalesce completely. 

 The form is otherwise perhaps rather to be considered as a young 

 f. torosa. It is most frequently found in small numbers associated 

 with the latter. 



