No. 3] REMARKS ON NORTHERN LITHOTHAMNIA. 83 



same time developed a new overgrowing hypothallium and new 

 thickening layers, forming a new crust upon the primary one. 

 Even three or four crusts, one above the other, are to be met 

 with. But the oldest of these crusts are mostly falling to decay 

 or crumbling. Thus things are going on, animals and boring algae 

 carrying on their work of destruction and mostly from the inside 

 or from the side turning downward of the alga, while on the 

 other hand the latter is continually forming new layers of tissue 

 and covering the parts attacked. In so doing the alga constantly 

 increases in size, while the crust itself does not grow much thicker 

 than mentioned above. There are found specimens of Ph. inve- 

 stiens which are up to 0,5 m. in diameter, but most frequently 

 they are smaller, obverse-cup-shaped or flattened hemispherical, or 

 irregular. Cp. Norw. Lithoth. pi. 1. In such coarse specimens of 

 the species are often formed subhemispheric or wartlike processes 

 0,5 — 1 cm. in diameter or more. They are, however, more or 

 less hollow and have only come out as the result of the growing 

 over and the covering up extraneous objects, or of the attacks 

 of animals with a consequent brisker formation of tissue. Some- 

 times the whole specimen is hollow with a small opening and very 

 light of weight, while another is comparatively heavy, the encom- 

 passed object being undissolved or the specimen being filled with 

 boring mussels. Thus I have seen specimens only 3 — 4 cm. in 

 diameter filled with small boring mussels, which had nearly dis- 

 solved the encompassed calcareous alga and was about to begin 

 its attack on Ph. investiens itself. It sometimes happens that 

 Ph. investiens, when occurring in banks oiL. glaciate and growing 

 over the latter, is overgrown itself by the said species. But this 

 seems only to occur in conditions particularly unfavourable to the 

 development of Ph. investiens. In old specimens of the species 

 in question which have become inverted cup-shaped or more or 

 less convex-concave, also the inside or the part turning downward 

 of the alga is often covered with new layers of tissue, or a new 

 crust is formed. The same can be the case with specimens which 

 have become water-worn in the part turning upwards. Such spe- 

 cimens will even increase considerably in thickness and finally 



