M. FOSLIE. | 1905 



In the northern areas are often found lying loose on the 

 bottom densely branching roundish knolls of Lithothamnia, which 

 are more or less hollow. This is in most cases caused by attacks 

 of animals, chiefly of boring muscles. They attack the interior 

 of the alga, which is here gradually destroyed and crumbled, while 

 the peripheric parts continue growing, and the specimen even some- 

 times may increase rather considerably in size. I have seen spe- 

 cimens of up to about 10 cm. in diameter which only formed a 

 cover of coalescing branches, about 1 cm. thick, round a number 

 of boring muscles which completely filled up the interior of the alga, 

 or in other cases the interior of the alga formed a cavity with 

 distinct scars from attacks of animals. In such specimens the 

 branches at last become shorter and are more apt to coalesce 

 than elsewhere. In most cases, however, an opening is gradually 

 formed, which may be the direct consequence of the destructive 

 action of the animals, or the breaking through of the comparatively 

 thin peripheric portion is owing to mechanical causes. The opening 

 is mostly formed in the turning down part of the plant. Then an 

 inverted cup-shaped specimen is seen, which by continued destruc- 

 tion becomes inverted crateriform, or plate-shaped, or more or less 

 concave-convex, with the part turning downwards showing worn, 

 truncate and fastigiate ends of branches. Cp. Contrib. II, pi. 2. 

 The latter form, as a rule, rises from specimens which earlier or 

 more typically developed formed masses depressed globular or sub- 

 hemispheric. In most cases the alga still continues growing. Under 

 certain circumstances the turning down and worn part of such 

 specimens will be partly (very seldom wholly) covered with a thin 

 crust developed from the peripheric and live parts. From this crust 

 new branches will grow out, when the specimen is lying on a 

 hard bottom or on other calcareous algae (e. g. in a bank) in such 

 a way that it only in part touches the substratum and allows the 

 light to penetrate below it. In similar condition of things the 

 cavity can be almost filled up with recent ramification from the 

 peripheric parts of an inverted goblet-shaped specimen without the 

 incrustation mentioned. Such specimens then again approach their 

 earlier depressed-globular shape. On the other hand, by continued 



