M. FOSLIE. [1905 



pi. 13, fig. 2. The surface affected with the attrition often conveys 

 the impression of crustlike formation or almost orbicular discs up 

 to about 1 cm. in diameter, but this is only owing to the fact 

 that the branches partly torn away have inwardly coalesced more 

 than in the outward parts of the plant. Cp. Norw. Lithoth. 1. c. 

 Such portions of the alga oftener prove to be dead; but under 

 certain circumstances new layers of tissue extend themselves from 

 the adjacent parts still live over parts of the dead portions, forming 

 a thin crust over the latter. From this crust are then developed 

 short verruciform branches or somewhat elongated ones, often, 

 however, only from the edge of the said discs. These branches 

 are more or less diverging from the normal ramification of the 

 species. In some localities a great deal of specimens will appear 

 with bigger or smaller specks caused by friction. This state of 

 things bears to a certain degree a considerable resemblance to the 

 state of the alga, when affected by the attacks of lower animals. 1 ) 

 There is every probability that the currents also play the most 

 important part when some branching species are often forming 

 compressed globular masses, whereas the typically developed ones 

 are spherical. The same is, most likely, the case with the flabellate 

 forms or with such ones whose systems of branches often spread 

 like fans almost in one plane. A great deal of the branching Li- 

 thothamnia exhibit forms of that kind. There are species of which,, 

 in certain localities, almost only such forms are developed, e. g. 

 Lithothamnion calcareum f. compressa in Dalkey Sound near Dublin,, 

 the first place where this form was collected by M'Calla. Besides, 

 I have even seen densely branched and attached specimens of up 

 to ten cm. in diameter with branches more or less compressed, 

 all the branch systems being fairly much bent in one and the same 

 direction. In this case there can be no doubt that the current has 



r ) Also mollusks are specked by friction, when occurring in places with strong 

 currents. Thus Mr. Nordgaard says of a kind of muscle (Modiola 

 modiolus): „Some shells were even thoroughly torn so that the mantle-leaf 

 (kappebladet) was quite laid bare." Cp. 0. Nordgaard. Bidrag til 

 Oskjaellets Biologi. Norsk Fiskeritidende. Bergen 1901. Pag. 545. 



