No. 3] REMARKS ON NORTHERN LITHOTHAMNIA. 9 



predominantly gone in one direction so as to determine the em- 

 phatically unilateral bend of the branch-systems. 



A similar state of things is found with varying forms of se- 

 veral crustlike species, which are partly growing in shallower water 

 than the branching ones. If the habitat is much exposed, the mo- 

 tion of the waves will exercise a considerable influence on the 

 development of the alga, in some measure corresponding to the 

 influence of the current in places where the tidal waters run strongly. 

 Thus, by way of example, Lithothamnion Sonderi generally occurs 

 in the upper part of the sublitoral region in a depth of 3 — 10 

 fathoms. Here it develops excrescences densely verruciform, or 

 at any rate shows an uneven surface. On the other hand, when 

 the species occasionally is growing in the litoral region in much 

 exposed places at the open sea, it may become quite smooth and 

 slightly shining. It then assumes so diverging an appearance that 

 even in a fertile state, if it is not closely examined, it may be 

 confounded with Lithothamnion Iceve otherwise rather different. 

 A similar state of things is seen in Phymatolithon polymorphum 

 and in other species. 



In the tropics, where not only crustlike, but also branching 

 calcareous alga? seem to occur in a larger number in the litoral 

 region than in the northern areas, these algae are in exposed 

 places often water-worn. If at the same time they are much en- 

 cumbered by extraneous bodies, rise is given to forms much 

 stunted and almost unrecognizable. In such places other algae, 

 or even Corallinacese only a little incrusted with carbonate of lime, 

 will assume the form of roundish masses densely heaped together 

 and much diverging from specimens typically developed, e. g. 

 Mastophora macrocarpa. Cp. Web. and Fosl. Corall. Siboga-Exp. 

 p. 71, fig. 27. A similar state of things also occurs in exposed 

 localities in the northern areas, where e. g. Corattina officinalis 

 may assume a form nearly corresponding, f. hemisphairica Fosl. 



It is evident from what has been stated above that the great 

 variation of the Lithothamnia is often in essential points owing to 

 extrinsic conditions. By the coincidence of certain circumstances 

 they will — as is mentioned — assume the most peculiar forms, 



