54 M. FOSLIE. [1905 



— partly or exclusively — two-parted sporangia, whereas else- 

 where they always develop four-parted ones. 1 ) 



The most southern boreal area where sporangia two-parted 

 are developed is in the Mediterranean and the Adriatic. Here is 

 found a form of the epiphytic Lithoph. macrocarpum. Within the 

 cold boreal area and the arctic one, the distribution of almost all 

 species with sporangia four-parted proves to be more circumscribed 

 than that of species with two-parted ones. They occur particularly 

 in the southern part of these areas, while in the northern part 

 species with sporangia two-parted occur in great majority. On the 

 other hand, from the tropics, as far as I am aware, there is as 

 yet found no calcareous alga of the section in question with spor- 

 angia two-parted. This difference of development is probably 

 owing to certain biological conditions, and particularly to the un- 

 favourable states of light in which these algae are growing in the 

 arctic and antarctic areas. Still these algae occur in large numbers, 

 at any rate in the arctic area and in the cold boreal one, being often 

 very vigorously developed and attaining to a considerable size. 

 On the other hand, reproductive organs, as I have mentioned 

 above, are comparatively rarely developed, especially by the spe- 

 cies branching, but this seems frequently to be the case also in 

 warm areas. 



It seems to be necessary to set aside the irregularities mentioned 

 as to the parting of sporangia in certain species, and to give it 

 the value of character of species in others. Otherwise, we might 

 have to unite species, which, when vegetatively typically developed, 

 do not seem to belong to the same series of forms, but which 

 are, in their extreme forms, undistinguishable except by the parting 

 of sporangia. On the other hand, we shall in this way have spe- 

 cies stated which vegetatively show the same development and 

 are in most cases quite like each other except as to the parting 

 of sporangia, e. g. L. soriferum and L. tophiforme in the sense 

 here taken. The latter species, indeed, is comprehended in the L. 



J ) Antarctic Lithothamnia are, as yet, but little known, but as a matter of 

 fact, Lithoth. couhnanicum Fosl. from Coulman Island near South Victoria 

 Land bears only two-parted sporangia. 



