with rather rapid tides densely branched and typically developed 

 globular or hemispherical individuals, especially of the larger forms, 

 very often assume a more depressed shape; and the apex of the 

 branches become truncate or assume disc-shaped, not seldom con- 

 fluent or nearly confluent and even crustlike expansions, while the 

 part turning towards the bottom bears branches of another and 

 apparently typical shape. On the other hand, in more sparsely 

 branched individuals living in sounds with rapid tides, the apex 

 of most of the branches are often denudated or biten off. This I 

 have not seen on greater depths than about 5 fathom, although 

 in certain localities it probably also takes place farther down. 



It appears not to be excluded, that hybrids arise from som'e 

 of the species growing together in banks. The Lithothamnia ge- 

 nerally grow gregarious in great masses, sometimes and most 

 often only one or two species, sometimes more, together. I have 

 seen widely extended banks, once even as far as about 3 kilometer 

 in length, composed of millions of individuals of up to four or 

 five species, not only of the smaller forms, but large ones up to 

 nearly 2 feet in diameter. It has, however, not been possible in 

 this varying group of species to decide with any degree of cer- 

 tainty, whether hybrid forms really occur. I certainly possess spe- 

 cimens which, in my opinion, probably are hybrids, but these I 

 do not quote here, as on closer examination I did not arrive at 

 any satisfactory result. However, a solitary one is mentioned 

 under L. fruticulosum f. flexuosa. 



Specimens of more species are frequently to be found fastened 

 to one and the same substratum, growing together, covering each 

 other, or one fastened to and growing over the other. Branched 

 individuals growing together not unfrequently continue their growth 

 together even being loosened from the substratum, and without 

 any defined limit, not seldom looking as a solitary individual. 



The greatest difficulty in regard to the identifying especially 

 of many of the branched Lithothamnia consists, however, therein, 

 that proportionally very few specimens of these are to be found 

 with reproductive organs. In some species the very greatest number 

 of the specimens gathered in summer have been sterile, only now 



