99 



describes under the above name is in my opinion the most extreme 

 form of a species, which in some respects rather differs in its 

 development from most other Lithothamnia, so far as these are 

 hitherto known. I have, at least, not been able to draw any limit 

 between this form, which I propose to name f. Rosenvingii, and 

 the above f. Oranii, and it probably includes more forms, the latter 

 perhaps tåken in too wide a sense. 



Description of the form of the species. The form Oranii is 

 at first fastened to shells or smaller stones, forming a very thin 

 crust scarcely up to 0.5 mm. in thickness and closely adherent to 

 the substratum. The peripherical portion of this crust is thinner 

 than the internal, feebly zonated, with a whitish brim in specimens 

 not fully encompassing the object, to which it is fastened. Small 

 excrescences appear at an early stage of development, at first in 

 the central portion, later over the whole or nearly the whole crust. 

 At this stage it very much resembles younger individuals of L. 

 colliculosum. Afterwards the excrescences arise into erect, straight 

 or somewhat bent knobs or short branches, which are either simple, 

 bifid or trifid above, and at length getting more branched in a 

 rather irregular subdichotomous manner, in old individuals with 

 branch-systems of at least three orders, with rather short axes. 

 Pl. 17, fig. 1—7, pl. 22, fig. 1. At the same time the crust by 

 and by disappears, the plant loosens itself from the substratum 

 and lies free on the bottom, still plainly showing that it has been 

 attacked at first, though not in old and more driven specimens. 

 The branches are in older specimens more or less spreading and 

 seldom straight, terete or slightly compressed, either nearly cylin- 

 drical or, more frequently, slightly attenuating towards the apex, 

 with rounded or occasionalfy obtuse ends, about 2 mm. thick, 

 partly less partly a little more. They are frequently a little ana- 

 stomosed below, and the surface partly is nearly smooth, occasio- 

 nally with short, concentric striæ, partly rather uneven on account 

 of numerous local, scaly thickenings. The crust, so far as I have 

 seen, never increases in thickness after the branches are developed, 

 but if much attacked by animals in a younger stage a new crust 

 here and there is formed upon the older, even over short knobs, 



