116 



knobs, that frequently are about 2 — 3 mm. high and 2 — 5 mm. 

 thick, either thickened towards the tip, with obtuse or truncate 

 ends, or occasionally forming wart-like, subconical or subhemis- 

 pherical processes. New crusts are formed, one upon the other, 

 which eling more or less closely to the older one and its knobs, 

 and thereby also the knobs become thicker, but not always stretched 

 over the tip of the latter, or here the edges now and then form 

 somewhat irregular excrescences by trumbling each other. Pl. 17, 

 fig. 17 — 19. Especially in younger crusts a smaller or greater 

 part of the peripherical portion often is free, and this portion is 

 more or less distinctly zonated, with a whitish brim, and the down- 

 wards turned part of such a free portion frequently forms rather 

 distinct concentric ridges. This also now and then is due to new 

 crusts overlapping extraneous objects. 



In f. valida the crust is thicker than in the preceding, up to 

 5 mm. or more, with larger and subhemispherical processes, which 

 in general are 5 — 20 mm. in diameter and 2 — 6 mm. high. Pl. 

 17, fig. 20—21. This form reminds one much in habit of L. 

 loculosum Kjellm. Beringh. Algfl. p. 21, t. 1, fig. 1 — 2. It often 

 grows over shells of Balanidæ and Mytilus, and the new crusts 

 are rather closely united to one another, if not covering extraneous 

 objects. 



The form papillata appears to be less independent than any 

 of the other forms, and often passing into the one or other, but, 

 on the other hand, in its most extreme form it is rather differing. 

 The crust is of about the same thickness as in f. tuberculata, 

 partly, however, nearly even, partly more or less uneven and 

 provided with small processes, particularly in a younger stage with 

 concentric zonated peripherical portion and whitish brim. Pl. 17, 

 fig. 22 — 23. The surface of the frond is in younger individuals 

 of this as well as the other forms rather smooth and often even 

 shining. Magnifying shows, however, the surface to be uneven in 

 consequence of concentric and radiating striæ. These striæ are 

 not, or seldom, to be seen in older individuals, and in such ones 

 the surface never is quite smooth. 



On a radial section the cells of the upper thickening layers 



