96 
LAMINARIACEiE.— Thalassiopiiyllum. 
iv. 
naceous, soon drying ; that of the stipe and midrib more coriaceous, or cartilagi- 
nous, The colour is a darkish olive-green, becoming brown in age. The leaves, 
when full grown, are often ten or twelve feet in length, and two or three feet wide. 
Plate V. Fig. 1. A young frond of Agakum Turneri , the natural size ; fig. 2, 
part of a thin vertical slice, through a sorus and the outer coats of the frond ; fig. 
3, spores , in their perispores, from the sorus ; fig. 4, a spore isolated : — all the latter 
figures more or less highly magnified. 
2. Agarum pertusum, Mert. ; “ stipes compressed, coriaceous, continued as a 
scarcely widened midrib ; lamina membranaceous, its holes when young furnished 
with a margin raised at one side, and formed by openings in the bullated mem- 
brane.” J. Ag. Sp. Alg. 1. p. 142. Kiitz. Sp. Alg. p. 580. Post, and Rupr. t. 23. 
Hab. Newfoundland, De la Pylceie (fide J. Ag.) 
I am not acquainted with this species, which is said to have the holes much more 
irregular in shape and fewer in number than those of the preceding species ; also 
of more equal size, and smaller, rarely two lines in width ; and that they arise from 
the bursting of a bullated membrane. 
A third species (A. Gmelini Post, and Rupr. p. 11. t. 20, 21 J is described from, 
the Northern Pacific, characterised chiefly, as it would seem, by having a midrib 
twice as wide as the stipes, and holes witli undulated margins ; but I fear these 
characters can hardly be considered as alone sufficient to distinguish a species, for 
I find among a number of specimens picked up on Nahant Beach, great diversity 
in the comparative breadth of the midrib, and form of the holes. In some of my 
specimens, where the leaf measures 26 inches in length, the midrib is but two lines 
wide ; and in others of somewhat inferior superficies, it is at least five lines, the 
stipe being in the same specimens but two lines wide. I find similar variations 
in specimens collected at Halifax, and that it is impossible to fix limits between 
those with narrow, and those with wide stipes. It will remain to be seen whether 
observers on the shore can detect characters, existing at all ages, between those 
specimens with wide midribs and those with narrow. In many that I possess, the 
apex of the frond, both midrib and lamina, is strongly curved or hooked to one 
side, and this seems generally to occur in those with wide ribs. 
VIII. THALASSIOPHYLLUM. Post, and Rupr. 
Frond with subdistinct leaves ; the leafy expansions formed by the evolution of 
a lamina, spirally developed round a branching stipe ; each leafy-lobe ribless, 
