PI. cell, aph. 2. TH ALASSIOPHYTiE. 75. Halidrys. 39-1 
L. 75. HALIDRYS. Lyngbye. Sea-oak. 
Thallus compressed, confluent, forked, ribless ; vesicles 0, 
or innate; holdfasts shieldlike; sporidia intermixed with 
jointed fibres in roundish thecse immersed in pediceiled, 
lateral apothecia. 
1. Plalidrys nodosa. Knobbed sea-oak. 
Thallus pinnately branched. 
Fucus maritimus nodosus, Raii Syn. 48,41. 
Fucus nodosus, Lin. S. P. 1628; Engl. Rot. 570; Turn. Hist. >91. 
Halidrys nodosa, Lyngbye Hydr. Dan. 37. 
Sea-shore; perennial; winter. 
Thallus 3 or 4 feet long, flat; apothecia roundish, mostly 
solitary. 
(3. minor. Apothecia ovate, hardly wider than the pe- 
duncle. 
7 . siliquatus. Apothecia linear, lanceolate. 
2. Halidrys canaliculata. Channelled sea-oak. 
Thallus compressed, ribless, linear, grooved on one side, 
level-topped; edge not cut; tip 2-cut; apothecia oblong, 
2-cut, turned. 
Fucus pumilus dichotomus, segmentis ex una parte gibbosis, ex altera 
excavatis, Raii Syn. 43, 12. 
Fucus canal iculatus, Linn. Syst. Nat. 716 ; Engl. Bot . 823 ; Turn. Hist. 3. 
Fucus excisus, Lin. S. P. 1627. 
Fucus rotundus, Esper , 17. 
Sea-shore; perennial. 
Thalli aggregate, 2 to 4 inches long. 
3. Halidrys Mackaii. Mackay’s sea-oak. 
Thallus leatherlike, cylindrical, threadlike, forked, level- 
topped; tips blunt; vesicles scattered, innate, elliptical* 
solitary, wider than the thallus. 
Fucus Mackaii, Turn. Hist. 52; Engl. Bot. 1927. 
Sea-shore; perennial; winter. 
Thallus 6 to 12 inches high, low 7 er part compressed; 
angles of the fork roundish. 
LI. 71. MACKAIA. Mackay. 
Thallus cartilaginous, rod-like, shrubby ; upper part 
branched, narrow, roundish compressed, below compressed, 
broad, leaflike; holdfasts shieldlike; vesicles 0, or innate 
in the branches; sporidia in roundish thecse intermixed 
with jointed fibre*, or in terminal apothecia. 
