THE GENUS POLYSIPHONIA. 285 
I. ECORTICATE SPECIES. 
A. Four primary pericentral siphons. 
1. P. rystpiosa Crouan, Flor. Finist. 1867, p. 156 (non Grev.). 
Grammita Richardsoni Crouan in Desmaz. exs. 1209, 1842 (non Hook.). 
G. insidiosa Desmaz. exs. 1209 [corrected label]. 
P. havanensis var. insidiosa J. G. Ag. Spee. Alg. ii. 1863, p. 960. 
Icones. Crouan, Flor. Finist, 1867, pl. 24. fig. 154. Bornet & Thuret, 
Recherch. sur la Feeond. des Florid., Ann. Se. Nat. Bot., 5° ser. vii. 
1867, pl. 12. figs. 18-20. Kesiecatw. Crouan, Alg. mar. Finist. no. 293, 
1852. Desmazières, exs. no. 1209 (with fig.), 1842. Lloyd, Alg. Ouest Fr. 
no. 296, 1854. 
Descriprion.—//abit. Occurring in tufts, branched from the base. 
Branching pseudo-dielotomous, ramuli alternate or unilateral. Diameter 
of the branches variable, attenuate upwards. — S/phons. 4 pericentral and 
very small central. Colour. Greyish-brown, becoming black in drying. 
Anatomy. Articulations visible throughout the plant. Lower articulations 
and those of the ramuli once or twice as long as broad, those of the upper 
parts of main branches 3-4 times their breadth. Cell contents tending to 
adhere to inner wall of pericentral siphons, probably accounting for the 
greyish colour of the plant when fresh. Attachment organ. Plant attached 
to substratum by a number of rhizoids, each bearing a dise at the distal end. 
Rhizoids formed by lateral proliferation of pericentral siphons on creeping 
parts of lowest branches. Reproductive organs. Tetraspores borne in dis- 
torted ramuli. Antheridia cylindrical, obtuse at the apices. Üystocarps 
ovate, sessile on the ramuli. 
P. insidiosa has not been described before from this country. It is not 
mentioned by Harvey (28), Batters (9), or De Toni (22). Agardh regards 
P. insidiosa as a simple form of P. havanensis (2. p. 960), but Thuret 
(54. p. 85) does hot support this view. There appears to be sufficient 
difference both in habit and anatomical characters to confirm the view that 
P. insidiosa is a distinct species; moreover, its branches are much more 
variable in size than those of P. havanensis. P. insidiosa might be confused 
with some forms of P. rhunensis, but the antheridia of the former have longer 
stalks. Also the branches of P. rhunensis arise alternately and the articu- 
lations are shorter. 
Habitat. Occurring on muddy rocks, large stones, and on other algæ at 
extreme low-water mark. hare. 
British Records. Yarmouth, Studland, Swanage, Weymouth (leg. A. D. 
Cotton, 1908), and Torquay. 
Distribution in Europe. Occurring off the coast of N.W. France. 
