THE GREEN ALGAE OF NORTH AMERICA 213 



ence can be seen in the form and dimensions of the cells, except 

 that the membrane is thicker in the older parts and thinner in 

 the younger in var. varium, than in the type, in which the frond 

 is of nearly uniform thickness. 



ii. M. PUSCUM (Post, and Rupr.) Wittrock, 1866, p. 53, PI. 

 IV, fig. 13. Frond membranaceous, at first tubular, soon split- 

 ting, dull green, more or less lobed but not divided to the base ; 

 membrane 20-35 /* thick ; cells 4-6-angled, very closely set; in 

 cross section quadrate, with only slightly rounded corners ; 

 occupying nearly the entire thickness of the frond. Greenland, 

 Alaska. Northern Europe. 



Under this species are included two forms, which, with the 

 type, pass into each other with 110 dividing line, while they are 

 sharply marked off from all other species of the genus in nearly 

 eveiy respect. The very young plant is in the form of a closed 

 tube, which soon splits down one side, and spreads out to form 

 a flat membrane ; not splitting into several segments, as in the 

 Grevillei group. Compared with other species of Monostroma, 

 the fit sen in forms are thick and coarse, blackish when dried, 

 staining the paper on which they are mounted, and not adher- 

 ing well to it. 



Forma BLYTTII (Wittr.) Collins, 1903, p. 12 ; M. /?#'//// Witt- 

 rock, 1866, p. 49, PI. Ill, fig. ii ; Farlow, 1881, p. 41 ; M.fus- 

 cum, P. B.-A., No. 715. Frond deep green, blackish in drying, 

 60-70 /u, thick ; cells " palisade-form " in cross section. Fig. 72. 

 Greenland to R. I. ; Vancouver Island, Washington. 



A plant of late autumn and winter in New England, in tide 

 pools and also on pebbles in the sublitoral zone ; great quanti- 

 ties are sometimes washed ashore by storms, the stones on 

 which they grew remaining attached to them. 



Forma SPLENDENS (Wittr.) Collins, 1903, p. 12; P. B.-A., 

 No. 911 ; M. splendcns Wittrock, 1866, p. 50, PI. Ill, fig. 12. 

 Frond deep green, glossy, 50-55 p. thick, more deeply parted 

 than in the other forms ; cells similar to those of forma Blyttii, 

 or more rounded. Alaska to Vancouver I. Northern Asia. 



12. M. LEPTODERMUM Kjellman, 1877, p. 52,- PI. I, figs. 23 and 

 24; P. B.-A., No. 1272; M. zostericohnn Tilden, Amer. Algae, 

 No. 388. Frond cuneate-obovate or divided into segments of 

 that form ; cells squarish, in distinct series, longitudinal and 

 transveVse ; the former often radiate, the latter concentric ; mar- 



