210 TUFTS COLLEGE STUDIES, VOL. II, No. 3 



crisped edges ; membrane 20-25 /" thick ; cells seen superficially 

 parenchymatously united, chromatophore not filling the cell ; 

 cells angular, often arranged in twos, threes or fours ; in cross 

 section rounded, horizontally elongate, 15-18 p. high. Cell walls 

 persisting after emission of spores. Mass., R. I., Conn. 



Europe. 



A spring plant, nearly allied to M. Grevillei, but quite dis- 

 tinct in habit and texture. It has a thicker membrane, which 

 is also firmer and less lubricous. M. Grevillei forms at first a 

 globular or obovate sac, splitting into broad segments of indefi- 

 nite shape ; M. Lactuca at first is a longer sac, and divides into 

 long, sublinear laciniae, often with a tapering stipe-like base ; 

 when these laciniae are simple, the resemblance to Enteromorpka 

 linza is striking ; when they fork, which is often the case, the 

 resemblance to Ulva fasciata is equally marked ; there are some 

 fronds with quite distinct palmate division of the laciniae. 



4. M. ARCTICUM Wittrock, 1866, p. 44, PI. II, fig. 8 ; P. 

 B.-A., No. 910. Frond attached, at first saccate, soon splitting 

 into a few broad laciniae ; subradiately plicate, with crisped 

 margin ; pale green, becoming yellowish in drying ; membrane 

 25-45 M thick ; cells 4-6 angled, closely set, irregularly placed ; 

 in cross section either vertically or horizontally oval, 10-30 /j. 

 high. Greenland, Alaska. Northern Europe. 



A northern species, chiefly distinguished from M. Grevillci by 

 the thicker frond, which is saccate only in the earliest stages 

 and afterwards appears as a rather broad membrane, not split 

 up into strips, as is usual in M. Grevillei. M. saccodeum Kjell- 

 man, 1883, p. 296, PI. XXVIII, figs, i-io; M. cylindraceum 

 Kjellinan, 1883, p. 295, PI. XXX, and M. angicava Kjellman, 

 1883, p. 297, PI. XXIX, seem to be forms of M. arcticum. 



Var. intestiniforme (Rosenv.) nov. comb. ; M. Grevillei var. 

 intestiniforme Rosenvinge, 1893, p. 953, fig. 52. Frond tubular, 

 to 50 cm. long ; membrane 25-50 /x. thick ; cells rounded, closely 

 set, vertically oval in cross section, 15-20 yu, high ; usually ar- 

 ranged in more or less distinct series longitudinally. Green- 

 land. 



Resembles in habit M. Grevillei var. Vahlii, but in thickness 

 of membrane and size and shape of cells agrees with M. arcticum ; 

 it is rather persistently tubular, and except by careful ex- 

 amination of its structure, is liable to be mistaken for Entero- 

 morpha intestinalis . 



