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



Apparently not uncommon in quiet waters, especially in 

 ditches in marshes, where the water is sometimes brackish 

 rather than salt. At first it is attached to various objects, but 

 soon becomes loosened and floats freely, sometimes in such 

 abundance as to quite fill a ditch from bottom to surface. It 

 appears in spring, and continues, chiefly in the floating state, 

 through the summer. The arrangement of cells varies in plants 

 from the same locality, and even in different parts of the same 

 frond ; indeed, the shape of the cells, whether seen from above 

 or in cross section, is liable to vary in any species of Monostroma, 

 or in different stages of growth of the individual. 



9. M. OUATERNARIUM (Ku'tz.) Desmazieres, Plantes Crypt, 

 de France, Troiseme Serie, No. 603, 1859 ; Wittrock, 1866, p. 

 37, PL I, fig. 5; P. B.-A., No. 567; Ulva quatcrnaria Kiitz- 

 ing, 1856, p. 6, PI. XIII, fig. 2. Frond at first attached, soon 

 becoming free, soft and delicate, irregularly lobed and folded, 

 20-23 V" thick ; cells rounded, when active!}' dividing set closely 

 in threes and fours within the mother cell wall ; in cross section 

 semicircular or oval, 15-17 /A high. In brackish or fresh water, 

 Washington to Cal. ; various fresh water localities throughout 

 the western U. S. Europe. 



10. M. ORBICULATUM Thuret, 1854, P- 3 88 ; Wittrock, 1866, 

 p. 39, PI. II, fig. 6 ; Alg. Am. Bor., No. 173. Frond mem- 

 brauaceous, attached by fibrils, or later free ; soft and flaccid, 

 sub-orbicular or irregular in outline, often radially plicate, with 

 undulate margin, 30-40 ^ thick ; cells angular, varying much in 

 size and arrangement, often irregularly elongate, closely set, 

 but with chromatophore not occupying the whole cell ; in cross 

 section vertically oval, 25-30 ^ high. In fresh and brackish 

 water. Bermuda, Cal. Europe. 



L,ike other species, this varies much in color, the Bermuda 

 plant being a full green, the California plant quite pale. In the 

 Bermuda specimens the frond is somewhat thinner than the 

 type, and the radical fibrils are strongly developed. 



Var. VARIUM Collins, igoga, p. 26. Frond very much lobed 

 and plicate, forming a rosette-like expansion, attached at the 

 center ; frond 50-60 p. thick in the older part, diminishing to 16 

 P at the margin. On muddy shore near low water mark. 

 So. Mass. 



The much divided and very much folded frond is quite differ- 

 ent in appearance from the typical Af. orbiculatnm, but no differ- 



