2 Rhodora [JANUARY 
published by the writer in 1900 gives from New England four 
genera, with twenty-six species, and six varieties. The present 
paper gives four genera, with thirty-three species, and twenty-six 
varieties, fifty-nine species and varieties as compared with Harvey’s 
nine in 1856 and Farlow’s six in 1876. Of many of these species 
no description is to be found in English, and no complete or nearly 
complete list with descriptions is to be found in any language; so 
that a paper of the present scope would seem to be of use. The 
region covered is the North American continent, with Greenland and 
the Alaskan islands. Some indication is also given of extra-limital 
distribution, 
In distinction from some other green algae, also of wide distribu- 
tion and common occurrence, — the Cladophoras, for instance, in 
which specific distinctions are based on characters discernible with 
the naked eye or at most with a pocket lens,— few of the U/vaceae can 
be distinguished except by microscopic examination, The external 
form of the frond certainly counts for something, but only in connec- 
tion with characters of the individual cell. The type of structure is 
a flat membrane composed of a single layer of cells, side by side, all 
similar except at the base of the frond, where they are usually modi- 
fied for the purpose of giving a firmer attachment to the substratum. 
In manner of reproduction there is no distinction to be made, and, 
with the exception of the basal cells referred to, any cell may develop 
the zoospores which are the only form of reproduction known. 
Generic and specific distinctions are therefore based on vegetative 
characters. 
In some forms the membrane is developed very soon after the germ- 
ination of the spore, but in all species there is probably a filament- 
ous stage, though it may be only of a few cells. In the genus Mon- 
ostroma the filament develops into a sac of a more or less clavate form. 
In some species this soon splits, and thereafter the plant grows as a 
flat membrane; at the other extreme in this genus we have a persist- 
ent nearly cylindrical sac, never splitting, and opening at the top 
only in fructification. Between these two all intermediate forms can 
be found in the various species. When the sac continues perma- 
nently closed except from external causes, we have the genus Entero- 
morpha, in which the species range from broadly clavate to filiform, 
and from simple to much branched; in Æ. percursa the cells of the 
1 RHODORA, Vol. II, 41, 1900. 
435 
