a FF 
1903] © Collins, — The Ulvaceae of North America 3 
original monosiphonous filament divide longitudinally, forming two 
parallel series, and there is no further development. This is so dif- 
ferent from most Enteromorpha species that this has been often kept 
as a separate genus, Ze/ranema, Diplonema or Percursaria; but there 
are undoubted species of Zmwferomorpha in which the most of the 
frond is of the normal form, while the tips of the branches are of the 
Jetranema type; so that it seems better to consider this very simple 
form as merely a case where the development stops at an early stage. 
In most species of Ænteromorpha the tubular or saccate form con- 
tinues through life, with or without branching; but in Æ. Zinza the 
membrane on opposite sides adheres more or less, the frond becom- 
ing flat in the middle with open spaces at the edges. This type 
leads to Ua, in which the membranes adhere throughout, forming 
a flat expansion, two layers of cells in thickness. 
The specific distinctions are based somewhat on the general form 
of the frond, but more on the size and shape of the cells and their 
arrangement. The latter, as seen in a superficial view of the frond, 
ranges from Uwa Lactuca with irregular polygonal cells closely set 
in no definite order, to Za fulvescens with cells of circular outline, in 
groups of twos, fours, or their multiples, arranged in longitudinal 
series. The shape of the cells as shown in a cross section of the 
frond js also important, as well as the character of the individual cell 
wall and of the membrane generally. All our species are marine, 
but some extend into brackish, and rarely into quite fresh water. 
As the species now acknowledged have been largely formed by 
segregation from older species, and as in the earlier American litera- 
ture little or no attention was given to microscopic characters, all the 
forms being divided among a few common species, it is evident that 
where these common species are quoted in the older works there 
must be doubt as to which of our present species are meant. The 
result is quite a list of doubtful references, and also some uncertainty 
in cases where the writer has ventured to decide on what seems good 
authority, but without seeing actual specimens, But this is unavoid- 
able in such cases. In the following list the writer has endeavored 
to make as complete as possible the references to American literature, 
but no such attempt has been made with foreign references, and 
apart from the citation of the original description, Agardh's mono- 
graph and De Toni's Sylloge! are the principal works cited. Refer- 
1J. B. De Toni, Sylloge Algarum Omnium hucusque cognitarum. Vol. I. 
Patavii, 1889. 
E us 
